python-docs-fa/library/dataclasses.html at gh-pages · python/python-docs-fa

History

1226 lines (1139 loc) · 128 KB

Raw

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

172

173

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

195

196

197

198

199

200

201

202

203

204

205

206

207

208

209

210

211

212

213

214

215

216

217

218

219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

236

237

238

239

240

241

242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

262

263

264

265

266

267

268

269

270

271

272

273

274

275

276

277

278

279

280

281

282

283

284

285

286

287

288

289

290

291

292

293

294

295

296

297

298

299

300

301

302

303

304

305

306

307

308

309

310

311

312

313

314

315

316

317

318

319

320

321

322

323

324

325

326

327

328

329

330

331

332

333

334

335

336

337

338

339

340

341

342

343

344

345

346

347

348

349

350

351

352

353

354

355

356

357

358

359

360

361

362

363

364

365

366

367

368

369

370

371

372

373

374

375

376

377

378

379

380

381

382

383

384

385

386

387

388

389

390

391

392

393

394

395

396

397

398

399

400

401

402

403

404

405

406

407

408

409

410

411

412

413

414

415

416

417

418

419

420

421

422

423

424

425

426

427

428

429

430

431

432

433

434

435

436

437

438

439

440

441

442

443

444

445

446

447

448

449

450

451

452

453

454

455

456

457

458

459

460

461

462

463

464

465

466

467

468

469

470

471

472

473

474

475

476

477

478

479

480

481

482

483

484

485

486

487

488

489

490

491

492

493

494

495

496

497

498

499

500

501

502

503

504

505

506

507

508

509

510

511

512

513

514

515

516

517

518

519

520

521

522

523

524

525

526

527

528

529

530

531

532

533

534

535

536

537

538

539

540

541

542

543

544

545

546

547

548

549

550

551

552

553

554

555

556

557

558

559

560

561

562

563

564

565

566

567

568

569

570

571

572

573

574

575

576

577

578

579

580

581

582

583

584

585

586

587

588

589

590

591

592

593

594

595

596

597

598

599

600

601

602

603

604

605

606

607

608

609

610

611

612

613

614

615

616

617

618

619

620

621

622

623

624

625

626

627

628

629

630

631

632

633

634

635

636

637

638

639

640

641

642

643

644

645

646

647

648

649

650

651

652

653

654

655

656

657

658

659

660

661

662

663

664

665

666

667

668

669

670

671

672

673

674

675

676

677

678

679

680

681

682

683

684

685

686

687

688

689

690

691

692

693

694

695

696

697

698

699

700

701

702

703

704

705

706

707

708

709

710

711

712

713

714

715

716

717

718

719

720

721

722

723

724

725

726

727

728

729

730

731

732

733

734

735

736

737

738

739

740

741

742

743

744

745

746

747

748

749

750

751

752

753

754

755

756

757

758

759

760

761

762

763

764

765

766

767

768

769

770

771

772

773

774

775

776

777

778

779

780

781

782

783

784

785

786

787

788

789

790

791

792

793

794

795

796

797

798

799

800

801

802

803

804

805

806

807

808

809

810

811

812

813

814

815

816

817

818

819

820

821

822

823

824

825

826

827

828

829

830

831

832

833

834

835

836

837

838

839

840

841

842

843

844

845

846

847

848

849

850

851

852

853

854

855

856

857

858

859

860

861

862

863

864

865

866

867

868

869

870

871

872

873

874

875

876

877

878

879

880

881

882

883

884

885

886

887

888

889

890

891

892

893

894

895

896

897

898

899

900

901

902

903

904

905

906

907

908

909

910

911

912

913

914

915

916

917

918

919

920

921

922

923

924

925

926

927

928

929

930

931

932

933

934

935

936

937

938

939

940

941

942

943

944

945

946

947

948

949

950

951

952

953

954

955

956

957

958

959

960

961

962

963

964

965

966

967

968

969

970

971

972

973

974

975

976

977

978

979

980

981

982

983

984

985

986

987

988

989

990

991

992

993

994

995

996

997

998

999

1000

<!DOCTYPE html>

<head>

<title>dataclasses --- Data Classes — مستندات Python3.14.6</title><meta name="viewport" content="width=device-width, initial-scale=1.0">

title="جستجو در مستندات Python3.14.6"

href="../_static/opensearch.xml"/>

<style>

@media only screen {

table.full-width-table {

width: 100%;

}

</style>

</head>

<body>

<input type="checkbox" id="menuToggler" class="toggler__input" aria-controls="navigation"

aria-pressed="false" aria-expanded="false" role="button" aria-label="Menu">

</label>

</a>

</svg>

</form>

</nav>

Theme

<option value="light">Light</option>

</select>

</label>

<div>

<h3><a href="../contents.html">فهرست عناوین</a></h3>

<ul>

<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate">dataclasses</code> --- Data Classes</a><ul>

<li><a class="reference internal" href="#module-contents">Module contents</a></li>

<li><a class="reference internal" href="#post-init-processing">Post-init processing</a></li>

<li><a class="reference internal" href="#class-variables">Class variables</a></li>

<li><a class="reference internal" href="#init-only-variables">Init-only variables</a></li>

<li><a class="reference internal" href="#frozen-instances">Frozen instances</a></li>

<li><a class="reference internal" href="#inheritance">Inheritance</a></li>

<li><a class="reference internal" href="#re-ordering-of-keyword-only-parameters-in-init">Re-ordering of keyword-only parameters in <code class="xref py py-meth docutils literal notranslate">__init__()</code></a></li>

<li><a class="reference internal" href="#default-factory-functions">Default factory functions</a></li>

<li><a class="reference internal" href="#mutable-default-values">Mutable default values</a></li>

<li><a class="reference internal" href="#descriptor-typed-fields">Descriptor-typed fields</a></li>

</ul>

</li>

</ul>

</div>

<div>

<h4>موضوع قبلی</h4>

<a href="warnings.html"

title="فصل قبلی"><code class="xref py py-mod docutils literal notranslate">warnings</code> --- Warning control</a>

</div>

<div>

<h4>موضوع بعدی</h4>

<a href="contextlib.html"

title="فصل بعدی"><code class="xref py py-mod docutils literal notranslate">contextlib</code> --- Utilities for <code class="xref std std-keyword docutils literal notranslate">with</code>-statement contexts</a>

</div>

document.addEventListener('DOMContentLoaded', () => {

const title = document.querySelector('meta[property="og:title"]').content;

const elements = document.querySelectorAll('.improvepage');

const pageurl = window.location.href.split('?')[0];

elements.forEach(element => {

const url = new URL(element.href.split('?')[0].replace("-nojs", ""));

url.searchParams.set('pagetitle', title);

url.searchParams.set('pageurl', pageurl);

url.searchParams.set('pagesource', "library/dataclasses.rst");

element.href = url.toString();

});

</script>

<li><a href="../bugs.html">گزارش یک اشکال</a></li>

<li><a class="improvepage" href="../improve-page-nojs.html">بهبود این صفحه</a></li>

<li>

<a href="https://github.com/python/cpython/blob/main/Doc/library/dataclasses.rst?plain=1"

rel="nofollow">نمایش منبع

</a>

</li>

<li>

<a href="https://github.com/python/python-docs-fa/blob/3.14/library/dataclasses.po?plain=1"

rel="nofollow">نمایش منبع ترجمه</a>

</li>

</ul>

</div>

</nav>

</div>

<h3>ناوبری</h3>

<ul>

<a href="../genindex.html" title="فهرست کلی"

accesskey="I">فهرست</a></li>

<a href="../py-modindex.html" title="نمایه ی ماژول های پایتون"

>ماژول ها</a> |</li>

<a href="contextlib.html" title="contextlib --- Utilities for with-statement contexts"

accesskey="N">بعدی</a> |</li>

<a href="warnings.html" title="warnings --- Warning control"

accesskey="P">قبلی</a> |</li>

<li><a href="https://www.python.org/">Python</a> »</li>

</li>

<li>

</li>

<a href="../index.html">3.14.6 Documentation</a> »

</li>

<li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li>

<li class="nav-item nav-item-2"><a href="python.html" accesskey="U">Python Runtime Services</a> »</li>

<li class="nav-item nav-item-this"><a href=""><code class="xref py py-mod docutils literal notranslate">dataclasses</code> --- Data Classes</a></li>

</form>

</div>

</li>

Theme

<option value="light">Light</option>

</select>

</label> |</li>

</ul>

</div>

<h1><code class="xref py py-mod docutils literal notranslate">dataclasses</code> --- Data Classes<a class="headerlink" href="#module-dataclasses" title="Link to this heading">¶</a></h1>

Source code: <a class="extlink-source reference external" href="https://github.com/python/cpython/tree/3.14/Lib/dataclasses.py">Lib/dataclasses.py</a>

This module provides a decorator and functions for automatically

adding generated <a class="reference internal" href="../glossary.html#term-special-method">special methods</a> such as <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> and

<a class="reference internal" href="../reference/datamodel.html#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate">__repr__()</code></a> to user-defined classes. It was originally described

in <a class="pep reference external" href="https://peps.python.org/pep-0557/">PEP 557</a>.

The member variables to use in these generated methods are defined

using <a class="pep reference external" href="https://peps.python.org/pep-0526/">PEP 526</a> type annotations. For example, this code:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>from dataclasses import dataclass

@dataclass

class InventoryItem:

"""Class for keeping track of an item in inventory."""

name: str

unit_price: float

quantity_on_hand: int = 0

def total_cost(self) -> float:

return self.unit_price * self.quantity_on_hand

</pre></div>

</div>

will add, among other things, a <code class="xref py py-meth docutils literal notranslate">__init__()</code> that looks like:

self.name = name

self.unit_price = unit_price

self.quantity_on_hand = quantity_on_hand

</pre></div>

</div>

Note that this method is automatically added to the class: it is not

directly specified in the <code class="xref py py-class docutils literal notranslate">InventoryItem</code> definition shown above.

Added in version 3.7.

</div>

<h2>Module contents<a class="headerlink" href="#module-contents" title="Link to this heading">¶</a></h2>

@dataclasses.dataclass(<abbr title="Keyword-only parameters separator (PEP 3102)">*</abbr>, init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False, match_args=True, kw_only=False, slots=False, weakref_slot=False)<a class="headerlink" href="#dataclasses.dataclass" title="Link to this definition">¶</a></dt>

<dd>This function is a <a class="reference internal" href="../glossary.html#term-decorator">decorator</a> that is used to add generated

<a class="reference internal" href="../glossary.html#term-special-method">special methods</a> to classes, as described below.

The <code class="docutils literal notranslate">@dataclass</code> decorator examines the class to find

<code class="docutils literal notranslate">field</code>s. A <code class="docutils literal notranslate">field</code> is defined as a class variable that has a

<a class="reference internal" href="../glossary.html#term-variable-annotation">type annotation</a>. With two

exceptions described below, nothing in <code class="docutils literal notranslate">@dataclass</code>

examines the type specified in the variable annotation.

The order of the fields in all of the generated methods is the

order in which they appear in the class definition.

The <code class="docutils literal notranslate">@dataclass</code> decorator will add various "dunder" methods to

the class, described below. If any of the added methods already

exist in the class, the behavior depends on the parameter, as documented

below. The decorator returns the same class that it is called on; no new

class is created.

If <code class="docutils literal notranslate">@dataclass</code> is used just as a simple decorator with no parameters,

it acts as if it has the default values documented in this

signature. That is, these three uses of <code class="docutils literal notranslate">@dataclass</code> are

equivalent:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class C:

...

@dataclass()

class C:

...

@dataclass(init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False,

match_args=True, kw_only=False, slots=False, weakref_slot=False)

class C:

...

</pre></div>

</div>

The parameters to <code class="docutils literal notranslate">@dataclass</code> are:

<ul>

<li>init: If true (the default), a <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> method will be

generated.

If the class already defines <code class="xref py py-meth docutils literal notranslate">__init__()</code>, this parameter is

ignored.

</li>

<li>repr: If true (the default), a <a class="reference internal" href="../reference/datamodel.html#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate">__repr__()</code></a> method will be

generated. The generated repr string will have the class name and

the name and repr of each field, in the order they are defined in

the class. Fields that are marked as being excluded from the repr

are not included. For example:

<code class="docutils literal notranslate">InventoryItem(name='widget', unit_price=3.0, quantity_on_hand=10)</code>.

If the class already defines <code class="xref py py-meth docutils literal notranslate">__repr__()</code>, this parameter is

ignored.

</li>

<li>eq: If true (the default), an <a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate">__eq__()</code></a> method will be

generated. This method compares the class as if it were a tuple

of its fields, in order. Both instances in the comparison must

be of the identical type.

If the class already defines <code class="xref py py-meth docutils literal notranslate">__eq__()</code>, this parameter is

ignored.

</li>

<li>order: If true (the default is <code class="docutils literal notranslate">False</code>), <a class="reference internal" href="../reference/datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate">__lt__()</code></a>,

<a class="reference internal" href="../reference/datamodel.html#object.__le__" title="object.__le__"><code class="xref py py-meth docutils literal notranslate">__le__()</code></a>, <a class="reference internal" href="../reference/datamodel.html#object.__gt__" title="object.__gt__"><code class="xref py py-meth docutils literal notranslate">__gt__()</code></a>, and <a class="reference internal" href="../reference/datamodel.html#object.__ge__" title="object.__ge__"><code class="xref py py-meth docutils literal notranslate">__ge__()</code></a> methods will be

generated. These compare the class as if it were a tuple of its

fields, in order. Both instances in the comparison must be of the

identical type. If order is true and eq is false, a

<a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> is raised.

If the class already defines any of <code class="xref py py-meth docutils literal notranslate">__lt__()</code>,

<code class="xref py py-meth docutils literal notranslate">__le__()</code>, <code class="xref py py-meth docutils literal notranslate">__gt__()</code>, or <code class="xref py py-meth docutils literal notranslate">__ge__()</code>, then

<a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> is raised.

</li>

<li>unsafe_hash: If true, force <code class="docutils literal notranslate">dataclasses</code> to create a

<a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate">__hash__()</code></a> method, even though it may not be safe to do so.

Otherwise, generate a <code class="xref py py-meth docutils literal notranslate">__hash__()</code> method according to how

eq and frozen are set.

The default value is <code class="docutils literal notranslate">False</code>.

<code class="xref py py-meth docutils literal notranslate">__hash__()</code> is used by built-in <a class="reference internal" href="functions.html#hash" title="hash"><code class="xref py py-meth docutils literal notranslate">hash()</code></a>, and when objects are

added to hashed collections such as dictionaries and sets. Having a

<code class="xref py py-meth docutils literal notranslate">__hash__()</code> implies that instances of the class are immutable.

Mutability is a complicated property that depends on the programmer's

intent, the existence and behavior of <code class="xref py py-meth docutils literal notranslate">__eq__()</code>, and the values of

the eq and frozen flags in the <code class="docutils literal notranslate">@dataclass</code> decorator.

By default, <code class="docutils literal notranslate">@dataclass</code> will not implicitly add a <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate">__hash__()</code></a>

method unless it is safe to do so. Neither will it add or change an

existing explicitly defined <code class="xref py py-meth docutils literal notranslate">__hash__()</code> method. Setting the class

attribute <code class="docutils literal notranslate">__hash__ = None</code> has a specific meaning to Python, as

described in the <code class="xref py py-meth docutils literal notranslate">__hash__()</code> documentation.

If <code class="xref py py-meth docutils literal notranslate">__hash__()</code> is not explicitly defined, or if it is set to <code class="docutils literal notranslate">None</code>,

then <code class="docutils literal notranslate">@dataclass</code> may add an implicit <code class="xref py py-meth docutils literal notranslate">__hash__()</code> method.

Although not recommended, you can force <code class="docutils literal notranslate">@dataclass</code> to create a

<code class="xref py py-meth docutils literal notranslate">__hash__()</code> method with <code class="docutils literal notranslate">unsafe_hash=True</code>. This might be the case

if your class is logically immutable but can still be mutated.

This is a specialized use case and should be considered carefully.

Here are the rules governing implicit creation of a <code class="xref py py-meth docutils literal notranslate">__hash__()</code>

method. Note that you cannot both have an explicit <code class="xref py py-meth docutils literal notranslate">__hash__()</code>

method in your dataclass and set <code class="docutils literal notranslate">unsafe_hash=True</code>; this will result

in a <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>.

If eq and frozen are both true, by default <code class="docutils literal notranslate">@dataclass</code> will

generate a <code class="xref py py-meth docutils literal notranslate">__hash__()</code> method for you. If eq is true and

frozen is false, <code class="xref py py-meth docutils literal notranslate">__hash__()</code> will be set to <code class="docutils literal notranslate">None</code>, marking it

unhashable (which it is, since it is mutable). If eq is false,

<code class="xref py py-meth docutils literal notranslate">__hash__()</code> will be left untouched meaning the <code class="xref py py-meth docutils literal notranslate">__hash__()</code>

method of the superclass will be used (if the superclass is

<a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate">object</code></a>, this means it will fall back to id-based hashing).

</li>

<li>frozen: If true (the default is <code class="docutils literal notranslate">False</code>), assigning to fields will

generate an exception. This emulates read-only frozen instances.

See the <a class="reference internal" href="#dataclasses-frozen">discussion</a> below.

If <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate">__setattr__()</code></a> or <a class="reference internal" href="../reference/datamodel.html#object.__delattr__" title="object.__delattr__"><code class="xref py py-meth docutils literal notranslate">__delattr__()</code></a> is defined in the class

and frozen is true, then <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> is raised.

</li>

<li>match_args: If true (the default is <code class="docutils literal notranslate">True</code>), the

<a class="reference internal" href="../reference/datamodel.html#object.__match_args__" title="object.__match_args__"><code class="xref py py-attr docutils literal notranslate">__match_args__</code></a> tuple will be created from the list of

non keyword-only parameters to the generated <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> method (even if

<code class="xref py py-meth docutils literal notranslate">__init__()</code> is not generated, see above). If false, or if

<code class="xref py py-attr docutils literal notranslate">__match_args__</code> is already defined in the class, then

<code class="xref py py-attr docutils literal notranslate">__match_args__</code> will not be generated.</li>

</ul>

Added in version 3.10.

</div>

</div></blockquote>

<ul>

<li>kw_only: If true (the default value is <code class="docutils literal notranslate">False</code>), then all

fields will be marked as keyword-only. If a field is marked as

keyword-only, then the only effect is that the <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a>

parameter generated from a keyword-only field must be specified

with a keyword when <code class="xref py py-meth docutils literal notranslate">__init__()</code> is called. See the <a class="reference internal" href="../glossary.html#term-parameter">parameter</a>

glossary entry for details. Also see the

<a class="reference internal" href="#dataclasses.KW_ONLY" title="dataclasses.KW_ONLY"><code class="xref py py-const docutils literal notranslate">KW_ONLY</code></a> section.

Keyword-only fields are not included in <code class="xref py py-attr docutils literal notranslate">__match_args__</code>.

</li>

</ul>

Added in version 3.10.

</div>

</div></blockquote>

<li>slots: If true (the default is <code class="docutils literal notranslate">False</code>), <a class="reference internal" href="../reference/datamodel.html#object.__slots__" title="object.__slots__"><code class="xref py py-attr docutils literal notranslate">__slots__</code></a> attribute

will be generated and new class will be returned instead of the original one.

If <code class="xref py py-attr docutils literal notranslate">__slots__</code> is already defined in the class, then <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>

is raised.</li>

</ul>

هشدار

Passing parameters to a base class <a class="reference internal" href="../reference/datamodel.html#object.__init_subclass__" title="object.__init_subclass__"><code class="xref py py-meth docutils literal notranslate">__init_subclass__()</code></a>

when using <code class="docutils literal notranslate">slots=True</code> will result in a <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>.

Either use <code class="docutils literal notranslate">__init_subclass__</code> with no parameters

or use default values as a workaround.

See <a class="reference external" href="https://github.com/python/cpython/issues/91126">gh-91126</a> for full details.

</div>

Added in version 3.10.

</div>

تغییر داده شده در نسخه 3.11: If a field name is already included in the <code class="xref py py-attr docutils literal notranslate">__slots__</code>

of a base class, it will not be included in the generated <code class="xref py py-attr docutils literal notranslate">__slots__</code>

to prevent <a class="reference internal" href="../reference/datamodel.html#datamodel-note-slots">overriding them</a>.

Therefore, do not use <code class="xref py py-attr docutils literal notranslate">__slots__</code> to retrieve the field names of a

dataclass. Use <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate">fields()</code></a> instead.

To be able to determine inherited slots,

base class <code class="xref py py-attr docutils literal notranslate">__slots__</code> may be any iterable, but not an iterator.

</div>

</div></blockquote>

<li>weakref_slot: If true (the default is <code class="docutils literal notranslate">False</code>), add a slot

named "__weakref__", which is required to make an instance

<a class="reference internal" href="weakref.html#weakref.ref" title="weakref.ref"><code class="xref py py-func docutils literal notranslate">weakref-able</code></a>.

It is an error to specify <code class="docutils literal notranslate">weakref_slot=True</code>

without also specifying <code class="docutils literal notranslate">slots=True</code>.</li>

</ul>

Added in version 3.11.

</div>

</div></blockquote>

<code class="docutils literal notranslate">field</code>s may optionally specify a default value, using normal

Python syntax:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class C:

a: int # 'a' has no default value

b: int = 0 # assign a default value for 'b'

</pre></div>

</div>

In this example, both <code class="xref py py-attr docutils literal notranslate">a</code> and <code class="xref py py-attr docutils literal notranslate">b</code> will be included in the added

<a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> method, which will be defined as:

</pre></div>

</div>

<a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> will be raised if a field without a default value

follows a field with a default value. This is true whether this

occurs in a single class, or as a result of class inheritance.

</dd></dl>

dataclasses.field(<abbr title="Keyword-only parameters separator (PEP 3102)">*</abbr>, default=MISSING, default_factory=MISSING, init=True, repr=True, hash=None, compare=True, metadata=None, kw_only=MISSING, doc=None)<a class="headerlink" href="#dataclasses.field" title="Link to this definition">¶</a></dt>

<dd>For common and simple use cases, no other functionality is

required. There are, however, some dataclass features that

require additional per-field information. To satisfy this need for

additional information, you can replace the default field value

with a call to the provided <code class="xref py py-func docutils literal notranslate">field()</code> function. For example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class C:

mylist: list[int] = field(default_factory=list)

c = C()

c.mylist += [1, 2, 3]

</pre></div>

</div>

As shown above, the <a class="reference internal" href="#dataclasses.MISSING" title="dataclasses.MISSING"><code class="xref py py-const docutils literal notranslate">MISSING</code></a> value is a sentinel object used to

detect if some parameters are provided by the user. This sentinel is

used because <code class="docutils literal notranslate">None</code> is a valid value for some parameters with

a distinct meaning. No code should directly use the <code class="xref py py-const docutils literal notranslate">MISSING</code> value.

The parameters to <code class="xref py py-func docutils literal notranslate">field()</code> are:

<ul>

<li>default: If provided, this will be the default value for this

field. This is needed because the <code class="xref py py-func docutils literal notranslate">field()</code> call itself

replaces the normal position of the default value.</li>

<li>default_factory: If provided, it must be a zero-argument

callable that will be called when a default value is needed for

this field. Among other purposes, this can be used to specify

fields with mutable default values, as discussed below. It is an

error to specify both default and default_factory.</li>

<li>init: If true (the default), this field is included as a

parameter to the generated <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> method.</li>

<li>repr: If true (the default), this field is included in the

string returned by the generated <a class="reference internal" href="../reference/datamodel.html#object.__repr__" title="object.__repr__"><code class="xref py py-meth docutils literal notranslate">__repr__()</code></a> method.</li>

<li>hash: This can be a bool or <code class="docutils literal notranslate">None</code>. If true, this field is

included in the generated <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate">__hash__()</code></a> method. If false,

this field is excluded from the generated <code class="xref py py-meth docutils literal notranslate">__hash__()</code>.

If <code class="docutils literal notranslate">None</code> (the default), use the value of compare: this would

normally be the expected behavior, since a field should be included

in the hash if it's used for comparisons. Setting this value to anything

other than <code class="docutils literal notranslate">None</code> is discouraged.

One possible reason to set <code class="docutils literal notranslate">hash=False</code> but <code class="docutils literal notranslate">compare=True</code>

would be if a field is expensive to compute a hash value for,

that field is needed for equality testing, and there are other

fields that contribute to the type's hash value. Even if a field

is excluded from the hash, it will still be used for comparisons.

</li>

<li>compare: If true (the default), this field is included in the

generated equality and comparison methods (<a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate">__eq__()</code></a>,

<li>metadata: This can be a mapping or <code class="docutils literal notranslate">None</code>. <code class="docutils literal notranslate">None</code> is treated as

an empty dict. This value is wrapped in

<a class="reference internal" href="types.html#types.MappingProxyType" title="types.MappingProxyType"><code class="xref py py-func docutils literal notranslate">MappingProxyType()</code></a> to make it read-only, and exposed

on the <a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate">Field</code></a> object. It is not used at all by Data

Classes, and is provided as a third-party extension mechanism.

Multiple third-parties can each have their own key, to use as a

namespace in the metadata.</li>

<li>kw_only: If true, this field will be marked as keyword-only.

This is used when the generated <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> method's

parameters are computed.

Keyword-only fields are also not included in <code class="xref py py-attr docutils literal notranslate">__match_args__</code>.

</li>

</ul>

Added in version 3.10.

</div>

</div></blockquote>

<li>doc: optional docstring for this field.</li>

</ul>

Added in version 3.14.

</div>

</div></blockquote>

If the default value of a field is specified by a call to

<code class="xref py py-func docutils literal notranslate">field()</code>, then the class attribute for this field will be

replaced by the specified default value. If default is not

provided, then the class attribute will be deleted. The intent is

that after the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-deco docutils literal notranslate">@dataclass</code></a> decorator runs, the class

attributes will all contain the default values for the fields, just

as if the default value itself were specified. For example,

after:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class C:

x: int

y: int = field(repr=False)

z: int = field(repr=False, default=10)

t: int = 20

</pre></div>

</div>

The class attribute <code class="xref py py-attr docutils literal notranslate">C.z</code> will be <code class="docutils literal notranslate">10</code>, the class attribute

<code class="xref py py-attr docutils literal notranslate">C.t</code> will be <code class="docutils literal notranslate">20</code>, and the class attributes <code class="xref py py-attr docutils literal notranslate">C.x</code> and

<code class="xref py py-attr docutils literal notranslate">C.y</code> will not be set.

</dd></dl>

class dataclasses.Field<a class="headerlink" href="#dataclasses.Field" title="Link to this definition">¶</a></dt>

<dd><code class="xref py py-class docutils literal notranslate">Field</code> objects describe each defined field. These objects

are created internally, and are returned by the <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate">fields()</code></a>

module-level method (see below). Users should never instantiate a

<code class="xref py py-class docutils literal notranslate">Field</code> object directly. Its documented attributes are:

<li><code class="xref py py-attr docutils literal notranslate">name</code>: The name of the field.</li>

<li><code class="xref py py-attr docutils literal notranslate">type</code>: The type of the field.</li>

<li><code class="xref py py-attr docutils literal notranslate">default</code>, <code class="xref py py-attr docutils literal notranslate">default_factory</code>, <code class="xref py py-attr docutils literal notranslate">init</code>, <code class="xref py py-attr docutils literal notranslate">repr</code>, <code class="xref py py-attr docutils literal notranslate">hash</code>,

<code class="xref py py-attr docutils literal notranslate">compare</code>, <code class="xref py py-attr docutils literal notranslate">metadata</code>, and <code class="xref py py-attr docutils literal notranslate">kw_only</code> have the identical

meaning and values as they do in the <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate">field()</code></a> function.</li>

</ul>

Other attributes may exist, but they are private and must not be

inspected or relied on.

</dd></dl>

class dataclasses.InitVar<a class="headerlink" href="#dataclasses.InitVar" title="Link to this definition">¶</a></dt>

<dd><code class="docutils literal notranslate">InitVar[T]</code> type annotations describe variables that are <a class="reference internal" href="#dataclasses-init-only-variables">init-only</a>. Fields annotated with <code class="xref py py-class docutils literal notranslate">InitVar</code>

are considered pseudo-fields, and thus are neither returned by the

<a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate">fields()</code></a> function nor used in any way except adding them as

parameters to <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> and an optional

<a class="reference internal" href="#dataclasses.__post_init__" title="dataclasses.__post_init__"><code class="xref py py-meth docutils literal notranslate">__post_init__()</code></a>.

</dd></dl>

dataclasses.fields(class_or_instance)<a class="headerlink" href="#dataclasses.fields" title="Link to this definition">¶</a></dt>

<dd>Returns a tuple of <a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate">Field</code></a> objects that define the fields for this

dataclass. Accepts either a dataclass, or an instance of a dataclass.

Raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> if not passed a dataclass or instance of one.

Does not return pseudo-fields which are <code class="docutils literal notranslate">ClassVar</code> or <code class="docutils literal notranslate">InitVar</code>.

</dd></dl>

dataclasses.asdict(obj, <abbr title="Keyword-only parameters separator (PEP 3102)">*</abbr>, dict_factory=dict)<a class="headerlink" href="#dataclasses.asdict" title="Link to this definition">¶</a></dt>

<dd>Converts the dataclass obj to a dict (by using the

factory function dict_factory). Each dataclass is converted

to a dict of its fields, as <code class="docutils literal notranslate">name: value</code> pairs. dataclasses, dicts,

lists, and tuples are recursed into. Other objects are copied with

<a class="reference internal" href="copy.html#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal notranslate">copy.deepcopy()</code></a>.

Example of using <code class="xref py py-func docutils literal notranslate">asdict()</code> on nested dataclasses:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class Point:

x: int

y: int

@dataclass

class C:

mylist: list[Point]

p = Point(10, 20)

assert asdict(p) == {'x': 10, 'y': 20}

c = C([Point(0, 0), Point(10, 4)])

assert asdict(c) == {'mylist': [{'x': 0, 'y': 0}, {'x': 10, 'y': 4}]}

</pre></div>

</div>

To create a shallow copy, the following workaround may be used:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>{field.name: getattr(obj, field.name) for field in fields(obj)}

</pre></div>

</div>

<code class="xref py py-func docutils literal notranslate">asdict()</code> raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> if obj is not a dataclass

instance.

</dd></dl>

dataclasses.astuple(obj, <abbr title="Keyword-only parameters separator (PEP 3102)">*</abbr>, tuple_factory=tuple)<a class="headerlink" href="#dataclasses.astuple" title="Link to this definition">¶</a></dt>

<dd>Converts the dataclass obj to a tuple (by using the

factory function tuple_factory). Each dataclass is converted

to a tuple of its field values. dataclasses, dicts, lists, and

tuples are recursed into. Other objects are copied with

<a class="reference internal" href="copy.html#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal notranslate">copy.deepcopy()</code></a>.

Continuing from the previous example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>assert astuple(p) == (10, 20)

assert astuple(c) == ([(0, 0), (10, 4)],)

</pre></div>

</div>

To create a shallow copy, the following workaround may be used:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>tuple(getattr(obj, field.name) for field in dataclasses.fields(obj))

</pre></div>

</div>

<code class="xref py py-func docutils literal notranslate">astuple()</code> raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> if obj is not a dataclass

instance.

</dd></dl>

dataclasses.make_dataclass(cls_name, fields, <abbr title="Keyword-only parameters separator (PEP 3102)">*</abbr>, bases=(), namespace=None, init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False, match_args=True, kw_only=False, slots=False, weakref_slot=False, module=None, decorator=dataclass)<a class="headerlink" href="#dataclasses.make_dataclass" title="Link to this definition">¶</a></dt>

<dd>Creates a new dataclass with name cls_name, fields as defined

in fields, base classes as given in bases, and initialized

with a namespace as given in namespace. fields is an

iterable whose elements are each either <code class="docutils literal notranslate">name</code>, <code class="docutils literal notranslate">(name, type)</code>,

or <code class="docutils literal notranslate">(name, type, Field)</code>. If just <code class="docutils literal notranslate">name</code> is supplied,

<a class="reference internal" href="typing.html#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal notranslate">typing.Any</code></a> is used for <code class="docutils literal notranslate">type</code>. The values of init,

repr, eq, order, unsafe_hash, frozen,

match_args, kw_only, slots, and weakref_slot have

the same meaning as they do in <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-deco docutils literal notranslate">@dataclass</code></a>.

If module is defined, the <code class="xref py py-attr docutils literal notranslate">__module__</code> attribute

of the dataclass is set to that value.

By default, it is set to the module name of the caller.

The decorator parameter is a callable that will be used to create the dataclass.

It should take the class object as a first argument and the same keyword arguments

as <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-deco docutils literal notranslate">@dataclass</code></a>. By default, the <code class="xref py py-deco docutils literal notranslate">@dataclass</code>

function is used.

This function is not strictly required, because any Python

mechanism for creating a new class with <a class="reference internal" href="../reference/datamodel.html#object.__annotations__" title="object.__annotations__"><code class="xref py py-attr docutils literal notranslate">__annotations__</code></a> can

then apply the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-deco docutils literal notranslate">@dataclass</code></a> function to convert that class to

a dataclass. This function is provided as a convenience. For

example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>C = make_dataclass('C',

[('x', int),

'y',

('z', int, field(default=5))],

namespace={'add_one': lambda self: self.x + 1})

</pre></div>

</div>

Is equivalent to:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class C:

x: int

y: 'typing.Any'

z: int = 5

def add_one(self):

return self.x + 1

</pre></div>

</div>

Added in version 3.14: Added the decorator parameter.

</div>

</dd></dl>

dataclasses.replace(obj, <abbr title="Positional-only parameter separator (PEP 570)">/</abbr>, **changes)<a class="headerlink" href="#dataclasses.replace" title="Link to this definition">¶</a></dt>

<dd>Creates a new object of the same type as obj, replacing

fields with values from changes. If obj is not a Data

Class, raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>. If keys in changes are not

field names of the given dataclass, raises <code class="xref py py-exc docutils literal notranslate">TypeError</code>.

The newly returned object is created by calling the <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a>

method of the dataclass. This ensures that

Init-only variables without default values, if any exist, must be

specified on the call to <code class="xref py py-func docutils literal notranslate">replace()</code> so that they can be passed to

<code class="xref py py-meth docutils literal notranslate">__init__()</code> and <a class="reference internal" href="#dataclasses.__post_init__" title="dataclasses.__post_init__"><code class="xref py py-meth docutils literal notranslate">__post_init__()</code></a>.

It is an error for changes to contain any fields that are

defined as having <code class="docutils literal notranslate">init=False</code>. A <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> will be raised

in this case.

Be forewarned about how <code class="docutils literal notranslate">init=False</code> fields work during a call to

<code class="xref py py-func docutils literal notranslate">replace()</code>. They are not copied from the source object, but

rather are initialized in <a class="reference internal" href="#dataclasses.__post_init__" title="dataclasses.__post_init__"><code class="xref py py-meth docutils literal notranslate">__post_init__()</code></a>, if they're

initialized at all. It is expected that <code class="docutils literal notranslate">init=False</code> fields will

be rarely and judiciously used. If they are used, it might be wise

to have alternate class constructors, or perhaps a custom

<code class="xref py py-func docutils literal notranslate">replace()</code> (or similarly named) method which handles instance

copying.

Dataclass instances are also supported by generic function <a class="reference internal" href="copy.html#copy.replace" title="copy.replace"><code class="xref py py-func docutils literal notranslate">copy.replace()</code></a>.

</dd></dl>

dataclasses.is_dataclass(obj)<a class="headerlink" href="#dataclasses.is_dataclass" title="Link to this definition">¶</a></dt>

<dd>Return <code class="docutils literal notranslate">True</code> if its parameter is a dataclass (including subclasses of a

dataclass, but not including <a class="reference internal" href="stdtypes.html#types-genericalias">generic aliases</a>)

or an instance of one, otherwise return <code class="docutils literal notranslate">False</code>.

If you need to know if a class is an instance of a dataclass (and

not a dataclass itself), then add a further check for <code class="docutils literal notranslate">not

isinstance(obj, type)</code>:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>def is_dataclass_instance(obj):

return is_dataclass(obj) and not isinstance(obj, type)

</pre></div>

</div>

</dd></dl>

dataclasses.MISSING<a class="headerlink" href="#dataclasses.MISSING" title="Link to this definition">¶</a></dt>

<dd>A sentinel value signifying a missing default or default_factory.

</dd></dl>

dataclasses.KW_ONLY<a class="headerlink" href="#dataclasses.KW_ONLY" title="Link to this definition">¶</a></dt>

<dd>A sentinel value used as a type annotation. Any fields after a

pseudo-field with the type of <code class="xref py py-const docutils literal notranslate">KW_ONLY</code> are marked as

keyword-only fields. Note that a pseudo-field of type

<code class="xref py py-const docutils literal notranslate">KW_ONLY</code> is otherwise completely ignored. This includes the

name of such a field. By convention, a name of <code class="docutils literal notranslate">_</code> is used for a

<code class="xref py py-const docutils literal notranslate">KW_ONLY</code> field. Keyword-only fields signify

the class is instantiated.

In this example, the fields <code class="docutils literal notranslate">y</code> and <code class="docutils literal notranslate">z</code> will be marked as keyword-only fields:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class Point:

x: float

_: KW_ONLY

y: float

z: float

p = Point(0, y=1.5, z=2.0)

</pre></div>

</div>

In a single dataclass, it is an error to specify more than one

field whose type is <code class="xref py py-const docutils literal notranslate">KW_ONLY</code>.

Added in version 3.10.

</div>

</dd></dl>

exception dataclasses.FrozenInstanceError<a class="headerlink" href="#dataclasses.FrozenInstanceError" title="Link to this definition">¶</a></dt>

<dd>Raised when an implicitly defined <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate">__setattr__()</code></a> or

<a class="reference internal" href="../reference/datamodel.html#object.__delattr__" title="object.__delattr__"><code class="xref py py-meth docutils literal notranslate">__delattr__()</code></a> is called on a dataclass which was defined with

<code class="docutils literal notranslate">frozen=True</code>. It is a subclass of <a class="reference internal" href="exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate">AttributeError</code></a>.

</dd></dl>

</section>

<h2>Post-init processing<a class="headerlink" href="#post-init-processing" title="Link to this heading">¶</a></h2>

dataclasses.__post_init__()<a class="headerlink" href="#dataclasses.__post_init__" title="Link to this definition">¶</a></dt>

<dd>When defined on the class, it will be called by the generated

However, if any <code class="docutils literal notranslate">InitVar</code> fields are defined, they will also be

passed to <code class="xref py py-meth docutils literal notranslate">__post_init__()</code> in the order they were defined in the

class. If no <code class="xref py py-meth docutils literal notranslate">__init__()</code> method is generated, then

<code class="xref py py-meth docutils literal notranslate">__post_init__()</code> will not automatically be called.

Among other uses, this allows for initializing field values that

depend on one or more other fields. For example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class C:

a: float

b: float

c: float = field(init=False)

def __post_init__(self):

self.c = self.a + self.b

</pre></div>

</div>

</dd></dl>

The <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> method generated by <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-deco docutils literal notranslate">@dataclass</code></a> does not call base

class <code class="xref py py-meth docutils literal notranslate">__init__()</code> methods. If the base class has an <code class="xref py py-meth docutils literal notranslate">__init__()</code> method

that has to be called, it is common to call this method in a

<div class="highlight-python3 notranslate"><div class="highlight"><pre>class Rectangle:

def __init__(self, height, width):

self.height = height

self.width = width

@dataclass

class Square(Rectangle):

side: float

def __post_init__(self):

super().__init__(self.side, self.side)

</pre></div>

</div>

Note, however, that in general the dataclass-generated <code class="xref py py-meth docutils literal notranslate">__init__()</code> methods

don't need to be called, since the derived dataclass will take care of

initializing all fields of any base class that is a dataclass itself.

See the section below on init-only variables for ways to pass

parameters to <code class="xref py py-meth docutils literal notranslate">__post_init__()</code>. Also see the warning about how

<a class="reference internal" href="#dataclasses.replace" title="dataclasses.replace"><code class="xref py py-func docutils literal notranslate">replace()</code></a> handles <code class="docutils literal notranslate">init=False</code> fields.

</section>

<h2>Class variables<a class="headerlink" href="#class-variables" title="Link to this heading">¶</a></h2>

One of the few places where <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-deco docutils literal notranslate">@dataclass</code></a> actually inspects the type

of a field is to determine if a field is a class variable as defined

in <a class="pep reference external" href="https://peps.python.org/pep-0526/">PEP 526</a>. It does this by checking if the type of the field is

<a class="reference internal" href="typing.html#typing.ClassVar" title="typing.ClassVar"><code class="xref py py-data docutils literal notranslate">typing.ClassVar</code></a>. If a field is a <code class="docutils literal notranslate">ClassVar</code>, it is excluded

from consideration as a field and is ignored by the dataclass

mechanisms. Such <code class="docutils literal notranslate">ClassVar</code> pseudo-fields are not returned by the

module-level <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate">fields()</code></a> function.

</section>

<h2>Init-only variables<a class="headerlink" href="#init-only-variables" title="Link to this heading">¶</a></h2>

Another place where <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-deco docutils literal notranslate">@dataclass</code></a> inspects a type annotation is to

determine if a field is an init-only variable. It does this by seeing

if the type of a field is of type <a class="reference internal" href="#dataclasses.InitVar" title="dataclasses.InitVar"><code class="xref py py-class docutils literal notranslate">InitVar</code></a>. If a field

is an <code class="xref py py-class docutils literal notranslate">InitVar</code>, it is considered a pseudo-field called an init-only

field. As it is not a true field, it is not returned by the

parameters to the generated <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> method, and are passed to

the optional <a class="reference internal" href="#dataclasses.__post_init__" title="dataclasses.__post_init__"><code class="xref py py-meth docutils literal notranslate">__post_init__()</code></a> method. They are not otherwise used

by dataclasses.

For example, suppose a field will be initialized from a database, if a

value is not provided when creating the class:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class C:

i: int

j: int | None = None

database: InitVar[DatabaseType | None] = None

def __post_init__(self, database):

if self.j is None and database is not None:

self.j = database.lookup('j')

c = C(10, database=my_database)

</pre></div>

</div>

In this case, <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate">fields()</code></a> will return <a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate">Field</code></a> objects for <code class="xref py py-attr docutils literal notranslate">i</code> and

<code class="xref py py-attr docutils literal notranslate">j</code>, but not for <code class="xref py py-attr docutils literal notranslate">database</code>.

</section>

<h2>Frozen instances<a class="headerlink" href="#frozen-instances" title="Link to this heading">¶</a></h2>

It is not possible to create truly immutable Python objects. However,

by passing <code class="docutils literal notranslate">frozen=True</code> to the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-deco docutils literal notranslate">@dataclass</code></a> decorator you can

emulate immutability. In that case, dataclasses will add

<a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate">__setattr__()</code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__delattr__" title="object.__delattr__"><code class="xref py py-meth docutils literal notranslate">__delattr__()</code></a> methods to the class. These

methods will raise a <a class="reference internal" href="#dataclasses.FrozenInstanceError" title="dataclasses.FrozenInstanceError"><code class="xref py py-exc docutils literal notranslate">FrozenInstanceError</code></a> when invoked.

There is a tiny performance penalty when using <code class="docutils literal notranslate">frozen=True</code>:

must use <code class="xref py py-meth docutils literal notranslate">object.__setattr__()</code>.

</section>

<h2>Inheritance<a class="headerlink" href="#inheritance" title="Link to this heading">¶</a></h2>

When the dataclass is being created by the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-deco docutils literal notranslate">@dataclass</code></a> decorator,

it looks through all of the class's base classes in reverse MRO (that

is, starting at <a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate">object</code></a>) and, for each dataclass that it finds,

adds the fields from that base class to an ordered mapping of fields.

After all of the base class fields are added, it adds its own fields

to the ordered mapping. All of the generated methods will use this

combined, calculated ordered mapping of fields. Because the fields

are in insertion order, derived classes override base classes. An

example:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class Base:

x: Any = 15.0

y: int = 0

@dataclass

class C(Base):

z: int = 10

x: int = 15

</pre></div>

</div>

The final list of fields is, in order, <code class="xref py py-attr docutils literal notranslate">x</code>, <code class="xref py py-attr docutils literal notranslate">y</code>, <code class="xref py py-attr docutils literal notranslate">z</code>. The final

type of <code class="xref py py-attr docutils literal notranslate">x</code> is <a class="reference internal" href="functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a>, as specified in class <code class="xref py py-class docutils literal notranslate">C</code>.

The generated <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> method for <code class="xref py py-class docutils literal notranslate">C</code> will look like:

</pre></div>

</div>

</section>

<h2>Re-ordering of keyword-only parameters in <code class="xref py py-meth docutils literal notranslate">__init__()</code><a class="headerlink" href="#re-ordering-of-keyword-only-parameters-in-init" title="Link to this heading">¶</a></h2>

After the parameters needed for <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> are computed, any

keyword-only parameters are moved to come after all regular

(non-keyword-only) parameters. This is a requirement of how

keyword-only parameters are implemented in Python: they must come

after non-keyword-only parameters.

In this example, <code class="xref py py-attr docutils literal notranslate">Base.y</code>, <code class="xref py py-attr docutils literal notranslate">Base.w</code>, and <code class="xref py py-attr docutils literal notranslate">D.t</code> are keyword-only

fields, and <code class="xref py py-attr docutils literal notranslate">Base.x</code> and <code class="xref py py-attr docutils literal notranslate">D.z</code> are regular fields:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class Base:

x: Any = 15.0

_: KW_ONLY

y: int = 0

w: int = 1

@dataclass

class D(Base):

z: int = 10

t: int = field(kw_only=True, default=0)

</pre></div>

</div>

The generated <code class="xref py py-meth docutils literal notranslate">__init__()</code> method for <code class="xref py py-class docutils literal notranslate">D</code> will look like:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>def __init__(self, x: Any = 15.0, z: int = 10, *, y: int = 0, w: int = 1, t: int = 0):

</pre></div>

</div>

Note that the parameters have been re-ordered from how they appear in

the list of fields: parameters derived from regular fields are

followed by parameters derived from keyword-only fields.

The relative ordering of keyword-only parameters is maintained in the

re-ordered <code class="xref py py-meth docutils literal notranslate">__init__()</code> parameter list.

</section>

<h2>Default factory functions<a class="headerlink" href="#default-factory-functions" title="Link to this heading">¶</a></h2>

If a <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate">field()</code></a> specifies a default_factory, it is called with

zero arguments when a default value for the field is needed. For

example, to create a new instance of a list, use:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>mylist: list = field(default_factory=list)

</pre></div>

</div>

If a field is excluded from <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate">__init__()</code></a> (using <code class="docutils literal notranslate">init=False</code>)

and the field also specifies default_factory, then the default

factory function will always be called from the generated

<code class="xref py py-meth docutils literal notranslate">__init__()</code> function. This happens because there is no other

way to give the field an initial value.

</section>

<h2>Mutable default values<a class="headerlink" href="#mutable-default-values" title="Link to this heading">¶</a></h2>

Python stores default member variable values in class attributes.

Consider this example, not using dataclasses:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>class C:

x = []

def add(self, element):

self.x.append(element)

o1 = C()

o2 = C()

o1.add(1)

o2.add(2)

assert o1.x == [1, 2]

assert o1.x is o2.x

</pre></div>

</div>

Note that the two instances of class <code class="xref py py-class docutils literal notranslate">C</code> share the same class

variable <code class="xref py py-attr docutils literal notranslate">x</code>, as expected.

Using dataclasses, if this code was valid:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass

class D:

x: list = [] # This code raises ValueError

</pre></div>

</div>

it would generate code similar to:

<div class="highlight-python3 notranslate"><div class="highlight"><pre>class D:

x = []

def __init__(self, x=x):

self.x = x

assert D().x is D().x

</pre></div>

</div>

This has the same issue as the original example using class <code class="xref py py-class docutils literal notranslate">C</code>.

That is, two instances of class <code class="xref py py-class docutils literal notranslate">D</code> that do not specify a value

for <code class="xref py py-attr docutils literal notranslate">x</code> when creating a class instance will share the same copy

of <code class="xref py py-attr docutils literal notranslate">x</code>. Because dataclasses just use normal Python class

creation they also share this behavior. There is no general way

for Data Classes to detect this condition. Instead, the

<a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-deco docutils literal notranslate">@dataclass</code></a> decorator will raise a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> if it

detects an unhashable default parameter. The assumption is that if

a value is unhashable, it is mutable. This is a partial solution,

but it does protect against many common errors.

View remainder of file in raw view

Navigation Menu

Uh oh!

Uh oh!

FilesExpand file tree

dataclasses.html

Latest commit

History

dataclasses.html

File metadata and controls