Walrus operator in Python interpreter

Question

When I use the walrus operator as below in the Python(3.9.6) interpreter,

>>> walrus:=True

I get a syntax error:

  File "<stdin>", line 1
    walrus := True
                  ^
SyntaxError: invalid syntax

How is this different from the following?

>>> print(walrus := True)

Assignment expressions must be enclosed in parens. They work inside function calls like this, though, similar to how bare generator expressions must be enclosed but can be passed in a function call, e.g. sum(x for x in some_iterable). — ddejohn
– ddejohn, Commented Sep 15, 2021 at 15:46
This is just how it's degined. See: python.org/dev/peps/pep-0572/#exceptional-cases — gen_Eric
– gen_Eric, Commented Sep 15, 2021 at 15:47

ShadowRanger · Accepted Answer · 2022-04-18 13:54:47Z

3

It's different because the Python core developers were very ambivalent about violating the Zen of Python guideline "There should be one-- and preferably only one --obvious way to do it", and chose to make it inconvenient to replace most uses of plain = with := without adding additional parentheses to the expression.

Rather than allowing := to replace = in all contexts, they specifically prohibited unparenthesized top-level use of the walrus:

Unparenthesized assignment expressions are prohibited at the top level of an expression statement.
y := f(x)  # INVALID
(y := f(x))  # Valid, though not recommended
This rule is included to simplify the choice for the user between an assignment statement and an assignment expression – there is no syntactic position where both are valid.

In many cases where := is prohibited, you can make it valid by adding otherwise unnecessary parentheses around the expression, so:

(walrus:=True)

works just fine, but it's enough of a pain that the assumption is that most people will stick to the simpler and more Pythonic:

walrus = True

in that scenario.

answered Apr 18, 2022 at 13:54

ShadowRanger

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2 Comments

Antonios Sarikas Over a year ago

What is "the top level of an expression statement"?

ShadowRanger Over a year ago

@AntoniosSarikas: An expression statement is an expression used as a statement. Top-level means "it's the outermost expression (in the expression statement). The example is demonstrating this; without the parentheses, the walrus is at the top level, with the parentheses, it's not. Sorry I can't give a more rigorous definition off-the-cuff, but this is easier as "you know it when you see it".

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