pantsbuild · Eric-Arellano · Oct 12, 2020 · Sep 18, 2020 · Oct 12, 2020 · Oct 12, 2020
diff --git a/.travis.yml b/.travis.yml
@@ -22,6 +22,6 @@ install:
 
 script:
   # We also smoke test that our release process will work by running `package`.
-  - ./pants lint test package '::'
+  - ./pants lint typecheck test package '::'
   - ./pants run helloworld/main.py
   - ./pants run helloworld/main_py2.py
diff --git a/README.md b/README.md
@@ -111,17 +111,23 @@ Try these out in this repo!
 ## List targets
 
 ```
-./pants list helloworld::  # All targets.
+./pants list ::  # All targets.
 ./pants list 'helloworld/**/*.py'  # Just targets containing Python code.
 ```
 
 ## Run linters and formatters
 
 ```
-./pants lint helloworld::
+./pants lint ::
 ./pants fmt 'helloworld/**/*.py'
 ```
 
+## Run MyPy
+
+```
+./pants typecheck ::
+```
+
 ## Run tests
 
 ```

diff --git a/.flake8 → build-support/.flake8 b/.flake8 → build-support/.flake8
diff --git a/build-support/mypy.ini b/build-support/mypy.ini
@@ -0,0 +1,34 @@
+[mypy]
+# Optionals
+no_implicit_optional = True
+
+# Strictness
+allow_untyped_globals = False
+allow_redefinition = False
+implicit_reexport = False
+strict_equality = True
+
+# Warnings
+warn_unused_ignores = True
+warn_no_return = True
+warn_return_any = True
+warn_redundant_casts = True
+warn_unreachable = True
+
+# Error output
+show_column_numbers = True
+show_error_context = True
+show_error_codes = True
+show_traceback = True
+pretty = True
+color_output = True
+error_summary = True
+
+[mypy-colors]
+ignore_missing_imports = True
+
+[mypy-translate]
+ignore_missing_imports = True
+
+[mypy-pytest]
+ignore_missing_imports = True
diff --git a/helloworld/greet/__init__.py b/helloworld/greet/__init__.py
diff --git a/helloworld/main.py b/helloworld/main.py
@@ -9,7 +9,9 @@
 
 def say_hello() -> None:
     config = load_config()
-    greeter = Greeter(languages=config.languages, greetings=config.greetings)
+    greeter = Greeter(
+        languages=list(config.languages), greetings=list(config.greetings)
+    )
     sentence = greeter.greet("world")
     print(green(sentence))
 

diff --git a/helloworld/util/__init__.py b/helloworld/util/__init__.py
diff --git a/helloworld/util/lang.py b/helloworld/util/lang.py
@@ -2,7 +2,7 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 import random
-from typing import List
+from typing import List, cast
 
 from translate import Translator
 
@@ -18,7 +18,7 @@ def translate(self, lang: str, phrase: str) -> str:
         if lang not in self._langs:
             raise self.UnknownLanguage(lang)
         translator = Translator(lang)
-        return translator.translate(phrase)
+        return cast(str, translator.translate(phrase))
 
     def translate_to_random_language(self, phrase: str) -> str:
         return self.translate(self._pick_random_language(), phrase)

diff --git a/helloworld/util/proto/BUILD b/helloworld/util/proto/BUILD
@@ -3,4 +3,14 @@
 
 # `name` defaults to the name of this directory, i.e., `proto`.
 # `sources` defaults to ["*.proto"].
-protobuf_library()
+protobuf_library(dependencies=[":init"])
+
+
+# Note that we need an `__init__.py` file for MyPy to recognize our generated code as a valid
+# module. We include this in the dependencies for the `protobuf_library` to ensure the
+# `__init__.py` is always used.
+#
+# Often, you won't want to put `__init__.py` files in your Protobuf directories; see
+# https://www.pantsbuild.org/docs/protobuf#protobuf-and-source-roots for how to change where the
+# protobuf code is generated.
+python_library(name="init")
diff --git a/helloworld/util/proto/__init__.py b/helloworld/util/proto/__init__.py
diff --git a/pants.toml b/pants.toml
@@ -13,6 +13,7 @@ backend_packages.add = [
   'pants.backend.python.lint.black',
   'pants.backend.python.lint.flake8',
   'pants.backend.python.lint.isort',
+  'pants.backend.python.typecheck.mypy',
 ]
 
 # This ensures that the result of `./pants package` is always unique. It will become the default in
@@ -34,9 +35,13 @@ requirement_constraints = "constraints.txt"
 [python-protobuf]
 # See https://www.pantsbuild.org/docs/protobuf.
 runtime_dependencies = ["//:protobuf"]
+mypy_plugin = true
 
 [flake8]
-config = ".flake8"
+config = "build-support/.flake8"
 
 [isort]
 config = [".isort.cfg"]
+
+[mypy]
+config = "build-support/mypy.ini"