Set up mypy pre-commit hook for incremental adoption (#4510)

.pre-commit-config.yaml

@@ -10,6 +10,16 @@ repos:
       - id: flake8
         additional_dependencies: [flake8-bugbear]
 
+  - repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v0.910
+    hooks:
+      - id: mypy
+        pass_filenames: false
+        additional_dependencies:
+          - types-requests
+        args:
+          - poetry
+
   - repo: https://github.com/timothycrosley/isort
     rev: 5.7.0
     hooks:

mypy.ini

@@ -6,3 +6,50 @@ strict_optional = True
 warn_unused_ignores = True
 warn_redundant_casts = True
 warn_unused_configs = True
+
+# The following whitelist is used to allow for incremental adoption
+# of Mypy. Modules should be removed from this whitelist as and when
+# their respective type errors have been addressed. No new modules
+# should be added to this whitelist.
+# see https://github.com/python-poetry/poetry/pull/4510.
+
+[mypy-poetry.config.*]
+ignore_errors = True
+
+[mypy-poetry.console.*]
+ignore_errors = True
+
+[mypy-poetry.factory.*]
+ignore_errors = True
+
+[mypy-poetry.inspection.*]
+ignore_errors = True
+
+[mypy-poetry.installation.*]
+ignore_errors = True
+
+[mypy-poetry.locations.*]
+ignore_errors = True
+
+[mypy-poetry.mixology.*]
+ignore_errors = True
+
+[mypy-poetry.packages.*]
+ignore_errors = True
+
+[mypy-poetry.plugins.*]
+ignore_errors = True
+
+[mypy-poetry.publishing.*]
+ignore_errors = True
+
+[mypy-poetry.puzzle.*]
+ignore_errors = True
+
+[mypy-poetry.repositories.*]
+ignore_errors = True
+
+[mypy-poetry.utils.*]
+ignore_errors = True
+
+# end of whitelist

poetry/__init__.py

 from pkgutil import extend_path
+from typing import List
 
 
-__path__ = extend_path(__path__, __name__)
+__path__: List[str] = extend_path(__path__, __name__)

poetry/poetry.py

 from pathlib import Path
 from typing import TYPE_CHECKING
 from typing import List
+from typing import Optional
 
 from poetry.__version__ import __version__
 from poetry.config.source import Source
@@ -35,7 +36,7 @@ class Poetry(BasePoetry):
         self._locker = locker
         self._config = config
         self._pool = Pool()
-        self._plugin_manager = None
+        self._plugin_manager: Optional[PluginManager] = None
 
     @property
     def locker(self) -> "Locker":