Use mypy for static type checking

.pre-commit-config.yaml

@@ -17,6 +17,13 @@ repos:
       language_version: python3
       additional_dependencies:
       - flake8-pytest-style
+  - repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v0.790
+    hooks:
+      - id: mypy
+        # Avoid error: Duplicate module named 'setup'
+        # https://github.com/python/mypy/issues/4008
+        exclude: ^tests/test_data/
   - repo: https://github.com/PyCQA/bandit
     rev: 1.7.0
     hooks:

piptools/logging.py

@@ -12,7 +12,7 @@
 class LogContext:
     stream = sys.stderr
 
-    def __init__(self, verbosity=0, indent_width=2):
+    def __init__(self, verbosity: int = 0, indent_width: int = 2):
         self.verbosity = verbosity
         self.current_indent = 0
         self._indent_width = indent_width

piptools/scripts/compile.py

@@ -3,6 +3,7 @@
 import sys
 import tempfile
 import warnings
+from typing import Any
 
 from click import Command
 from click.utils import safecall
@@ -25,7 +26,7 @@
 DEFAULT_REQUIREMENTS_OUTPUT_FILE = "requirements.txt"
 
 
-def _get_default_option(option_name):
+def _get_default_option(option_name: str) -> Any:
     """
     Get default value of the pip's option (including option from pip.conf)
     by a given option name.

piptools/utils.py

@@ -43,7 +43,7 @@ def key_from_req(req):
     return key
 
 
-def comment(text):
+def comment(text: str) -> str:
     return style(text, fg="green")
 
 

setup.cfg

@@ -69,3 +69,17 @@ pytest-parametrize-values-row-type = tuple
 
 [isort]
 profile = black
+
+[mypy]
+disallow_any_generics = true
+disallow_incomplete_defs = true
+disallow_subclassing_any = true
+disallow_untyped_calls = true
+disallow_untyped_decorators = true
+ignore_missing_imports = true
+no_implicit_optional = true
+strict_equality = true
+warn_redundant_casts = true
+warn_return_any = true
+warn_unused_configs = true
+warn_unused_ignores = true