Add type annotations to package

Adding type annotations will allow packages consuming certifi to have
access to its typing information. The py.typed data file is include for
PEP 561 compliance.

If the API is ever expanded, the mypy type checker will ensure it is
also typed. For example, right now, typeshed is incomplete after
certifi.contents() was added:

https://github.com/python/typeshed/blob/ac2ef6e8c963a45841ad91e068e06748caa1fce6/stubs/certifi/certifi.pyi

Author: jdufresne

Diff for: .github/workflows/ci.yml

@@ -6,6 +6,18 @@ on:
   pull_request: {}
 
 jobs:
+  mypy:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Python
+        uses: actions/setup-python@v2
+      - name: Install dependencies
+        run: pip install mypy
+      - name: Run mypy
+        run: mypy --strict certifi
+
   test:
     runs-on: ubuntu-latest
     strategy:

Diff for: certifi/__init__.py

@@ -1,3 +1,4 @@
 from .core import contents, where
 
+__all__ = ["contents", "where"]
 __version__ = "2021.10.08"

Diff for: certifi/core.py

@@ -7,14 +7,16 @@
 This module returns the installation location of cacert.pem or its contents.
 """
 import os
+import types
+from typing import Union
 
 try:
     from importlib.resources import path as get_path, read_text
 
     _CACERT_CTX = None
     _CACERT_PATH = None
 
-    def where():
+    def where() -> str:
         # This is slightly terrible, but we want to delay extracting the file
         # in cases where we're inside of a zipimport situation until someone
         # actually calls where(), but we don't want to re-extract the file
@@ -40,21 +42,29 @@ def where():
 
 
 except ImportError:
+    Package = Union[types.ModuleType, str]
+    Resource = Union[str, "os.PathLike"]
+
     # This fallback will work for Python versions prior to 3.7 that lack the
     # importlib.resources module but relies on the existing `where` function
     # so won't address issues with environments like PyOxidizer that don't set
     # __file__ on modules.
-    def read_text(_module, _path, encoding="ascii"):
+    def read_text(
+        package: Package,
+        resource: Resource,
+        encoding: str = 'utf-8',
+        errors: str = 'strict'
+    ) -> str:
         with open(where(), "r", encoding=encoding) as data:
             return data.read()
 
     # If we don't have importlib.resources, then we will just do the old logic
     # of assuming we're on the filesystem and munge the path directly.
-    def where():
+    def where() -> str:
         f = os.path.dirname(__file__)
 
         return os.path.join(f, "cacert.pem")
 
 
-def contents():
+def contents() -> str:
     return read_text("certifi", "cacert.pem", encoding="ascii")

Diff for: certifi/py.typed

@@ -7,9 +7,14 @@
 
 
 class TestCertifi(unittest.TestCase):
-    def test_cabundle_exists(self):
+    def test_cabundle_exists(self) -> None:
         assert os.path.exists(certifi.where())
 
-    def test_read_contents(self):
+    def test_read_contents(self) -> None:
         content = certifi.contents()
         assert "-----BEGIN CERTIFICATE-----" in content
+
+    def test_py_typed_exists(self) -> None:
+        assert os.path.exists(
+            os.path.join(os.path.dirname(certifi.__file__), 'py.typed')
+        )

Diff for: certifi/tests/test_certify.py

@@ -43,7 +43,7 @@
         'certifi',
     ],
     package_dir={'certifi': 'certifi'},
-    package_data={'certifi': ['*.pem']},
+    package_data={'certifi': ['*.pem', 'py.typed']},
     # data_files=[('certifi', ['certifi/cacert.pem'])],
     include_package_data=True,
     zip_safe=False,