Correctly match package/module names in import hook (#144)

The previously used regular expression tried to handle both exact
matches and and prefix matches in one go, using this approach:

    re.compile(r'^%s\.?' % pkg)

However, this is incorrect, since the literal dot is optional in the
pattern, causing longer matches to also get included. For example, ‘foo’
should match ‘foo’ and ‘foo.bar’, but it also incorrectly matches
‘foobar’:

    >>> re.compile(r'^foo\.?').match('foobar')
    <_sre.SRE_Match object; span=(0, 3), match='foo'>

In practice, a command like this (using the pytest plugin as an example)
is supposed to check the ‘flask’ package and any modules below it:

    pytest --typeguard-packages=flask

... but in reality it also checks other packages, such as
‘flask_sqlalchemy’ and ‘flask_redis’, if those happen to be installed.

This can be easily fixed by not using regular expression, but simple
string matching instead.

tests/test_importhook.py

@@ -6,7 +6,7 @@
 
 import pytest
 
-from typeguard.importhook import install_import_hook
+from typeguard.importhook import TypeguardFinder, install_import_hook
 
 this_dir = Path(__file__).parent
 dummy_module_path = this_dir / 'dummymodule.py'
@@ -99,3 +99,20 @@ def test_inner_class_classmethod(dummymodule):
 def test_inner_class_staticmethod(dummymodule):
     retval = dummymodule.Outer.create_inner_staticmethod()
     assert retval.__class__.__qualname__ == 'Outer.Inner'
+
+
+def test_package_name_matching():
+    """
+    The path finder only matches configured (sub)packages.
+    """
+    packages = ["ham", "spam.eggs"]
+    dummy_original_pathfinder = None
+    finder = TypeguardFinder(packages, dummy_original_pathfinder)
+
+    assert finder.should_instrument("ham")
+    assert finder.should_instrument("ham.eggs")
+    assert finder.should_instrument("spam.eggs")
+
+    assert not finder.should_instrument("spam")
+    assert not finder.should_instrument("ha")
+    assert not finder.should_instrument("spam_eggs")

typeguard/importhook.py

@@ -1,5 +1,4 @@
 import ast
-import re
 import sys
 from importlib.machinery import SourceFileLoader
 from importlib.abc import MetaPathFinder
@@ -94,7 +93,7 @@ class TypeguardFinder(MetaPathFinder):
     """
 
     def __init__(self, packages, original_pathfinder):
-        self._package_exprs = [re.compile(r'^%s\.?' % pkg) for pkg in packages]
+        self.packages = packages
         self._original_pathfinder = original_pathfinder
 
     def find_spec(self, fullname, path=None, target=None):
@@ -113,8 +112,8 @@ def should_instrument(self, module_name: str) -> bool:
         :param module_name: full name of the module that is about to be imported (e.g. ``xyz.abc``)
 
         """
-        for expr in self._package_exprs:
-            if expr.match(module_name):
+        for package in self.packages:
+            if module_name == package or module_name.startswith(package + '.'):
                 return True
 
         return False