refactor: a better way to have maybe-importable third-party modules

Author: nedbat

coverage/misc.py

@@ -83,14 +83,16 @@ def import_third_party(modname):
         modname (str): the name of the module to import.
 
     Returns:
-        The imported module, or None if the module couldn't be imported.
+        The imported module, and a boolean indicating if the module could be imported.
+
+    If the boolean is False, the module returned is not the one you want: don't use it.
 
     """
     with sys_modules_saved():
         try:
-            return importlib.import_module(modname)
+            return importlib.import_module(modname), True
         except ImportError:
-            return None
+            return sys, False
 
 
 def nice_pair(pair):

coverage/tomlconfig.py

@@ -5,7 +5,6 @@
 
 import os
 import re
-import sys
 
 from typing import Any, Callable, Dict, Iterable, List, Optional, Tuple, Type, TypeVar
 
@@ -17,9 +16,10 @@
 
 if env.PYVERSION >= (3, 11, 0, "alpha", 7):
     import tomllib      # pylint: disable=import-error
+    has_tomllib = True
 else:
     # TOML support on Python 3.10 and below is an install-time extra option.
-    tomllib = import_third_party("tomli")
+    tomllib, has_tomllib = import_third_party("tomli")
 
 
 class TomlDecodeError(Exception):
@@ -51,7 +51,7 @@ def read(self, filenames: Iterable[str]) -> List[str]:
                 toml_text = fp.read()
         except OSError:
             return []
-        if sys.version_info >= (3, 11) or tomllib is not None:
+        if has_tomllib:
             try:
                 self.data = tomllib.loads(toml_text)
             except tomllib.TOMLDecodeError as err:

tests/helpers.py

@@ -15,12 +15,10 @@
 import textwrap
 import warnings
 
-from types import ModuleType
 from typing import (
     cast,
     Any, Callable, Generator, Iterable, List, Optional, Set, Tuple, Type, Union,
 )
-from unittest import mock
 
 import pytest
 
@@ -284,24 +282,6 @@ def change_dir(new_dir: str) -> Generator[None, None, None]:
         os.chdir(old_dir)
 
 
-def without_module(using_module: ModuleType, missing_module_name: str) -> mock._patch[Any]:
-    """
-    Hide a module for testing.
-
-    Use this in a test function to make an optional module unavailable during
-    the test::
-
-        with without_module(product.something, 'tomli'):
-            use_toml_somehow()
-
-    Arguments:
-        using_module: a module in which to hide `missing_module_name`.
-        missing_module_name (str): the name of the module to hide.
-
-    """
-    return mock.patch.object(using_module, missing_module_name, None)
-
-
 def assert_count_equal(a: Iterable[Union[int, str]], b: Iterable[Union[int, str]]) -> None:
     """
     A pytest-friendly implementation of assertCountEqual.

tests/test_config.py

@@ -15,7 +15,6 @@
 from coverage.tomlconfig import TomlConfigParser
 
 from tests.coveragetest import CoverageTest, UsingModulesMixin
-from tests.helpers import without_module
 
 
 class ConfigTest(CoverageTest):
@@ -713,7 +712,7 @@ def test_nocoveragerc_file_when_specified(self) -> None:
 
     def test_no_toml_installed_no_toml(self) -> None:
         # Can't read a toml file that doesn't exist.
-        with without_module(coverage.tomlconfig, 'tomllib'):
+        with mock.patch.object(coverage.tomlconfig, "has_tomllib", False):
             msg = "Couldn't read 'cov.toml' as a config file"
             with pytest.raises(ConfigError, match=msg):
                 coverage.Coverage(config_file="cov.toml")
@@ -722,7 +721,7 @@ def test_no_toml_installed_no_toml(self) -> None:
     def test_no_toml_installed_explicit_toml(self) -> None:
         # Can't specify a toml config file if toml isn't installed.
         self.make_file("cov.toml", "# A toml file!")
-        with without_module(coverage.tomlconfig, 'tomllib'):
+        with mock.patch.object(coverage.tomlconfig, "has_tomllib", False):
             msg = "Can't read 'cov.toml' without TOML support"
             with pytest.raises(ConfigError, match=msg):
                 coverage.Coverage(config_file="cov.toml")
@@ -735,7 +734,7 @@ def test_no_toml_installed_pyproject_toml(self) -> None:
             [tool.coverage.run]
             xyzzy = 17
             """)
-        with without_module(coverage.tomlconfig, 'tomllib'):
+        with mock.patch.object(coverage.tomlconfig, "has_tomllib", False):
             msg = "Can't read 'pyproject.toml' without TOML support"
             with pytest.raises(ConfigError, match=msg):
                 coverage.Coverage()
@@ -748,7 +747,7 @@ def test_no_toml_installed_pyproject_toml_shorter_syntax(self) -> None:
             [tool.coverage]
             run.parallel = true
             """)
-        with without_module(coverage.tomlconfig, 'tomllib'):
+        with mock.patch.object(coverage.tomlconfig, "has_tomllib", False):
             msg = "Can't read 'pyproject.toml' without TOML support"
             with pytest.raises(ConfigError, match=msg):
                 coverage.Coverage()
@@ -761,7 +760,7 @@ def test_no_toml_installed_pyproject_no_coverage(self) -> None:
             [tool.something]
             xyzzy = 17
             """)
-        with without_module(coverage.tomlconfig, 'tomllib'):
+        with mock.patch.object(coverage.tomlconfig, "has_tomllib", False):
             cov = coverage.Coverage()
             # We get default settings:
             assert not cov.config.timid

tests/test_misc.py

@@ -118,7 +118,8 @@ def test_success(self):
         # Make sure we don't have pytest in sys.modules before we start.
         del sys.modules["pytest"]
         # Import pytest
-        mod = import_third_party("pytest")
+        mod, has = import_third_party("pytest")
+        assert has
         # Yes, it's really pytest:
         assert mod.__name__ == "pytest"
         print(dir(mod))
@@ -127,8 +128,8 @@ def test_success(self):
         assert "pytest" not in sys.modules
 
     def test_failure(self):
-        mod = import_third_party("xyzzy")
-        assert mod is None
+        _, has = import_third_party("xyzzy")
+        assert not has
         assert "xyzzy" not in sys.modules
 
 

tests/test_testing.py

@@ -12,14 +12,13 @@
 import pytest
 
 import coverage
-from coverage import tomlconfig
 from coverage.exceptions import CoverageWarning
 from coverage.files import actual_path
 
 from tests.coveragetest import CoverageTest
 from tests.helpers import (
     arcs_to_arcz_repr, arcz_to_arcs, assert_count_equal, assert_coverage_warnings,
-    CheckUniqueFilenames, re_lines, re_lines_text, re_line, without_module,
+    CheckUniqueFilenames, re_lines, re_lines_text, re_line,
 )
 
 
@@ -356,16 +355,6 @@ def _same_python_executable(e1, e2):
     return False                                        # pragma: only failure
 
 
-def test_without_module():
-    toml1 = tomlconfig.tomllib
-    with without_module(tomlconfig, 'tomllib'):
-        toml2 = tomlconfig.tomllib
-    toml3 = tomlconfig.tomllib
-
-    assert toml1 is toml3 is not None
-    assert toml2 is None
-
-
 class ArczTest(CoverageTest):
     """Tests of arcz/arcs helpers."""