Handle 'mocker.patch' being used without source code

Fix #169

CHANGELOG.rst

@@ -1,3 +1,11 @@
+1.12.1 (2019-11-20)
+-------------------
+
+* Fix error if ``mocker.patch`` is used in code where the source file
+  is not available, for example stale ``.pyc`` files (`#169`_).
+
+.. _#169: https://github.com/pytest-dev/pytest-mock/issues/169#issuecomment-555729265
+
 1.12.0 (2019-11-19)
 -------------------
 

src/pytest_mock/plugin.py

@@ -160,6 +160,9 @@ def _enforce_no_with_context(self, stack):
             caller = stack[2]
             frame = caller[0]
             info = inspect.getframeinfo(frame)
+            if info.code_context is None:
+                # no source code available (#169)
+                return
             code_context = " ".join(info.code_context).strip()
 
             if code_context.startswith("with mocker."):

tests/test_pytest_mock.py

@@ -753,3 +753,41 @@ def test_abort_patch_context_manager(mocker):
     )
 
     assert str(excinfo.value) == expected_error_msg
+
+
+def test_abort_patch_context_manager_with_stale_pyc(testdir):
+    """Ensure we don't trigger an error in case the frame where mocker.patch is being
+    used doesn't have a 'context' (#169)"""
+    import compileall
+
+    py_fn = testdir.makepyfile(
+        c="""
+        class C:
+            x = 1
+
+        def check(mocker):
+            mocker.patch.object(C, "x", 2)
+            assert C.x == 2
+    """
+    )
+    testdir.syspathinsert()
+
+    testdir.makepyfile(
+        """
+        from c import check
+        def test_foo(mocker):
+            check(mocker)
+    """
+    )
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines("* 1 passed *")
+
+    kwargs = {"legacy": True} if sys.version_info[0] >= 3 else {}
+    assert compileall.compile_file(str(py_fn), **kwargs)
+
+    pyc_fn = str(py_fn) + "c"
+    assert os.path.isfile(pyc_fn)
+
+    py_fn.remove()
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines("* 1 passed *")