Replace file based mocking by replacing __import__.

tests/utils/time_type_tests.py

@@ -1,6 +1,6 @@
 import sys
 import unittest
-import os
+import builtins
 import contextlib
 import importlib
 import fractions
@@ -14,23 +14,29 @@
 import qupulse.utils.types as qutypes
 
 
-def mock_missing_gmpy2(exit_stack: contextlib.ExitStack):
-    if os.path.exists('gmpy2.py'):
-        raise RuntimeError('Cannot mock missing gmpy2 due to existing file')
+@contextlib.contextmanager
+def mock_missing_module(module_name: str):
+    exit_stack = contextlib.ExitStack()
 
-    with open('gmpy2.py', 'w') as gmpy_file:
-        exit_stack.callback(os.remove, 'gmpy2.py')
+    if module_name in sys.modules:
+        # temporarily remove gmpy2 from the imported modules
 
-        gmpy_file.write("raise ImportError()")
+        temp_modules = sys.modules.copy()
+        del temp_modules[module_name]
+        exit_stack.enter_context(mock.patch.dict(sys.modules, temp_modules))
 
-    if 'gmpy2' in sys.modules:
-        modules_patcher = mock.patch.dict(sys.modules,
-                                          values=((name, module)
-                                                  for name, module in sys.modules.items()
-                                                  if name != 'gmpy2'),
-                                          clear=True)
-        modules_patcher.__enter__()
-        exit_stack.push(modules_patcher)
+    original_import = builtins.__import__
+
+    def mock_import(name, *args, **kwargs):
+        if name == module_name:
+            raise ImportError(name)
+        else:
+            return original_import(name, *args, **kwargs)
+
+    exit_stack.enter_context(mock.patch('builtins.__import__', mock_import))
+
+    with exit_stack:
+        yield
 
 
 class TestTimeType(unittest.TestCase):
@@ -39,25 +45,22 @@ class TestTimeType(unittest.TestCase):
     @property
     def fallback_qutypes(self):
         if not self._fallback_qutypes:
-            exit_stack = contextlib.ExitStack()
-
-            with exit_stack:
-
-                if gmpy2:
-                    # create a local file that raises ImportError on import
-                    mock_missing_gmpy2(exit_stack)
-
+            if gmpy2:
+                with mock_missing_module('gmpy2'):
                     self._fallback_qutypes = importlib.reload(qutypes)
 
-                else:
-                    self._fallback_qutypes = qutypes
+            else:
+                self._fallback_qutypes = qutypes
         return self._fallback_qutypes
 
     def test_fraction_fallback(self):
         self.assertIs(fractions.Fraction, self.fallback_qutypes.TimeType)
 
     @unittest.skipIf(gmpy2 is None, "gmpy2 not available.")
     def test_default_time_from_float(self):
+        # assert mocking did no permanent damage
+        self.assertIs(gmpy2.mpq, qutypes.TimeType)
+
         self.assertEqual(qutypes.time_from_float(123/931), gmpy2.mpq(123, 931))
 
         self.assertEqual(qutypes.time_from_float(1000000/1000001, 1e-5), gmpy2.mpq(1))