Add some support for loading fake modules

 - works only if open_code patch mode is not off
 - see #1079

CHANGES.md

@@ -14,6 +14,10 @@ The released versions correspond to PyPI releases.
 
 ## Unreleased
 
+### Enhancements
+* added some support for loading fake modules in `AUTO` patch mode
+  using `importlib.import_module` (see [#1079](../../issues/1079))
+
 ### Performance
 * avoid reloading `tempfile` in Posix systems
 

docs/usage.rst

@@ -587,6 +587,20 @@ set ``patch_open_code`` to ``PatchMode.AUTO``:
   def test_something(fs):
       assert foo()
 
+In this mode, it is also possible to import modules created in the fake filesystem
+using `importlib.import_module`. Make sure that the `sys.path` contains the parent path in this case:
+
+.. code:: python
+
+  @patchfs(patch_open_code=PatchMode.AUTO)
+  def test_fake_import(fs):
+      fake_module_path = Path("/") / "site-packages" / "fake_module.py"
+      self.fs.create_file(fake_module_path, contents="x = 5")
+      sys.path.insert(0, str(fake_module_path.parent))
+      module = importlib.import_module("fake_module")
+      assert module.x == 5
+
+
 .. _patch_default_args:
 
 patch_default_args

pyfakefs/fake_filesystem_unittest.py

@@ -49,7 +49,12 @@
 import sys
 import tempfile
 import tokenize
+import unittest
+import warnings
+from importlib import reload
 from importlib.abc import Loader, MetaPathFinder
+from importlib.machinery import ModuleSpec
+from importlib.util import spec_from_file_location, module_from_spec
 from types import ModuleType, TracebackType, FunctionType
 from typing import (
     Any,
@@ -66,10 +71,13 @@
     ItemsView,
     Sequence,
 )
-import unittest
-import warnings
 from unittest import TestSuite
 
+from pyfakefs import fake_filesystem, fake_io, fake_os, fake_open, fake_path, fake_file
+from pyfakefs import fake_filesystem_shutil
+from pyfakefs import fake_legacy_modules
+from pyfakefs import fake_pathlib
+from pyfakefs import mox3_stubout
 from pyfakefs.fake_filesystem import (
     set_uid,
     set_gid,
@@ -79,17 +87,8 @@
 )
 from pyfakefs.fake_os import use_original_os
 from pyfakefs.helpers import IS_PYPY
-from pyfakefs.mox3_stubout import StubOutForTesting
-
-from importlib.machinery import ModuleSpec
-from importlib import reload
-
-from pyfakefs import fake_filesystem, fake_io, fake_os, fake_open, fake_path, fake_file
-from pyfakefs import fake_legacy_modules
-from pyfakefs import fake_filesystem_shutil
-from pyfakefs import fake_pathlib
-from pyfakefs import mox3_stubout
 from pyfakefs.legacy_packages import pathlib2, scandir
+from pyfakefs.mox3_stubout import StubOutForTesting
 
 OS_MODULE = "nt" if sys.platform == "win32" else "posix"
 PATH_MODULE = "ntpath" if sys.platform == "win32" else "posixpath"
@@ -1225,14 +1224,32 @@ def cleanup(self) -> None:
                 del sys.modules[name]
 
     def needs_patch(self, name: str) -> bool:
-        """Check if the module with the given name shall be replaced."""
+        """Checks if the module with the given name shall be replaced."""
         if name not in self.modules:
             self._loaded_module_names.add(name)
             return False
         if name in sys.modules and type(sys.modules[name]) is self.modules[name]:
             return False
         return True
 
+    def fake_module_path(self, name: str) -> str:
+        """Checks if the module with the given name is a module existing in the fake
+        filesystem and returns its path in this case.
+        """
+        fs = self._patcher.fs
+        # we assume that the module name is the absolute module path
+        if fs is not None:
+            base_path = name.replace(".", fs.path_separator)
+            for path in sys.path:
+                module_path = fs.joinpaths(path, base_path)
+                py_module_path = module_path + ".py"
+                if fs.exists(py_module_path):
+                    return py_module_path
+                init_path = fs.joinpaths(module_path, "__init__.py")
+                if fs.exists(init_path):
+                    return init_path
+        return ""
+
     def find_spec(
         self,
         fullname: str,
@@ -1242,6 +1259,15 @@ def find_spec(
         """Module finder."""
         if self.needs_patch(fullname):
             return ModuleSpec(fullname, self)
+        if self._patcher.patch_open_code != PatchMode.OFF:
+            # handle modules created in the fake filesystem
+            module_path = self.fake_module_path(fullname)
+            if module_path:
+                spec = spec_from_file_location(fullname, module_path)
+                if spec:
+                    module = module_from_spec(spec)
+                    sys.modules[fullname] = module
+                    return ModuleSpec(fullname, self)
         return None
 
     def load_module(self, fullname: str) -> ModuleType:

pyfakefs/tests/fake_filesystem_unittest_test.py

@@ -951,5 +951,33 @@ def test_write_tmp_windows(self):
         self.check_write_tmp_after_reset(OSType.WINDOWS)
 
 
+@unittest.skipIf(sys.version_info < (3, 8), "Not available before Python 3.8")
+class FakeImportTest(fake_filesystem_unittest.TestCase):
+    """Checks that a fake module can be imported in AUTO patch mode."""
+
+    def setUp(self):
+        self.setUpPyfakefs(patch_open_code=PatchMode.AUTO)
+
+    def test_simple_fake_import(self):
+        fake_module_path = Path("/") / "site-packages" / "fake_module.py"
+        self.fs.create_file(fake_module_path, contents="number = 42")
+        sys.path.insert(0, str(fake_module_path.parent))
+        module = importlib.import_module("fake_module")
+        del sys.path[0]
+        assert module.__name__ == "fake_module"
+        assert module.number == 42
+
+    def test_fake_import_dotted_module(self):
+        fake_pkg_path = Path("/") / "site-packages"
+        self.fs.create_file(fake_pkg_path / "fakepkg" / "__init__.py")
+        fake_module_path = fake_pkg_path / "fakepkg" / "fake_module.py"
+        self.fs.create_file(fake_module_path, contents="number = 42")
+        sys.path.insert(0, str(fake_pkg_path))
+        module = importlib.import_module("fakepkg.fake_module")
+        del sys.path[0]
+        assert module.__name__ == "fakepkg.fake_module"
+        assert module.number == 42
+
+
 if __name__ == "__main__":
     unittest.main()