Preserve filesystem subclass types in context managers

CHANGELOG.md

@@ -15,6 +15,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
   Closes [#459](https://github.com/PyFilesystem/pyfilesystem2/issues/459).
 
 ### Fixed
+- Preserved concrete filesystem types when entering context managers.
+  Related to [#580](https://github.com/PyFilesystem/pyfilesystem2/issues/580).
 - Elaborated documentation of `filter_dirs` and `exclude_dirs` in `fs.walk.Walker`.
   Closes [#371](https://github.com/PyFilesystem/pyfilesystem2/issues/371).
 - Fixes a backward incompatibility where `fs.move.move_file` raises `DestinationExists`

CONTRIBUTORS.md

@@ -20,6 +20,7 @@ Many thanks to the following developers for contributing to this project:
 - [George Macon](https://github.com/gmacon)
 - [Giampaolo Cimino](https://github.com/gpcimino)
 - [@Hoboneer](https://github.com/Hoboneer)
+- [Hugohong258](https://github.com/Hugohong258)
 - [Jen Hagg](https://github.com/jenhagg)
 - [Joseph Atkins-Turkish](https://github.com/Spacerat)
 - [Joshua Tauberer](https://github.com/JoshData)

fs/base.py

@@ -118,7 +118,7 @@ def __del__(self):
         self.close()
 
     def __enter__(self):
-        # type: (...) -> FS
+        # type: (_F) -> _F
         """Allow use of filesystem as a context manager."""
         return self
 

tests/test_context_typing.py

@@ -0,0 +1,41 @@
+"""Check preservation of concrete filesystem types in context managers."""
+from __future__ import unicode_literals
+
+import unittest
+
+try:
+    from mypy import api as mypy_api
+except ImportError:
+    mypy_api = None
+
+
+class TestContextTyping(unittest.TestCase):
+    @unittest.skipIf(mypy_api is None, "mypy is not installed")
+    def test_context_manager_preserves_subclass_type(self):
+        source = """
+from fs.memoryfs import MemoryFS
+from fs.osfs import OSFS
+from fs.tempfs import TempFS
+
+def accepts_temp(fs):
+    # type: (TempFS) -> None
+    pass
+
+with TempFS() as temp:
+    accepts_temp(temp)
+
+with OSFS(".") as osfs:
+    concrete = osfs  # type: OSFS
+
+class CustomFS(MemoryFS):
+    def custom_method(self):
+        # type: () -> int
+        return 1
+
+with CustomFS() as custom:
+    custom.custom_method()
+"""
+        output, error, status = mypy_api.run(
+            ["--follow-imports=silent", "--no-incremental", "-c", source]
+        )
+        self.assertEqual(status, 0, output + error)