Allow custom subfs (#314)

* Implementation - don't know if it works

* Add test

* More documentation

* Change documentation for opendir

* Make a test for my original problem

* Run black

* Back out .gitignore changes

Author: rkhwaja

fs/base.py

@@ -101,6 +101,9 @@ class FS(object):
     # most FS will use default walking algorithms
     walker_class = Walker
 
+    # default to SubFS, used by opendir and should be returned by makedir(s)
+    subfs_class = None
+
     def __init__(self):
         # type: (...) -> None
         """Create a filesystem. See help(type(self)) for accurate signature.
@@ -1180,7 +1183,7 @@ def opendir(
             factory (callable, optional): A callable that when invoked
                 with an FS instance and ``path`` will return a new FS object
                 representing the sub-directory contents. If no ``factory``
-                is supplied then `~fs.subfs.SubFS` will be used.
+                is supplied then `~fs.subfs_class` will be used.
 
         Returns:
             ~fs.subfs.SubFS: A filesystem representing a sub-directory.
@@ -1192,7 +1195,7 @@ def opendir(
         """
         from .subfs import SubFS
 
-        _factory = factory or SubFS
+        _factory = factory or self.subfs_class or SubFS
 
         if not self.getbasic(path).is_dir:
             raise errors.DirectoryExpected(path=path)

tests/test_subfs.py

@@ -3,8 +3,11 @@
 import os
 import shutil
 import tempfile
+import unittest
 
 from fs import osfs
+from fs.subfs import SubFS
+from fs.memoryfs import MemoryFS
 from fs.path import relpath
 from .test_osfs import TestOSFS
 
@@ -26,3 +29,44 @@ def tearDown(self):
     def _get_real_path(self, path):
         _path = os.path.join(self.temp_dir, "__subdir__", relpath(path))
         return _path
+
+
+class CustomSubFS(SubFS):
+    """Just a custom class to change the type"""
+
+    def custom_function(self, custom_path):
+        fs, delegate_path = self.delegate_path(custom_path)
+        fs.custom_function(delegate_path)
+
+
+class CustomSubFS2(SubFS):
+    """Just a custom class to change the type"""
+
+
+class CustomFS(MemoryFS):
+    subfs_class = CustomSubFS
+
+    def __init__(self):
+        super(CustomFS, self).__init__()
+        self.custom_path = None
+
+    def custom_function(self, custom_path):
+        self.custom_path = custom_path
+
+
+class TestCustomSubFS(unittest.TestCase):
+    """Test customization of the SubFS returned from opendir etc"""
+
+    def test_opendir(self):
+        fs = CustomFS()
+        fs.makedir("__subdir__")
+        subfs = fs.opendir("__subdir__")
+        # By default, you get the fs's defined custom SubFS
+        assert isinstance(subfs, CustomSubFS)
+
+        subfs.custom_function("filename")
+        assert fs.custom_path == "/__subdir__/filename"
+
+        # Providing the factory explicitly still works
+        subfs = fs.opendir("__subdir__", factory=CustomSubFS2)
+        assert isinstance(subfs, CustomSubFS2)