Fix path pattern construction on Windows

Author: shreve

nbgrader/coursedir.py

@@ -1,5 +1,6 @@
 import os
 import re
+import sys
 import datetime
 import typing
 from pathlib import Path
@@ -367,6 +368,10 @@ def find_assignments(self,
         # Convert to a Path and back to a string to remove any instances of `/.`
         pattern = str(Path(self.directory_structure.format(**pattern_args)))
 
+        if sys.platform == 'win32':
+            # Escape backslashes on Windows
+            pattern = pattern.replace('\\', r'\\')
+
         for dir in dirs:
             match = re.match(pattern, str(dir.relative_to(self.root)))
             if match:

nbgrader/tests/test_coursedir.py

@@ -1,5 +1,6 @@
 import pytest
 import os
+import re
 
 from traitlets.config import Config
 
@@ -24,7 +25,8 @@ def test_coursedir_format_path(conf):
     expected = os.path.join(coursedir.root, "step", "student_id", "assignment_id")
     assert coursedir.format_path("step", "student_id", "assignment_id") == expected
 
-    expected = os.path.join(coursedir.root.replace("-", r"\-"), "step", "student_id", "assignment_id")
+    escaped = re.escape(coursedir.root)
+    expected = os.path.join(escaped, "step", "student_id", "assignment_id")
     assert coursedir.format_path("step", "student_id", "assignment_id", escape=True) == expected