Added 'relativefrom' method to path.py

fs/multifs.py

@@ -127,7 +127,7 @@ class MultiFS(FS):
 
     def _priority_sort(self):
         """Sort filesystems by priority order"""
-        priority_order = sorted(self.fs_lookup.keys(), key=lambda n:self.fs_priorities[n], reverse=True)
+        priority_order = sorted(self.fs_lookup.keys(), key=lambda n: self.fs_priorities[n], reverse=True)
         self.fs_sequence = [self.fs_lookup[name] for name in priority_order]
 
     @synchronize
@@ -181,7 +181,7 @@ class MultiFS(FS):
 
         """
         if name not in self.fs_lookup:
-            raise ValueError("No filesystem called '%s'"%name)
+            raise ValueError("No filesystem called '%s'" % name)
         fs = self.fs_lookup[name]
         self.fs_sequence.remove(fs)
         del self.fs_lookup[name]

fs/path.py

@@ -396,6 +396,29 @@ def frombase(path1, path2):
     return path2[len(path1):]
 
 
+def relativefrom(base, path):
+    """Return a path relative from a given base path,
+    i.e. insert backrefs as appropriate to reach the path from the base.
+
+    :param base_path: Path to a directory
+    :param path: Path you wish to make relative
+
+
+    >>> relativefrom("foo/bar", "baz/index.html")
+    '../baz/index.html'
+
+    """
+    base = list(iteratepath(base))
+    path = list(iteratepath(path))
+
+    while base and path and base[0] == path[0]:
+        base.pop(0)
+        path.pop(0)
+
+    # If you multiply a list by a negative number, you get an empty list!
+    return u'/'.join([u'..'] * len(base) + path)
+
+
 class PathMap(object):
     """Dict-like object with paths for keys.
 
@@ -626,3 +649,8 @@ def iswildcard(path):
 
 if __name__ == "__main__":
     print recursepath('a/b/c')
+
+    print relativefrom('/', '/foo')
+    print relativefrom('/foo/bar', '/foo/baz')
+    print relativefrom('/foo/bar/baz', '/foo/egg')
+    print relativefrom('/foo/bar/baz/egg', '/foo/egg')

fs/tests/test_path.py

@@ -150,6 +150,21 @@ class TestPathFunctions(unittest.TestCase):
         self.assertFalse(iswildcard('img.jpg'))
         self.assertFalse(iswildcard('foo/bar'))
 
+    def test_realtivefrom(self):
+        tests = [('/', '/foo.html', 'foo.html'),
+                 ('/foo', '/foo/bar.html', 'bar.html'),
+                 ('/foo/bar/', '/egg.html', '../../egg.html'),
+                 ('/a/b/c/d', 'e', '../../../../e'),
+                 ('/a/b/c/d', 'a/d', '../../../d'),
+                 ('/docs/', 'tags/index.html', '../tags/index.html'),
+                 ('foo/bar', 'baz/index.html', '../../baz/index.html'),
+                 ('', 'a', 'a'),
+                 ('a', 'b/c', '../b/c')
+                 ]
+
+        for base, path, result in tests:
+            self.assertEqual(relativefrom(base, path), result)
+
 
 class Test_PathMap(unittest.TestCase):