Allow partials and other non-function callables as importer callbacks

sass.py

@@ -13,7 +13,6 @@
 from __future__ import absolute_import
 
 import collections
-import functools
 import inspect
 import io
 import os
@@ -195,19 +194,21 @@ def _to_bytes(obj):
 
 
 def _importer_callback_wrapper(func):
-    if PY2:
-        argspec = inspect.getargspec(func)
-    else:
-        argspec = inspect.getfullargspec(func)
-
-    num_args = len(argspec.args)
-
-    @functools.wraps(func)
     def inner(path, prev):
-        if num_args == 2:
-            ret = func(path.decode('UTF-8'), prev.decode('UTF-8'))
+        path, prev = path.decode('UTF-8'), prev.decode('UTF-8')
+        num_args = getattr(inner, '_num_args', None)
+        if num_args is None:
+            try:
+                ret = func(path, prev)
+            except TypeError:
+                inner._num_args = 1
+                ret = func(path)
+            else:
+                inner._num_args = 2
+        elif num_args == 2:
+            ret = func(path, prev)
         else:
-            ret = func(path.decode('UTF-8'))
+            ret = func(path)
         return _normalize_importer_return_value(ret)
     return inner
 

sasstests.py

@@ -3,6 +3,7 @@
 
 import base64
 import contextlib
+import functools
 import glob
 import json
 import io
@@ -358,6 +359,23 @@ def importer(path, prev):
         )
         assert ret == 'a{color:red}\n'
 
+    def test_importer_prev_path_partial(self):
+        def importer(a_css, path, prev):
+            assert path in ('a', 'b')
+            if path == 'a':
+                assert prev == 'stdin'
+                return ((path, '@import "b";'),)
+            elif path == 'b':
+                assert prev == 'a'
+                return ((path, a_css),)
+
+        ret = sass.compile(
+            string='@import "a";',
+            importers=((0, functools.partial(importer, 'a { color: red; }')),),
+            output_style='compressed',
+        )
+        assert ret == 'a{color:red}\n'
+
     def test_importer_does_not_handle_returns_None(self):
         def importer_one(path):
             if path == 'one':