Fix bug when MultipartEncoder is asked to encode zero parts

requests_toolbelt/multipart/encoder.py

@@ -190,7 +190,7 @@ def _load(self, amount):
         part = self._current_part or self._next_part()
         while amount == -1 or amount > 0:
             written = 0
-            if not part.bytes_left_to_write():
+            if part and not part.bytes_left_to_write():
                 written += self._write(b'\r\n')
                 written += self._write_boundary()
                 part = self._next_part()

tests/test_multipart_encoder.py

@@ -311,6 +311,12 @@ def test_accepts_custom_headers(self):
         output = m.read().decode('utf-8')
         assert output.index('X-My-Header: my-value\r\n') > 0
 
+    def test_no_parts(self):
+        fields = []
+        boundary = '--90967316f8404798963cce746a4f4ef9'
+        m = MultipartEncoder(fields=fields, boundary=boundary)
+        output = m.read().decode('utf-8')
+        assert output == '----90967316f8404798963cce746a4f4ef9--\r\n'
 
 if __name__ == '__main__':
     unittest.main()