Make multipart.boundary a str property

aiohttp/multipart.py

@@ -686,7 +686,7 @@ def _boundary_value(self):
         #                  / DIGIT / ALPHA
         #                  ; any VCHAR, except delimiters
         # VCHAR           = %x21-7E
-        value = self.boundary
+        value = self._boundary
         if re.match(self._valid_tchar_regex, value):
             return value.decode('ascii')  # cannot fail
 
@@ -701,7 +701,7 @@ def _boundary_value(self):
 
     @property
     def boundary(self):
-        return self._boundary
+        return self._boundary.decode('ascii')
 
     def append(self, obj, headers=None):
         if headers is None:

docs/multipart_reference.rst

@@ -165,7 +165,11 @@ Multipart reference
 
    .. attribute:: boundary
 
-      The byte-string representation of the boundary.
+      The string (:class:`str`) representation of the boundary.
+
+      .. versionchanged:: 3.0
+
+         Property type was changed from :class:`bytes` to :class:`str`.
 
    .. method:: append(obj, headers=None)
 

tests/test_multipart.py

@@ -762,7 +762,7 @@ async def test_reading_skips_prelude(self):
 
 async def test_writer(writer):
     assert writer.size == 0
-    assert writer.boundary == b':'
+    assert writer.boundary == ':'
 
 
 async def test_writer_serialize_io_chunk(buf, stream, writer):