Issue #11311: StringIO.readline(0) now returns an empty string as all other

file-like objects.

Lib/StringIO.py

@@ -158,7 +158,7 @@ class StringIO:
             newpos = self.len
         else:
             newpos = i+1
-        if length is not None and length > 0:
+        if length is not None and length >= 0:
             if self.pos + length < newpos:
                 newpos = self.pos + length
         r = self.buf[self.pos:newpos]

Lib/test/test_StringIO.py

@@ -28,6 +28,8 @@ class TestGenericStringIO(unittest.TestCase):
         eq = self.assertEqual
         self.assertRaises(TypeError, self._fp.seek)
         eq(self._fp.read(10), self._line[:10])
+        eq(self._fp.read(0), '')
+        eq(self._fp.readline(0), '')
         eq(self._fp.readline(), self._line[10:] + '\n')
         eq(len(self._fp.readlines(60)), 2)
         self._fp.seek(0)

Misc/NEWS

@@ -205,6 +205,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #11311: StringIO.readline(0) now returns an empty string as all other
+  file-like objects.
+
 - Issue #16800: tempfile.gettempdir() no longer left temporary files when
   the disk is full.  Original patch by Amir Szekely.