(original thread is https://mail.python.org/pipermail/python-list/2017-March/720391.html)

https://github.com/python/cpython/commit/4897300276d870f99459c82b937f0ac22450f0b6

this commit doubles sizeof set object created by set_merge().
It is used by constructor of set and frozenset.

$ /usr/bin/python3
Python 3.5.2+ (default, Sep 22 2016, 12:18:14)
[GCC 6.2.0 20160927] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> s = set(range(10))
>>> sys.getsizeof(frozenset(s))
736

$ python3
Python 3.6.0 (default, Dec 30 2016, 20:49:54)
[GCC 6.2.0 20161005] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import  sys
>>> s = set(range(10))
>>> sys.getsizeof(frozenset(s))
1248