Multiple assignment and dict

so_dangerous_comma.py

#!/usr/bin/env python

def key_first():
    dct = {}
    key = 'foo'  
    key, dct[key] = 'some_new_key', 'bar';
    return dct

def dict_first():
    dct = {}
    key = 'foo'
    dct[key], key = 'bar', 'some_new_key';
    return dct

assert key_first().keys() == dict_first().keys()  # AssertionError
# Because multiple assignment is not atomic
# In `key_first` when we assigning value by `key` it already has been changed