conina:~ graham$ python
Python 2.7.3 (v2.7.3:70274d53c1dd, Apr  9 2012, 20:52:43)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> dict({'a':'aa'},b='bb')
{'a': 'aa', 'b': 'bb'}
>>> dict({'a':'aa'},a='bb')
Segmentation fault: 11
conina:~ graham$



From the docs:
[[
If keyword arguments are given, the keyword arguments and their values are added to the dictionary created from the positional argument. If a key being added is already present, the value from the keyword argument replaces the value from the positional argument.
]]