Parameter Usage Style

bad.py

>>> def init(headers={}):
...     print "headers before: %r" % headers
...     headers['foo'] = 'bar'
...     print "headers after: %r" % headers
...     
... 
>>> init()
headers before: {}
headers after: {'foo': 'bar'}
>>> init()
headers before: {'foo': 'bar'}
headers after: {'foo': 'bar'}
>>> 

bad2.py

>>> mydict = {}
>>> def init(headers=mydict):
...     print "before: %r" % headers
...     headers['foo'] = 'bar'
...     print "after: %r" % headers
...     
... 
>>> print mydict
{}
>>> init()
before: {}
after: {'foo': 'bar'}
>>> print mydict
{'foo': 'bar'}
>>> 

good.py

>>> def init(headers=None):
...     headers = headers or {}
...     print "before: %r" % headers
...     headers['foo'] = 'bar'
...     print "after: %r" % headers
...     
... 
>>> init()
before: {}
after: {'foo': 'bar'}
>>> init()
before: {}
after: {'foo': 'bar'}
>>>

Trivial nitpick: line 2 of good.py will change any falsey value to an empty dict. In most cases that's OK, but it's a bit safer to say if headers is None: headers={} explicitly. It's more wordy to be sure, but people tripped up by the "non-local dict kwarg" ought to be made aware of this, too.

@donspaulding Yes, that's always a potential gotcha, though in this case I felt it was worth using for example clarity, and because the kwarg is intended to be a dict -- another type of empty value is likely to be wrong anyways, and an empty dict would evaluate to the amusing but harmless expression {} or {}.

(You could argue that in a case where somebody gave e.g. 0 to a kwarg expecting a dict, we'd want it to ValueError or similar instead of silently casting to empty-dict, but I'm not sure that eventuality is worth changing things up for. Maybe use headers = {} if headers is None else headers, or just a two line `if block. A bit more verbose but also more correct.