Parameter Usage Style

gistfile1.py

# Which of these

def __init__(self, uri, http_method=u'GET', body={}, headers={}):
    # do stuff with body and headers directly
    headers['Content-Type'] = u'x-www-url-formencoded'
    # etc...


# VERSUS #

def __init__(self, uri, http_method=u'GET', body=None, headers=None):
    body = body or {}
    headers = headers or {}

You should do what @dstufft recommends :)

I prefer the shorter version of @dstufft

def __init__(self, uri, http_method="GET", body=None, headers=None):
    body = body or {}
    headers = headers or {}

@tebeka it might be simpler, but its useless extra cost, and is implicit behavior of overwriting the structure that you passed in

@dcramer I don't follow. If you passed an empty structure, then a new empty one will be created which is not a big issue. The idea is not to mutate default arguments.
If you really care, you can probably use body = {} if body is None else body if you want to check for None explicitly.