This is a gotcha I ran across in some production code that is obvious in retrospect. I was profiling the code to find places where we were calling an_expensive_database_function and came across code like this:

def doit(*args, **kwargs):
     value = kwargs.get('key', an_expensive_database_function())

The original author probably assumed that if 'key' was present in the kwargs dictionary an_expensive_database_function wouldn't be called; that it would be short circuited in the same manner as Boolean expressions. But since get is a function the arguments are always evaluated on the way into the function. So in this case even if the value of an_expensive_database_function was already present in the kwargs dictionary the database function would be called again.

Here is a "look before you leap" solution:

def doit(*args, **kwargs):
    value = kwargs.get('key')
    if value is None:
         # assuming default value None isn't a valid value
         value = an_expensive_database_function()``

Here is the "easier to ask forgiveness than permission" solution:

def doit(*args, **kwargs):
    try:
        value = kwargs['key']
    except KeyError:
        value = an_expensive_database_function()``

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