This stackoverflow question covers the topic pretty well:
http://stackoverflow.com/q/1132941/1547030
From docs.python.org:
Default parameter values are evaluated when the function definition is
executed. This means that the expression is evaluated once, when the
function is defined, and that the same “pre-computed” value is used for
each call. This is especially important to understand when a default
parameter is a mutable object, such as a list or a dictionary: if the
function modifies the object (e.g. by appending an item to a list), the
default value is in effect modified. This is generally not what was
intended. A way around this is to use None as the default, and
explicitly test for it in the body of the function
