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[Pulp-list] Default values in method signatures



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This came up today with respect to the Filter __init__ method. The issue
surrounds the signature:  def __init__(self, id, type, description=None,
package_list=[]):

I first learned this was an issue when PyCharm started highlighting them
in our code base. From PyCharm's inspection description:

"Default argument values are evaluated only once at function definition
time, which means that modifying the default value of the argument will
affect all subsequent calls of the function. "

In other words, all Filter instances that don't pass in package_list
will point to the same list object. If they start to dork with its
contents, *all* Filter objects will reflect the package list.

A workaround could be:

def __init__(self, id, type, description=None, package_list=None):
    if not package_list:
        package_list = []
    self.package_list = package_list

That way a new list is created on each instantiation if one is not
passed in.

BTW, this is all over our code. I mentioned it back in August
(https://www.redhat.com/archives/pulp-list/2010-August/msg00030.html) so
please try to clean these up when you see them.


- -- 
Jay Dobies
RHCE# 805008743336126
Freenode: jdob
http://pulpproject.org
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