While the assignment of default parameter values is typically a good thing, it can go very wrong very quickly when mutable objects are used. That's because a new instance of the object _is not_ created for each function invocation. Instead, all invocations share the same instance, and the changes made for one caller are made for all!
== Noncompliant Code Example
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def get_attr_array(obj, arr=[]): # Noncompliant
props = (name for name in dir(obj) if not name.startswith('_'))
arr.extend(props) # after only a few calls, this is a big array!
return arr
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== Compliant Solution
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def get_attr_array(obj, arr):
props = (name for name in dir(obj) if not name.startswith('_'))
