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

----
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
----

== Compliant Solution

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def get_attr_array(obj, arr):
  props = (name for name in dir(obj) if not name.startswith('_'))
  arr.extend(props)
  return arr
----