== Why is this an issue?

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

[source,python]
----
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

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

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Remove the default value of "xxx".


endif::env-github,rspecator-view[]