== Why is this an issue?

A common anti-pattern is to check that a key does not exist in a dictionary before adding it with a corresponding value. This pattern works but is less readable than the equivalent call to the built-in dictionary method "setdefault()".


Note that if a default value is set for every key of the dictionary it is possible to use python's ``++defaultdict++`` instead.


This rule raises an issue when a key presence is checked before being set. It only raises an issue when the value is a hard-coded string, number, list, dictionary or tuple. Computed values will not raise an issue as they can have side-effects.


=== Noncompliant code example

[source,python]
----
if "key" not in my_dictionary:
    my_dictionary["key"] = ["a", "b", "c"]  # Noncompliant

if "key" not in my_dictionary:
    my_dictionary["key"] = generate_value()  # Compliant. No issue is raised as generate_value() might have some side-effect.
----


=== Compliant solution

[source,python]
----
my_dictionary.setdefault("key", ["a", "b", "c"])

# OR, if ["a", "b", "c"] is the default value for every key

from collections import defaultdict
my_dictionary = defaultdict(lambda: ["a", "b", "c"])
----

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

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

=== Message

Replace this key check and dictionary update with a call to "setdefault"


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