This rule raises an issue when .keys() is called explicitly on a dictionary for membership testing.

Why is this an issue?

In Python, dictionaries support direct membership testing without needing to explicitly call the .keys() method. When you write key in dict.keys(), you are being unnecessarily verbose.

The expression key in dict is the idiomatic Python way to check for key membership. It is:

• more readable: the intent is clearer and the code is shorter
• slightly more efficient: it avoids creating an intermediate dictionary view object
• more Pythonic: it follows Python’s principle of having one obvious way to do it

The .keys() method exists primarily for cases where you need to perform set operations on dictionary keys or when you need an explicit view object. For simple membership testing, it is redundant because dictionaries are designed to support the in operator directly.

What is the potential impact?

Unnecessarily verbose code patterns accumulate over a codebase and reduce readability. Following Python idioms makes code easier to review and maintain, and helps new contributors understand the codebase faster.

How to fix it

Remove the .keys() call and use direct membership testing on the dictionary. This is the standard Python idiom for checking if a key exists in a dictionary.

Code examples

Noncompliant code example

my_dict = {'a': 1, 'b': 2, 'c': 3}

if 'a' in my_dict.keys():  # Noncompliant
    print('Found')

Compliant solution

my_dict = {'a': 1, 'b': 2, 'c': 3}

if 'a' in my_dict:
    print('Found')

Resources

Documentation

• Python Documentation - Mapping Types — dict
• Python Documentation - Dictionary view objects