This rule raises an issue when dict(), list(), or tuple() are used to create empty collections or to initialize dictionaries with keyword arguments.

Why is this an issue?

Python provides concise literal syntax for creating empty dictionaries ({}), lists ([]), and tuples (()). It also offers a direct literal syntax for initializing dictionaries with key-value pairs (e.g., {'a': 1, 'b': 2}).

Using the function calls dict(), list(), and tuple() for these purposes is generally less preferred for a few reasons:

• Readability and Directness: Literals are often considered more "Pythonic" and directly express the intent of creating an empty container or a dictionary with specific initial values.
• Consistency: Python encourages the use of literals where they are clear and effective.
• Performance: Calling a function involves overhead, including name lookup for dict, list, or tuple in the current scope. While often negligible for single calls, using literals is a direct instruction to the interpreter and can be marginally faster, especially in performance-sensitive code or tight loops.

Specifically, the following patterns are discouraged:

• Using dict() to create an empty dictionary instead of {}.
• Using list() to create an empty list instead of [].
• Using tuple() to create an empty tuple instead of ().
• Using dict(key='value', …​) to initialize a dictionary instead of {'key': 'value', …​} when keys are simple strings valid as identifiers.

While the functional difference is minimal for creating empty collections or simple dictionaries, adopting literals promotes a more direct and idiomatic coding style.

How to fix it

To fix this replace the function calls dict(), list(), and tuple() with their equivalent literal syntax.

• To create an empty dictionary, replace dict() with {}.
• To create an empty list, replace list() with [].
• To create an empty tuple, replace tuple() with ().
• To initialize a dictionary with keyword arguments (where keys are simple strings that are valid identifiers), replace dict(key1=value1, key2=value2) with {'key1': value1, 'key2': value2}.

empty_d = dict() # Noncompliant: the dict constructor is used instead of the literal syntax

empty_d = {} # Compliant

Resources

Documentation

• Python Documentation - dict()
• Python Documentation - list()
• Python Documentation - tuple()