In Python for parameter typing, do you use the capital versions of dict and list?


Apologies, am new to Python so a very basic question. Below is an example line of my method definition. Where would I be using capital Dict/List and lowercase Dict/List?

Thanks in advance!

Example scenarios below

def execute_signals(self, parameter1: dict[list], parameter2: dict) -> list[dict]:

def execute_signals(self, parameter1: Dict[List], parameter2: Dict) -> List[Dict]:

def execute_signals(self, parameter1: dict[List], parameter2: dict) -> List[dict]:
Asked By: Sam



The List or Dict have to be imported from the typing module.
Before python 3.9 it was necessary, cause built-in list wouldn’t allow you to do something like this:


It would be a syntax error. It was introduced in Python 3.9, so if you are using that (or newer version) you can use lowercase.

It’s also worth to take a look at this:

Starting with Python 3.7, when from __future__ import annotations is used, function and variable annotations can parameterize standard collections directly.

Answered By: Gameplay

On Python 3.8 and earlier, the name of the collection type is capitalized, and the type is imported from the typing module.

Python 3.5 – 3.8 (also works in Python 3.9+):

from typing import List, Set, Dict, Tuple
x: List[int] = [1]
x: Set[int] = {6, 7}
x: Dict[str, float] = {"field": 2.0}
x: Tuple[int, str, float] = (3, "yes", 7.5)
x: Tuple[int, ...] = (1, 2, 3)

Python 3.9+:

x: list[int] = [1]
x: set[int] = {6, 7}
x: dict[str, float] = {"field": 2.0}
x: tuple[int, str, float] = (3, "yes", 7.5)
x: tuple[int, ...] = (1, 2, 3)

I recommend you read through the mypy’s docs about type hints.

Answered By: chubercik

In Python, the capital versions of Dict and List refer to the abstract base classes for dictionaries and lists, respectively. These classes are part of the module and are used primarily for type checking and duck typing.

However, in function annotations or parameter typing, it’s more common to use the lowercase versions of dict and list, as they refer to the concrete classes. For example:

def my_function(my_list: list, my_dict: dict) -> int:
    # function body here

This function takes a list and a dictionary as arguments, and returns an integer. Note that we are using the lowercase versions of list and dict for parameter typing.

Answered By: Kamyab
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