Posted on Dec 17, 2024

How to Use Typing Module to Annotate Function Definition with Input and Output Types in Python

The aim of this page📝 is to explain how to use type hints in Python, specifically for functions that return a list of dictionaries.
I am slowly going through David Baezley's Advanced Python mastery and - based on How to Code's systematic approach to program design - I am annotating functions with input and output types as that definition determines the shape of the function.

Type hints: Improve code readability and maintainability.
typing module: Provides more specific type annotations.
PEP 484: Introduced type hints in Python 3.5.
- See more: https://peps.python.org/pep-0484/#the-typing-module
Common types: List, Dict, Tuple, Union, Optional.
Specify list of dicts: Use List[Dict[str, int]] for return type.
Example from Advanced Python Mastery which reads a provided .csv files with Bus timetable of four columns and returns a list of dictionaries. Mostly, I want to specify that latter fact.

 from typing import List, Dict import csv def read_rides(filename: str) -> List[Dict]: rides = [] with open(filename, "r") as file: rows = csv.reader(file) headers = [row.strip() for row in next(rows)] print(f"ROW headers: {headers}") for row in rows: ride = {} for column_number, column_name in enumerate(headers): ride[column_name] = row[column_number].strip() rides.append(ride) return rides