How Do You Declare a Data Type in a Function in Python?


Larry Thompson

How Do You Declare a Data Type in a Function in Python?

When working with Python, it’s important to understand how to declare data types within functions. By explicitly declaring data types, you can improve code readability and catch potential errors before they occur.

In this article, we will explore different ways to declare data types in Python functions.

Using Type Hints

Python 3 introduced type hints, which allow developers to specify the expected data types of function parameters and return values. Although type hints are not enforced at runtime, they serve as a useful tool for documentation and static analysis tools such as linters and IDEs.

To declare a data type in a function using type hints, you can use the “->” arrow notation. Here’s an example:

def add_numbers(a: int, b: int) -> int:
    return a + b

In the above example, we declare that both “a” and “b” should be of type “int”. Additionally, we specify that the return value should also be of type “int”.

Type Checking with External Libraries

Python provides several external libraries that allow for more advanced type checking within functions. One popular library is mypy.

Mypy is a static type checker that can be integrated into your development workflow to catch potential type-related errors.

To use mypy with your codebase, you need to install it first using pip:

pip install mypy

Once installed, you can run mypy on your Python files by executing the following command:


Mypy will analyze your code and provide feedback on any potential type errors it finds. This can be incredibly helpful in catching bugs and ensuring the correctness of your code.

Using Assertions

Another way to declare data types in a function is by using assertions. Assertions allow you to check if certain conditions are met during runtime and raise an error if they are not.

In the context of type checking, assertions can be used to verify that the expected data types are being used.

Here’s an example of using assertions to declare data types:

def multiply_numbers(a, b):
    assert isinstance(a, int), "a should be of type int"
    assert isinstance(b, int), "b should be of type int"

    return a * b

In this example, we use the “isinstance” function to check if both “a” and “b” are of type “int”. If either of them is not an integer, an AssertionError will be raised with the provided error message.

This helps ensure that the function is used correctly with the expected data types.


In Python, declaring data types within functions can enhance code readability and catch potential errors early on. Type hints, external libraries like mypy, and assertions are all powerful tools at your disposal for declaring data types in functions.

By utilizing these techniques, you can write more robust and maintainable Python code.

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