PytestHow-ToBeginner · 3 min read

How to Assert Raises in pytest: Syntax and Examples

In pytest, use the pytest.raises() context manager to assert that a specific exception is raised during a block of code. Wrap the code that should raise the exception inside the with pytest.raises(ExceptionType): block to verify the error occurs.

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Syntax

The pytest.raises() function is used as a context manager to check if a block of code raises a specific exception.

ExceptionType: The type of exception you expect (e.g., ValueError).
The code inside the with block is the code expected to raise the exception.
If the exception is raised, the test passes; if not, the test fails.

python

import pytest

def function_that_raises():
    raise ExceptionType("Error message")

with pytest.raises(ExceptionType):
    # code that should raise ExceptionType
    function_that_raises()

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Example

This example shows how to test that a function raises a ValueError when given invalid input.

python

import pytest

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

def test_divide_by_zero():
    with pytest.raises(ValueError):
        divide(10, 0)

Output

============================= test session starts ============================== collected 1 item test_example.py . [100%] ============================== 1 passed in 0.03s ===============================

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Common Pitfalls

Common mistakes when using pytest.raises() include:

Not using it as a context manager with with, which will not catch the exception properly.
Expecting the wrong exception type, causing tests to fail unexpectedly.
Placing code that does not raise inside the with block, leading to false positives.

python

import pytest

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

def test_wrong_usage():
    # Wrong: not using 'with' - this will not catch the exception
    try:
        divide(10, 0)
    except ValueError:
        pass  # This is manual and not recommended

def test_correct_usage():
    with pytest.raises(ValueError):
        divide(10, 0)

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Quick Reference

Remember these tips when asserting exceptions in pytest:

Always use with pytest.raises(ExceptionType): as a context manager.
Check for the exact exception type you expect.
You can access the exception info with as exc_info if needed.

python

import pytest

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

def test_exception_message():
    with pytest.raises(ValueError) as exc_info:
        divide(10, 0)
    assert str(exc_info.value) == "Cannot divide by zero"

Output

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Key Takeaways

Use pytest.raises() as a context manager to assert exceptions.

Wrap only the code that should raise the exception inside the with block.

Specify the exact exception type to catch for accurate tests.

Access exception details with 'as exc_info' for further assertions.

Avoid manual try-except blocks; pytest.raises() is cleaner and more reliable.