How to write and report assertions in tests

Asserting with the assert statement

pytest allows you to use the standard Python assert for verifying expectations and values in Python tests. For example, you can write the following:

# content of
def f():
    return 3

def test_function():
    assert f() == 4

to assert that your function returns a certain value. If this assertion fails you will see the return value of the function call:

$ pytest
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-7.x.y, pluggy-1.x.y
rootdir: /home/sweet/project
collected 1 item F                                                    [100%]

================================= FAILURES =================================
______________________________ test_function _______________________________

    def test_function():
>       assert f() == 4
E       assert 3 == 4
E        +  where 3 = f() AssertionError
========================= short test summary info ==========================
FAILED - assert 3 == 4
============================ 1 failed in 0.12s =============================

pytest has support for showing the values of the most common subexpressions including calls, attributes, comparisons, and binary and unary operators. (See Demo of Python failure reports with pytest). This allows you to use the idiomatic python constructs without boilerplate code while not losing introspection information.

However, if you specify a message with the assertion like this:

assert a % 2 == 0, "value was odd, should be even"

then no assertion introspection takes places at all and the message will be simply shown in the traceback.

See Assertion introspection details for more information on assertion introspection.

Assertions about expected exceptions

In order to write assertions about raised exceptions, you can use pytest.raises() as a context manager like this:

import pytest

def test_zero_division():
    with pytest.raises(ZeroDivisionError):
        1 / 0

and if you need to have access to the actual exception info you may use:

def test_recursion_depth():
    with pytest.raises(RuntimeError) as excinfo:

        def f():

    assert "maximum recursion" in str(excinfo.value)

excinfo is an ExceptionInfo instance, which is a wrapper around the actual exception raised. The main attributes of interest are .type, .value and .traceback.

You can pass a match keyword parameter to the context-manager to test that a regular expression matches on the string representation of an exception (similar to the TestCase.assertRaisesRegex method from unittest):

import pytest

def myfunc():
    raise ValueError("Exception 123 raised")

def test_match():
    with pytest.raises(ValueError, match=r".* 123 .*"):

The regexp parameter of the match method is matched with the function, so in the above example match='123' would have worked as well.

There’s an alternate form of the pytest.raises() function where you pass a function that will be executed with the given *args and **kwargs and assert that the given exception is raised:

pytest.raises(ExpectedException, func, *args, **kwargs)

The reporter will provide you with helpful output in case of failures such as no exception or wrong exception.

Note that it is also possible to specify a “raises” argument to pytest.mark.xfail, which checks that the test is failing in a more specific way than just having any exception raised:

def test_f():

Using pytest.raises() is likely to be better for cases where you are testing exceptions your own code is deliberately raising, whereas using @pytest.mark.xfail with a check function is probably better for something like documenting unfixed bugs (where the test describes what “should” happen) or bugs in dependencies.

Assertions about expected warnings

You can check that code raises a particular warning using pytest.warns.

Making use of context-sensitive comparisons

pytest has rich support for providing context-sensitive information when it encounters comparisons. For example:

# content of
def test_set_comparison():
    set1 = set("1308")
    set2 = set("8035")
    assert set1 == set2

if you run this module:

$ pytest
=========================== test session starts ============================
platform linux -- Python 3.x.y, pytest-7.x.y, pluggy-1.x.y
rootdir: /home/sweet/project
collected 1 item F                                                    [100%]

================================= FAILURES =================================
___________________________ test_set_comparison ____________________________

    def test_set_comparison():
        set1 = set("1308")
        set2 = set("8035")
>       assert set1 == set2
E       AssertionError: assert {'0', '1', '3', '8'} == {'0', '3', '5', '8'}
E         Extra items in the left set:
E         '1'
E         Extra items in the right set:
E         '5'
E         Use -v to get more diff AssertionError
========================= short test summary info ==========================
FAILED - AssertionError: assert {'0'...
============================ 1 failed in 0.12s =============================

Special comparisons are done for a number of cases:

  • comparing long strings: a context diff is shown

  • comparing long sequences: first failing indices

  • comparing dicts: different entries

See the reporting demo for many more examples.

Defining your own explanation for failed assertions

It is possible to add your own detailed explanations by implementing the pytest_assertrepr_compare hook.

pytest_assertrepr_compare(config, op, left, right)[source]

Return explanation for comparisons in failing assert expressions.

Return None for no custom explanation, otherwise return a list of strings. The strings will be joined by newlines but any newlines in a string will be escaped. Note that all but the first line will be indented slightly, the intention is for the first line to be a summary.

  • config (Config) – The pytest config object.

  • op (str) – The operator, e.g. "==", "!=", "not in".

  • left (object) – The left operand.

  • right (object) – The right operand.

As an example consider adding the following hook in a file which provides an alternative explanation for Foo objects:

# content of
from test_foocompare import Foo

def pytest_assertrepr_compare(op, left, right):
    if isinstance(left, Foo) and isinstance(right, Foo) and op == "==":
        return [
            "Comparing Foo instances:",
            f"   vals: {left.val} != {right.val}",

now, given this test module:

# content of
class Foo:
    def __init__(self, val):
        self.val = val

    def __eq__(self, other):
        return self.val == other.val

def test_compare():
    f1 = Foo(1)
    f2 = Foo(2)
    assert f1 == f2

you can run the test module and get the custom output defined in the conftest file:

$ pytest -q
F                                                                    [100%]
================================= FAILURES =================================
_______________________________ test_compare _______________________________

    def test_compare():
        f1 = Foo(1)
        f2 = Foo(2)
>       assert f1 == f2
E       assert Comparing Foo instances:
E            vals: 1 != 2 AssertionError
========================= short test summary info ==========================
FAILED - assert Comparing Foo instances:
1 failed in 0.12s

Assertion introspection details

Reporting details about a failing assertion is achieved by rewriting assert statements before they are run. Rewritten assert statements put introspection information into the assertion failure message. pytest only rewrites test modules directly discovered by its test collection process, so asserts in supporting modules which are not themselves test modules will not be rewritten.

You can manually enable assertion rewriting for an imported module by calling register_assert_rewrite before you import it (a good place to do that is in your root

For further information, Benjamin Peterson wrote up Behind the scenes of pytest’s new assertion rewriting.

Assertion rewriting caches files on disk

pytest will write back the rewritten modules to disk for caching. You can disable this behavior (for example to avoid leaving stale .pyc files around in projects that move files around a lot) by adding this to the top of your file:

import sys

sys.dont_write_bytecode = True

Note that you still get the benefits of assertion introspection, the only change is that the .pyc files won’t be cached on disk.

Additionally, rewriting will silently skip caching if it cannot write new .pyc files, i.e. in a read-only filesystem or a zipfile.

Disabling assert rewriting

pytest rewrites test modules on import by using an import hook to write new pyc files. Most of the time this works transparently. However, if you are working with the import machinery yourself, the import hook may interfere.

If this is the case you have two options:

  • Disable rewriting for a specific module by adding the string PYTEST_DONT_REWRITE to its docstring.

  • Disable rewriting for all modules by using --assert=plain.