The writing and reporting of 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:

$ py.test
=========================== test session starts ============================
platform linux -- Python 3.4.1 -- py-1.4.27 -- pytest-2.7.1
rootdir: /tmp/doc-exec-87, inifile:
collected 1 items F

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

    def test_function():
>       assert f() == 4
E       assert 3 == 4
E        +  where 3 = f() AssertionError
========================= 1 failed in 0.01 seconds =========================

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 Advanced assertion introspection 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 a py.code.ExceptionInfo instance, which is a wrapper around the actual exception raised. The main attributes of interest are .type, .value and .traceback.

If you want to write test code that works on Python 2.4 as well, you may also use two other ways to test for an expected exception:

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

both of which execute the specified function with args and kwargs and asserts that the given ExpectedException is raised. 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.

Making use of context-sensitive comparisons

New in version 2.0.

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:

$ py.test
=========================== test session starts ============================
platform linux -- Python 3.4.1 -- py-1.4.27 -- pytest-2.7.1
rootdir: /tmp/doc-exec-87, inifile:
collected 1 items F

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

    def test_set_comparison():
        set1 = set("1308")
        set2 = set("8035")
>       assert set1 == set2
E       assert set(['0', '1', '3', '8']) == set(['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 the full diff AssertionError
========================= 1 failed in 0.01 seconds =========================

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 assertion comparison

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 sligthly, the intention is for the first line to be a summary.

As an example consider adding the following hook in a 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:',
               '   vals: %s != %s' % (left.val, right.val)]

now, given this test module:

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

$ py.test -q
================================= 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
1 failed in 0.01 seconds

Advanced assertion introspection

New in version 2.1.

Reporting details about a failing assertion is achieved either by rewriting assert statements before they are run or re-evaluating the assert expression and recording the intermediate values. Which technique is used depends on the location of the assert, pytest configuration, and Python version being used to run pytest. Note that for assert statements with a manually provided message, i.e. assert expr, message, no assertion introspection takes place and the manually provided message will be rendered in tracebacks.

By default, if the Python version is greater than or equal to 2.6, pytest rewrites assert statements in test modules. 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.


pytest rewrites test modules on import. It does this by using an import hook to write a new pyc files. Most of the time this works transparently. However, if you are messing with import yourself, the import hook may interfere. If this is the case, simply use --assert=reinterp or --assert=plain. Additionally, rewriting will fail silently if it cannot write new pycs, i.e. in a read-only filesystem or a zipfile.

If an assert statement has not been rewritten or the Python version is less than 2.6, pytest falls back on assert reinterpretation. In assert reinterpretation, pytest walks the frame of the function containing the assert statement to discover sub-expression results of the failing assert statement. You can force pytest to always use assertion reinterpretation by passing the --assert=reinterp option.

Assert reinterpretation has a caveat not present with assert rewriting: If evaluating the assert expression has side effects you may get a warning that the intermediate values could not be determined safely. A common example of this issue is an assertion which reads from a file:

assert != '...'

If this assertion fails then the re-evaluation will probably succeed! This is because will return an empty string when it is called the second time during the re-evaluation. However, it is easy to rewrite the assertion and avoid any trouble:

content =
assert content != '...'

All assert introspection can be turned off by passing --assert=plain.

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

New in version 2.1: Add assert rewriting as an alternate introspection technique.

Changed in version 2.1: Introduce the --assert option. Deprecate --no-assert and --nomagic.