03 PyTest Write and Report

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:

In [ ]:

# content of test_assert1.py
def f():
    return 3


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

# content of test_assert1.py 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_assert1.py

• pytest has support for showing the values of the most common subexpressions including calls, attributes, comparisons, and binary and unary operators.
• 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: (Never do it!)
  • 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.

Assertions about expected exceptions¶

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

In [1]:

import pytest

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

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:

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

In [2]:

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

        def f():
            f()

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

def test_recursion_depth(): with pytest.raises(RuntimeError) as excinfo: def f(): f() f() assert "maximum recursion" in str(excinfo.value)