Can I handle multiple asserts within a single Python pytest method?

Question

I am writing tests with pytest, and I ran into a problem: I have a test which testing some variable, and then I perform some heavy calculation, and after that I want to perform another test.

The problem is that if the first assert fails, the whole test fails, and pytest does not perform the second test. The code:

class TestSomething:
    def tests_method(self, some_variables):
        # Some actions that take a lot of time!
        assert some_var == 1
        # Some actions that take a lot of time!
        assert some_var == 2

I'm aware that this test method can be separated into 2 methods, but the performance issue here is crucial.

Is there a way I can run 2 asserts in one method?

Answer 1

Usually, I just let the test fail on the first assertion. However, if you really want to do more than one comparison, compare tuples. Here's a simple example:

def foo(x):
    return x + 1


def bar(y):
    return y - 1


def test_foo():
    # some expensive calculation                                                                                                                                    
    a = foo(10)

    # another expensive calculation                                                                                                                                 
    b = bar(10)

    assert (a, b) == (10, 9)

When I run that with pytest, it shows me both values:

$ pytest scratch.py
============================= test session starts =============================
platform linux2 -- Python 2.7.12, pytest-3.0.7, py-1.4.33, pluggy-0.4.0
rootdir: /home/don/workspace/scratch, inifile:
collected 1 items

scratch.py F

================================== FAILURES ===================================
__________________________________ test_foo ___________________________________

def test_foo():
# some expensive calculation
a = foo(10)

# another expensive calculation
b = bar(10)

>       assert (a, b) == (10, 9)
E       assert (11, 9) == (10, 9)
E         At index 0 diff: 11 != 10
E         Use -v to get the full diff

scratch.py:16: AssertionError
========================== 1 failed in 0.02 seconds ===========================

I also tried using and to combine comparisons, but that doesn't work because of short circuiting. For example, I tried this assertion:

assert a == 10 and b == 9

Pytest reported this failure:

>       assert a == 10 and b == 9
E       assert (11 == 10)

It doesn't report the value of b unless you use the --showlocals option.