We created a suite of tests for our mean function, but it was annoying to run them one at a time. It would be a lot better if there were some way to run them all at once, just reporting which tests fail and which succeed.
Thankfully, that exists. Recall our tests are in a file called test_mean.py:
from mean import *
def test_ints():
num_list = [1,2,3,4,5]
obs = mean(num_list)
exp = 3
assert obs == exp
def test_zero():
num_list=[0,2,4,6]
obs = mean(num_list)
exp = 3
assert obs == exp
def test_double():
# This one will fail in Python 2
num_list=[1,2,3,4]
obs = mean(num_list)
exp = 2.5
assert obs == exp
def test_long():
big = 100000000
obs = mean(range(1,big))
exp = big/2.0
assert obs == exp
def test_complex():
# given that complex numbers are an unordered field
# the arithmetic mean of complex numbers is meaningless
num_list = [2 + 3j, 3 + 4j, -32 - 2j]
obs = mean(num_list)
exp = NotImplemented
assert obs == exp
The framework we will use to run our test suite is pytest which can be run from the directory containing the tests:
%run pytest
================================================ test session starts ================================================
platform linux -- Python 3.6.3, pytest-3.2.1, py-1.4.34, pluggy-0.4.0
rootdir: /home/rjg20/training/arc-training/intro-testing, inifile:
collected 5 items
test_mean.py ....F
===================================================== FAILURES ======================================================
___________________________________________________ test_complex ____________________________________________________
def test_complex():
# given that complex numbers are an unordered field
# the arithmetic mean of complex numbers is meaningless
num_list = [2 + 3j, 3 + 4j, -32 - 2j]
> obs = mean(num_list)
test_mean.py:32:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
sample = [(2+3j), (3+4j), (-32-2j)]
def mean(sample):
'''
Takes a list of numbers, sample
and returns the mean.
'''
assert len(sample) != 0, "Unable to take the mean of an empty list"
for value in sample:
> assert isinstance(value,int) or isinstance(value,float), "Value in list is not a number."
E AssertionError: Value in list is not a number.
mean.py:12: AssertionError
======================================== 1 failed, 4 passed in 14.90 seconds ========================================
In the above case, the pytest package ‘sniffed-out’ the tests in the directory and ran them together to produce a report of the sum of the files and functions matching the regular expression [Tt]est[-_]*.
The major boon a testing framework provides is exactly that, a utility to find and run the tests automatically. With pytest, this is the command-line tool called pytest. When pytest is run, it will search all directories below where it was called, find all of the Python files in these directories whose names start or end with test, import them, and run all of the functions and classes whose names start with test or Test. This automatic registration of test code saves tons of human time and allows us to focus on what is important: writing more tests (and the code to pass them!).
When you run pytest, it will print a dot (.) on the screen for every test that passes, an F for every test that fails or where there was an unexpected error. In rarer situations you may also see an s indicating a skipped tests (because the test is not applicable on your system) or a x for a known failure (because the developers could not fix it promptly). After the dots, pytest will print summary information.
Without changing the tests, alter the mean.py file from the previous section until it passes. When it passes, py.test will produce results like the following:
%run pytest
collected 5 items
test_mean.py .....
========================== 5 passed in 2.68 seconds ===========================
Also try running pytest with the useful flags -s, -v, and -x. Check the pytest manual to see what each of these do if you cannot determine it from their behaviour.
As we write more code, we would write more tests, and pytest would produce more dots. Each passing test is a small, satisfying reward for having written quality scientific software. Now that you know how to write tests, let’s go into what can go wrong.