Test function appends NONE during end of unittest

Question

I've been working on some code to practice unit testing. The original is supposed to get all file names in a directory, and list the number of files with the same extension. When I run the same function using unittest, the test appends a None at the end, breaking the test.

#!/usr/local/bin/python3

import os
from glob import glob
from collections import Counter

directory = os.getcwd()
filetype = "*.*"

def lookUp(directory, filetype):
    """Returns filename in the current directory"""
    files = [os.path.basename(i) for i in
            glob(os.path.join(directory,filetype))]
    countExt = Counter([os.path.splitext(i)[1] for i in files])
    for i in countExt:
        print("There are %d file(s) with the %s extension" %
        (countExt[i], i))

returns this output:

There are 3 file(s) with the .html extension
There are 1 file(s) with the .txt extension
There are 2 file(s) with the .py extension
There are 3 file(s) with the .doc extension

and my unittest code:

#!/usr/local/bin/python3

import unittest
import FileHandling
import os
import tempfile
from glob import glob



class TestFileHandling(unittest.TestCase): #Defines TestHandling class

    def setUp(self): # define seUp function
        self.testdir = os.getcwd()
        #self.testdir = tempfile.mkdtemp("testdir") # creates a test directory
        os.chdir(self.testdir) # changes to test directory
        self.file_names = ["file1.txt", "file1.doc", "file2.doc", "file3.doc", "file1.html", "file2.html", "file3.html"] # name seven filenames
        for fn in self.file_names: # creates files
            joinedfile = os.path.join(self.testdir, fn) #joins the filename with the temp directory name
            f = open(joinedfile, "w") # create the file
            f.close() # close the file from writing



    def test_lookUp_text(self): # test function for lookUp
        print(FileHandling.lookUp(self.testdir, "*.*"))
        #print(os.getcwd())
        #self.assertEqual(files, expected,)



def tearDown(self):
    for fn in self.file_names:
        os.remove(joinedfile)

if __name__ == "__main__":
    unittest.main()

returns this output:

There are 2 file(s) with the .py extension
There are 3 file(s) with the .doc extension
There are 1 file(s) with the .txt extension
There are 3 file(s) with the .html extension
None
.
----------------------------------------------------------------------
Ran 1 test in 0.016s

OK

Why is there an additional None output at the end of the unittest output?

Answer 1

Your function doesn't use a return statement, so Python returns None (the default). You are printing that value. Note that your test isn't broken by it, your test is actually not testing anything, only running the code.

Simply remove your print() call:

def test_lookUp_text(self):
    FileHandling.lookUp(self.testdir, "*.*")

You should consider changing your function to return the a report to print rather than print directly, then your unit test more easily can make assertions about the right output having been produced.

You should also consider learning how the unittest.mock library could help you here; you could mock out the os.listdir() call so you don't actually have to create test files, and instead leave it to your mocked os.listdir() call to return a pre-determined list of strings.