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Unit Tests with __name__

In fact, we’ve already seen a prime example in this book of an instance where the __name__ check could be useful. In the section on arguments in Chapter 18, we coded a script that computed the minimum value from the set of arguments sent in:

def minmax(test, *args):
    res = args[0]
    for arg in args[1:]:
        if test(arg, res):
            res = arg
    return res

def lessthan(x, y): return x < y
def grtrthan(x, y): return x > y

print(minmax(lessthan, 4, 2, 1, 5, 6, 3))        # Self-test code
print(minmax(grtrthan, 4, 2, 1, 5, 6, 3))

This script includes self-test code at the bottom, so we can test it without having to retype everything at the interactive command line each time we run it. The problem with the way it is currently coded, however, is that the output of the self-test call will appear every time this file is imported from another file to be used as a tool—not exactly a user-friendly feature! To improve it, we can wrap up the self-test call in a __name__ check, so that it will be launched only when the file is run as a top-level script, not when it is imported:

print('I am:', __name__)

def minmax(test, *args):
    res = args[0]
    for arg in args[1:]:
        if test(arg, res):
            res = arg
    return res

def lessthan(x, y): return x < y
def grtrthan(x, y): return x > y

if __name__ == '__main__':
    print(minmax(lessthan, 4, 2, 1, 5, 6, 3))    # Self-test code
    print(minmax(grtrthan, 4, 2, 1, 5, 6, 3))

We’re also printing the value of __name__ at the top here to trace its value. Python creates and assigns this usage-mode variable as soon as it starts loading a file. When we run this file as a top-level script, its name is set to __main__, so its self-test code kicks in automatically:

% python min.py
I am: __main__
1
6

But, if we import the file, its name is not __main__, so we must explicitly call the function to make it run:

>>> import min
I am: min
>>> min.minmax(min.lessthan, 's', 'p', 'a', 'm')
'a'

Again, regardless of whether this is used for testing, the net effect is that we get to use our code in two different roles—as a library module of tools, or as an executable program.