Example: Trapping Constraints (but Not Errors!)

Assertions are typically used to verify program conditions during development. When displayed, their error message text automatically includes source code line information and the value listed in the assert statement. Consider the file asserter.py:

def f(x):
    assert x < 0, 'x must be negative'
    return x ** 2

% python
>>> import asserter
>>> asserter.f(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "asserter.py", line 2, in f
    assert x < 0, 'x must be negative'
AssertionError: x must be negative

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It’s important to keep in mind that assert is mostly intended for trapping user-defined constraints, not for catching genuine programming errors. Because Python traps programming errors itself, there is usually no need to code asserts to catch things like out-of-bounds indexes, type mismatches, and zero divides:

def reciprocal(x):
    assert x != 0              # A useless assert!
    return 1 / x               # Python checks for zero automatically

Such asserts are generally superfluous—because Python raises exceptions on errors automatically, you might as well let it do the job for you.[76] For another example of common assert usage, see the abstract superclass example in Chapter 28; there, we used assert to make calls to undefined methods fail with a message.

 

[76] In most cases, at least. As suggested earlier in the book, if a function has to perform long-running or unrecoverable actions before it reaches the place where an exception will be triggered, you still might want to test for errors. Even in this case, though, be careful not to make your tests overly specific or restrictive, or you will limit your code’s utility.