5.2.7 Warnings

  1. doctest is serious about requiring exact matches in expected output. If even a single character doesn't match, the test fails. This will probably surprise you a few times, as you learn exactly what Python does and doesn't guarantee about output. For example, when printing a dict, Python doesn't guarantee that the key-value pairs will be printed in any particular order, so a test like

    >>> foo()
    {"Hermione": "hippogryph", "Harry": "broomstick"}

    is vulnerable! One workaround is to do

    >>> foo() == {"Hermione": "hippogryph", "Harry": "broomstick"}

    instead. Another is to do

    >>> d = foo().items()
    >>> d.sort()
    >>> d
    [('Harry', 'broomstick'), ('Hermione', 'hippogryph')]

    There are others, but you get the idea.

    Another bad idea is to print things that embed an object address, like

    >>> id(1.0) # certain to fail some of the time

    Floating-point numbers are also subject to small output variations across platforms, because Python defers to the platform C library for float formatting, and C libraries vary widely in quality here.

    >>> 1./7  # risky
    >>> print 1./7 # safer
    >>> print round(1./7, 6) # much safer

    Numbers of the form I/2.**J are safe across all platforms, and I often contrive doctest examples to produce numbers of that form:

    >>> 3./4  # utterly safe

    Simple fractions are also easier for people to understand, and that makes for better documentation.

  2. Be careful if you have code that must only execute once.

    If you have module-level code that must only execute once, a more foolproof definition of _test() is

    def _test():
        import doctest, sys

  3. WYSIWYG isn't always the case, starting in Python 2.3. The string form of boolean results changed from '0' and '1' to 'False' and 'True' in Python 2.3. This makes it clumsy to write a doctest showing boolean results that passes under multiple versions of Python. In Python 2.3, by default, and as a special case, if an expected output block consists solely of '0' and the actual output block consists solely of 'False', that's accepted as an exact match, and similarly for '1' versus 'True'. This behavior can be turned off by passing the new (in 2.3) module constant DONT_ACCEPT_TRUE_FOR_1 as the value of testmod()'s new (in 2.3) optional optionflags argument. Some years after the integer spellings of booleans are history, this hack will probably be removed again.

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