cpython/Doc/lib/libdoctest.tex

\section{\module{doctest} ---
         Test docstrings represent reality}

\declaremodule{standard}{doctest}
\moduleauthor{Tim Peters}{tim_one@users.sourceforge.net}
\sectionauthor{Tim Peters}{tim_one@users.sourceforge.net}
\sectionauthor{Moshe Zadka}{moshez@debian.org}

\modulesynopsis{A framework for verifying examples in docstrings.}

The \module{doctest} module searches a module's docstrings for text that looks
like an interactive Python session, then executes all such sessions to verify
they still work exactly as shown.  Here's a complete but small example:

\begin{verbatim}
"""
This is module example.

Example supplies one function, factorial.  For example,

>>> factorial(5)
120
"""

def factorial(n):
    """Return the factorial of n, an exact integer >= 0.

    If the result is small enough to fit in an int, return an int.
    Else return a long.

    >>> [factorial(n) for n in range(6)]
    [1, 1, 2, 6, 24, 120]
    >>> [factorial(long(n)) for n in range(6)]
    [1, 1, 2, 6, 24, 120]
    >>> factorial(30)
    265252859812191058636308480000000L
    >>> factorial(30L)
    265252859812191058636308480000000L
    >>> factorial(-1)
    Traceback (most recent call last):
        ...
    ValueError: n must be >= 0

    Factorials of floats are OK, but the float must be an exact integer:
    >>> factorial(30.1)
    Traceback (most recent call last):
        ...
    ValueError: n must be exact integer
    >>> factorial(30.0)
    265252859812191058636308480000000L

    It must also not be ridiculously large:
    >>> factorial(1e100)
    Traceback (most recent call last):
        ...
    OverflowError: n too large
    """

\end{verbatim}
% allow LaTeX to break here.
\begin{verbatim}

    import math
    if not n >= 0:
        raise ValueError("n must be >= 0")
    if math.floor(n) != n:
        raise ValueError("n must be exact integer")
    if n+1 == n:  # catch a value like 1e300
        raise OverflowError("n too large")
    result = 1
    factor = 2
    while factor <= n:
        try:
            result *= factor
        except OverflowError:
            result *= long(factor)
        factor += 1
    return result

def _test():
    import doctest
    return doctest.testmod()

if __name__ == "__main__":
    _test()
\end{verbatim}

If you run \file{example.py} directly from the command line,
\module{doctest} works its magic:

\begin{verbatim}
$ python example.py
$
\end{verbatim}

There's no output!  That's normal, and it means all the examples
worked.  Pass \programopt{-v} to the script, and \module{doctest}
prints a detailed log of what it's trying, and prints a summary at the
end:

\begin{verbatim}
$ python example.py -v
Trying: factorial(5)
Expecting: 120
ok
Trying: [factorial(n) for n in range(6)]
Expecting: [1, 1, 2, 6, 24, 120]
ok
Trying: [factorial(long(n)) for n in range(6)]
Expecting: [1, 1, 2, 6, 24, 120]
ok\end{verbatim}

And so on, eventually ending with:

\begin{verbatim}
Trying: factorial(1e100)
Expecting:
    Traceback (most recent call last):
        ...
    OverflowError: n too large
ok
2 items passed all tests:
   1 tests in example
   8 tests in example.factorial
9 tests in 2 items.
9 passed and 0 failed.
Test passed.
$
\end{verbatim}

That's all you need to know to start making productive use of
\module{doctest}!  Jump in.

\subsection{Simple Usage}

The simplest (not necessarily the best) way to start using doctest is to
end each module \module{M} with:

\begin{verbatim}
def _test():
    import doctest
    return doctest.testmod()

if __name__ == "__main__":
    _test()
\end{verbatim}

\module{doctest} then examines docstrings in the module calling
\function{testmod()}.  If you want to test a different module, you can
pass that module object to \function{testmod()}.

Running the module as a script causes the examples in the docstrings
to get executed and verified:

\begin{verbatim}
python M.py
\end{verbatim}

This won't display anything unless an example fails, in which case the
failing example(s) and the cause(s) of the failure(s) are printed to stdout,
and the final line of output is
\\code{'***Test Failed*** \var{N} failures.'}, where \var{N} is the
number of examples that failed.

Run it with the \programopt{-v} switch instead:

\begin{verbatim}
python M.py -v
\end{verbatim}

and a detailed report of all examples tried is printed to standard
output, along with assorted summaries at the end.

You can force verbose mode by passing \code{verbose=True} to
\function{testmod()}, or
prohibit it by passing \code{verbose=False}.  In either of those cases,
\code{sys.argv} is not examined by \function{testmod()}.

In any case, \function{testmod()} returns a 2-tuple of ints \code{(\var{f},
\var{t})}, where \var{f} is the number of docstring examples that
failed and \var{t} is the total number of docstring examples
attempted.

\begin{funcdesc}{testmod}{\optional{m}\optional{, name}\optional{,
                          globs}\optional{, verbose}\optional{,
                          isprivate}\optional{, report}\optional{,
                          optionflags}\optional{, extraglobs}\optional{,
                          raise_on_error}}

  All arguments are optional, and all except for \var{m} should be
  specified in keyword form.

  Test examples in docstrings in functions and classes reachable
  from module \var{m} (or the current module if \var{m} is not supplied
  or is \code{None}), starting with \code{\var{m}.__doc__}.

  Also test examples reachable from dict \code{\var{m}.__test__}, if it
  exists and is not \code{None}.  \code{\var{m}.__test__} maps
  names (strings) to functions, classes and strings; function and class
  docstrings are searched for examples; strings are searched directly,
  as if they were docstrings.

  Only docstrings attached to objects belonging to module \var{m} are
  searched.

  Return \code{(\var{failure_count}, \var{test_count})}.

  Optional argument \var{name} gives the name of the module; by default,
  or if \code{None}, \code{\var{m}.__name__} is used.

  Optional argument \var{globs} gives a dict to be used as the globals
  when executing examples; by default, or if \code{None},
  \code{\var{m}.__dict__} is used.  A new shallow copy of this dict is
  created for each docstring with examples, so that each docstring's
  examples start with a clean slate.

  Optional argument \var{extraglobs} gives a dicti merged into the
  globals used to execute examples.  This works like
  \method{dict.update()}:  if \var{globs} and \var{extraglobs} have a
  common key, the associated value in \var{extraglobs} appears in the
  combined dict.  By default, or if \code{None}, no extra globals are
  used.  This is an advanced feature that allows parameterization of
  doctests.  For example, a doctest can be written for a base class, using
  a generic name for the class, then reused to test any number of
  subclasses by passing an \var{extraglobs} dict mapping the generic
  name to the subclass to be tested.

  Optional argument \var{verbose} prints lots of stuff if true, and prints
  only failures if false; by default, or if \code{None}, it's true
  if and only if \code{'-v'} is in \code{\module{sys}.argv}.

  Optional argument \var{report} prints a summary at the end when true,
  else prints nothing at the end.  In verbose mode, the summary is
  detailed, else the summary is very brief (in fact, empty if all tests
  passed).

  Optional argument \var{optionflags} or's together module constants,
  and defaults to 0.

%  Possible values:
%
%      DONT_ACCEPT_TRUE_FOR_1
%          By default, if an expected output block contains just "1",
%          an actual output block containing just "True" is considered
%          to be a match, and similarly for "0" versus "False".  When
%          DONT_ACCEPT_TRUE_FOR_1 is specified, neither substitution
%          is allowed.
%
%      DONT_ACCEPT_BLANKLINE
%          By default, if an expected output block contains a line
%          containing only the string "<BLANKLINE>", then that line
%          will match a blank line in the actual output.  When
%          DONT_ACCEPT_BLANKLINE is specified, this substitution is
%          not allowed.
%
%      NORMALIZE_WHITESPACE
%          When NORMALIZE_WHITESPACE is specified, all sequences of
%          whitespace are treated as equal.  I.e., any sequence of
%          whitespace within the expected output will match any
%          sequence of whitespace within the actual output.
%
%      ELLIPSIS
%          When ELLIPSIS is specified, then an ellipsis marker
%          ("...") in the expected output can match any substring in
%          the actual output.
%
%      UNIFIED_DIFF
%          When UNIFIED_DIFF is specified, failures that involve
%          multi-line expected and actual outputs will be displayed
%          using a unified diff.
%
%      CONTEXT_DIFF
%          When CONTEXT_DIFF is specified, failures that involve
%          multi-line expected and actual outputs will be displayed
%          using a context diff.

  Optional argument \var{raise_on_error} defaults to false.  If true,
  an exception is raised upon the first failure or unexpected exception
  in an example.  This allows failures to be post-mortem debugged.
  Default behavior is to continue running examples.

  Optional argument \var{isprivate} specifies a function used to
  determine whether a name is private.  The default function treats
  all names as public.  \var{isprivate} can be set to
  \code{\module{doctest}.is_private} to skip over names that are
  private according to Python's underscore naming convention.
  \deprecated{2.4}{\var{isprivate} was a stupid idea -- don't use it.
  If you need to skip tests based on name, filter the list returned by
  \code{DocTestFinder.find()} instead.}

%  """ [XX] This is no longer true:
%  Advanced tomfoolery:  testmod runs methods of a local instance of
%  class doctest.Tester, then merges the results into (or creates)
%  global Tester instance doctest.master.  Methods of doctest.master
%  can be called directly too, if you want to do something unusual.
%  Passing report=0 to testmod is especially useful then, to delay
%  displaying a summary.  Invoke doctest.master.summarize(verbose)
%  when you're done fiddling.

  \versionchanged[The parameter \var{optionflags} was added]{2.3}

  \versionchanged[Many new module constants for use with \var{optionflags}
                  were added]{2.4}

  \versionchanged[The parameters \var{extraglobs} and \var{raise_on_error}
                  were added]{2.4}
\end{funcdesc}


\subsection{Which Docstrings Are Examined?}

See the docstrings in \file{doctest.py} for all the details.  They're
unsurprising: the module docstring, and all function, class and method
docstrings are searched.  Optionally, the tester can be directed to
exclude docstrings attached to objects with private names.  Objects
imported into the module are not searched.

In addition, if \code{M.__test__} exists and "is true", it must be a
dict, and each entry maps a (string) name to a function object, class
object, or string.  Function and class object docstrings found from
\code{M.__test__} are searched even if the tester has been
directed to skip over private names in the rest of the module.
In output, a key \code{K} in \code{M.__test__} appears with name

\begin{verbatim}
<name of M>.__test__.K
\end{verbatim}

Any classes found are recursively searched similarly, to test docstrings in
their contained methods and nested classes.  While private names reached
from \module{M}'s globals can be optionally skipped, all names reached from
\code{M.__test__} are searched.

\subsection{What's the Execution Context?}

By default, each time \function{testmod()} finds a docstring to test, it uses
a \emph{copy} of \module{M}'s globals, so that running tests on a module
doesn't change the module's real globals, and so that one test in
\module{M} can't leave behind crumbs that accidentally allow another test
to work.  This means examples can freely use any names defined at top-level
in \module{M}, and names defined earlier in the docstring being run.

You can force use of your own dict as the execution context by passing
\code{globs=your_dict} to \function{testmod()} instead.  Presumably this
would be a copy of \code{M.__dict__} merged with the globals from other
imported modules.

\subsection{What About Exceptions?}

No problem, as long as the only output generated by the example is the
traceback itself.  For example:

\begin{verbatim}
>>> [1, 2, 3].remove(42)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: list.remove(x): x not in list
>>>
\end{verbatim}

Note that only the exception type and value are compared (specifically,
only the last line in the traceback).  The various ``File'' lines in
between can be left out (unless they add significantly to the documentation
value of the example).

\subsection{Advanced Usage}

Several module level functions are available for controlling how doctests
are run.

\begin{funcdesc}{debug}{module, name}
  Debug a single docstring containing doctests.

  Provide the \var{module} (or dotted name of the module) containing the
  docstring to be debugged and the \var{name} (within the module) of the
  object with the docstring to be debugged.

  The doctest examples are extracted (see function \function{testsource()}),
  and written to a temporary file.  The Python debugger, \refmodule{pdb},
  is then invoked on that file.
  \versionadded{2.3}
\end{funcdesc}

\begin{funcdesc}{testmod}{}
  This function provides the most basic interface to the doctests.
  It creates a local instance of class \class{Tester}, runs appropriate
  methods of that class, and merges the results into the global \class{Tester}
  instance, \code{master}.

  To get finer control than \function{testmod()} offers, create an instance
  of \class{Tester} with custom policies, or run methods of \code{master}
  directly.  See \code{Tester.__doc__} for details.
\end{funcdesc}

\begin{funcdesc}{testsource}{module, name}
  Extract the doctest examples from a docstring.

  Provide the \var{module} (or dotted name of the module) containing the
  tests to be extracted and the \var{name} (within the module) of the object
  with the docstring containing the tests to be extracted.

  The doctest examples are returned as a string containing Python
  code.  The expected output blocks in the examples are converted
  to Python comments.
  \versionadded{2.3}
\end{funcdesc}

\begin{funcdesc}{DocTestSuite}{\optional{module}}
  Convert doctest tests for a module to a
  \class{\refmodule{unittest}.TestSuite}.

  The returned \class{TestSuite} is to be run by the unittest framework
  and runs each doctest in the module.  If any of the doctests fail,
  then the synthesized unit test fails, and a \exception{DocTestTestFailure}
  exception is raised showing the name of the file containing the test and a
  (sometimes approximate) line number.

  The optional \var{module} argument provides the module to be tested.  It
  can be a module object or a (possibly dotted) module name.  If not
  specified, the module calling this function is used.

  Example using one of the many ways that the \refmodule{unittest} module
  can use a \class{TestSuite}:

  \begin{verbatim}
    import unittest
    import doctest
    import my_module_with_doctests

    suite = doctest.DocTestSuite(my_module_with_doctests)
    runner = unittest.TextTestRunner()
    runner.run(suite)
  \end{verbatim}

  \versionadded{2.3}
  \warning{This function does not currently search \code{M.__test__}
  and its search technique does not exactly match \function{testmod()} in
  every detail.  Future versions will bring the two into convergence.}
\end{funcdesc}


\subsection{How are Docstring Examples Recognized?}

In most cases a copy-and-paste of an interactive console session works
fine---just make sure the leading whitespace is rigidly consistent
(you can mix tabs and spaces if you're too lazy to do it right, but
\module{doctest} is not in the business of guessing what you think a tab
means).

\begin{verbatim}
>>> # comments are ignored
>>> x = 12
>>> x
12
>>> if x == 13:
...     print "yes"
... else:
...     print "no"
...     print "NO"
...     print "NO!!!"
...
no
NO
NO!!!
>>>
\end{verbatim}

Any expected output must immediately follow the final
\code{'>\code{>}>~'} or \code{'...~'} line containing the code, and
the expected output (if any) extends to the next \code{'>\code{>}>~'}
or all-whitespace line.

The fine print:

\begin{itemize}

\item Expected output cannot contain an all-whitespace line, since such a
  line is taken to signal the end of expected output.

\item Output to stdout is captured, but not output to stderr (exception
  tracebacks are captured via a different means).

\item If you continue a line via backslashing in an interactive session,
  or for any other reason use a backslash, you should use a raw
  docstring, which will preserve your backslahses exactly as you type
  them:

\begin{verbatim}
>>> def f(x):
...     r'''Backslashes in a raw docstring: m\n'''
>>> print f.__doc__
Backslashes in a raw docstring: m\n
\end{verbatim}

  Otherwise, the backslash will be interpreted as part of the string.
  E.g., the "\textbackslash" above would be interpreted as a newline
  character.  Alternatively, you can double each backslash in the
  doctest version (and not use a raw string):

\begin{verbatim}
>>> def f(x):
...     '''Backslashes in a raw docstring: m\\n'''
>>> print f.__doc__
Backslashes in a raw docstring: m\n
\end{verbatim}

\item The starting column doesn't matter:

\begin{verbatim}
  >>> assert "Easy!"
        >>> import math
            >>> math.floor(1.9)
            1.0
\end{verbatim}

and as many leading whitespace characters are stripped from the
expected output as appeared in the initial \code{'>\code{>}>~'} line
that triggered it.
\end{itemize}

\subsection{Warnings}

\begin{enumerate}

\item \module{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

% Hey! What happened to Monty Python examples?
% Tim: ask Guido -- it's his example!
\begin{verbatim}
>>> foo()
{"Hermione": "hippogryph", "Harry": "broomstick"}
>>>
\end{verbatim}

is vulnerable!  One workaround is to do

\begin{verbatim}
>>> foo() == {"Hermione": "hippogryph", "Harry": "broomstick"}
True
>>>
\end{verbatim}

instead.  Another is to do

\begin{verbatim}
>>> d = foo().items()
>>> d.sort()
>>> d
[('Harry', 'broomstick'), ('Hermione', 'hippogryph')]
\end{verbatim}

There are others, but you get the idea.

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

\begin{verbatim}
>>> id(1.0) # certain to fail some of the time
7948648
>>>
\end{verbatim}

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.

\begin{verbatim}
>>> 1./7  # risky
0.14285714285714285
>>> print 1./7 # safer
0.142857142857
>>> print round(1./7, 6) # much safer
0.142857
\end{verbatim}

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

\begin{verbatim}
>>> 3./4  # utterly safe
0.75
\end{verbatim}

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

\item 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 \function{_test()} is

\begin{verbatim}
def _test():
    import doctest, sys
    doctest.testmod()
\end{verbatim}

\item WYSIWYG isn't always the case, starting in Python 2.3.  The
  string form of boolean results changed from \code{'0'} and
  \code{'1'} to \code{'False'} and \code{'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 \code{'0'} and the actual output block consists solely of
  \code{'False'}, that's accepted as an exact match, and similarly for
  \code{'1'} versus \code{'True'}.  This behavior can be turned off by
  passing the new (in 2.3) module constant
  \constant{DONT_ACCEPT_TRUE_FOR_1} as the value of \function{testmod()}'s
  new (in 2.3) optional \var{optionflags} argument.  Some years after
  the integer spellings of booleans are history, this hack will
  probably be removed again.

\end{enumerate}


\subsection{Soapbox}

The first word in ``doctest'' is ``doc,'' and that's why the author
wrote \refmodule{doctest}: to keep documentation up to date.  It so
happens that \refmodule{doctest} makes a pleasant unit testing
environment, but that's not its primary purpose.

Choose docstring examples with care.  There's an art to this that
needs to be learned---it may not be natural at first.  Examples should
add genuine value to the documentation.  A good example can often be
worth many words.  If possible, show just a few normal cases, show
endcases, show interesting subtle cases, and show an example of each
kind of exception that can be raised.  You're probably testing for
endcases and subtle cases anyway in an interactive shell:
\refmodule{doctest} wants to make it as easy as possible to capture
those sessions, and will verify they continue to work as designed
forever after.

If done with care, the examples will be invaluable for your users, and
will pay back the time it takes to collect them many times over as the
years go by and things change.  I'm still amazed at how often one of
my \refmodule{doctest} examples stops working after a ``harmless''
change.

For exhaustive testing, or testing boring cases that add no value to the
docs, define a \code{__test__} dict instead.  That's what it's for.