"doctest" --- Test interactive Python examples
**********************************************

**Source code:** Lib/doctest.py

======================================================================

The "doctest" module searches for pieces of text that look like
interactive Python sessions, and then executes those sessions to
verify that they work exactly as shown.  There are several common ways
to use doctest:

* To check that a module's docstrings are up-to-date by verifying
  that all interactive examples still work as documented.

* To perform regression testing by verifying that interactive
  examples from a test file or a test object work as expected.

* To write tutorial documentation for a package, liberally
  illustrated with input-output examples.  Depending on whether the
  examples or the expository text are emphasized, this has the flavor
  of "literate testing" or "executable documentation".

Here's a complete but small example module:

   """
   This is the "example" module.

   The example module supplies one function, factorial().  For example,

   >>> factorial(5)
   120
   """

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

       >>> [factorial(n) for n in range(6)]
       [1, 1, 2, 6, 24, 120]
       >>> factorial(30)
       265252859812191058636308480000000
       >>> 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)
       265252859812191058636308480000000

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

       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:
           result *= factor
           factor += 1
       return result


   if __name__ == "__main__":
       import doctest
       doctest.testmod()

If you run "example.py" directly from the command line, "doctest"
works its magic:

   $ python example.py
   $

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

   $ 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

And so on, eventually ending with:

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

That's all you need to know to start making productive use of
"doctest"! Jump in.  The following sections provide full details.
Note that there are many examples of doctests in the standard Python
test suite and libraries. Especially useful examples can be found in
the standard test file "Lib/test/test_doctest.py".


Simple Usage: Checking Examples in Docstrings
=============================================

The simplest way to start using doctest (but not necessarily the way
you'll continue to do it) is to end each module "M" with:

   if __name__ == "__main__":
       import doctest
       doctest.testmod()

"doctest" then examines docstrings in module "M".

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

   python M.py

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 "***Test Failed*** N
failures.", where *N* is the number of examples that failed.

Run it with the "-v" switch instead:

   python M.py -v

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 "verbose=True" to "testmod()",
or prohibit it by passing "verbose=False".  In either of those cases,
"sys.argv" is not examined by "testmod()" (so passing "-v" or not has
no effect).

There is also a command line shortcut for running "testmod()".  You
can instruct the Python interpreter to run the doctest module directly
from the standard library and pass the module name(s) on the command
line:

   python -m doctest -v example.py

This will import "example.py" as a standalone module and run
"testmod()" on it.  Note that this may not work correctly if the file
is part of a package and imports other submodules from that package.

For more information on "testmod()", see section Basic API.


Simple Usage: Checking Examples in a Text File
==============================================

Another simple application of doctest is testing interactive examples
in a text file.  This can be done with the "testfile()" function:

   import doctest
   doctest.testfile("example.txt")

That short script executes and verifies any interactive Python
examples contained in the file "example.txt".  The file content is
treated as if it were a single giant docstring; the file doesn't need
to contain a Python program!   For example, perhaps "example.txt"
contains this:

   The ``example`` module
   ======================

   Using ``factorial``
   -------------------

   This is an example text file in reStructuredText format.  First import
   ``factorial`` from the ``example`` module:

       >>> from example import factorial

   Now use it:

       >>> factorial(6)
       120

Running "doctest.testfile("example.txt")" then finds the error in this
documentation:

   File "./example.txt", line 14, in example.txt
   Failed example:
       factorial(6)
   Expected:
       120
   Got:
       720

As with "testmod()", "testfile()" won't display anything unless an
example fails.  If an example does fail, then the failing example(s)
and the cause(s) of the failure(s) are printed to stdout, using the
same format as "testmod()".

By default, "testfile()" looks for files in the calling module's
directory. See section Basic API for a description of the optional
arguments that can be used to tell it to look for files in other
locations.

Like "testmod()", "testfile()"'s verbosity can be set with the "-v"
command-line switch or with the optional keyword argument *verbose*.

There is also a command line shortcut for running "testfile()".  You
can instruct the Python interpreter to run the doctest module directly
from the standard library and pass the file name(s) on the command
line:

   python -m doctest -v example.txt

Because the file name does not end with ".py", "doctest" infers that
it must be run with "testfile()", not "testmod()".

For more information on "testfile()", see section Basic API.


How It Works
============

This section examines in detail how doctest works: which docstrings it
looks at, how it finds interactive examples, what execution context it
uses, how it handles exceptions, and how option flags can be used to
control its behavior. This is the information that you need to know to
write doctest examples; for information about actually running doctest
on these examples, see the following sections.


Which Docstrings Are Examined?
------------------------------

The module docstring, and all function, class and method docstrings
are searched.  Objects imported into the module are not searched.

In addition, if "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
"M.__test__" are searched, and strings are treated as if they were
docstrings.  In output, a key "K" in "M.__test__" appears with name

   <name of M>.__test__.K

Any classes found are recursively searched similarly, to test
docstrings in their contained methods and nested classes.

**CPython implementation detail:** Prior to version 3.4, extension
modules written in C were not fully searched by doctest.


How are Docstring Examples Recognized?
--------------------------------------

In most cases a copy-and-paste of an interactive console session works
fine, but doctest isn't trying to do an exact emulation of any
specific Python shell.

   >>> # comments are ignored
   >>> x = 12
   >>> x
   12
   >>> if x == 13:
   ...     print("yes")
   ... else:
   ...     print("no")
   ...     print("NO")
   ...     print("NO!!!")
   ...
   no
   NO
   NO!!!
   >>>

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

The fine print:

* Expected output cannot contain an all-whitespace line, since such
  a line is taken to signal the end of expected output.  If expected
  output does contain a blank line, put "<BLANKLINE>" in your doctest
  example each place a blank line is expected.

* All hard tab characters are expanded to spaces, using 8-column tab
  stops. Tabs in output generated by the tested code are not modified.
  Because any hard tabs in the sample output *are* expanded, this
  means that if the code output includes hard tabs, the only way the
  doctest can pass is if the "NORMALIZE_WHITESPACE" option or
  directive is in effect. Alternatively, the test can be rewritten to
  capture the output and compare it to an expected value as part of
  the test.  This handling of tabs in the source was arrived at
  through trial and error, and has proven to be the least error prone
  way of handling them.  It is possible to use a different algorithm
  for handling tabs by writing a custom "DocTestParser" class.

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

* 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 backslashes exactly as you type
  them:

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

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

     >>> def f(x):
     ...     '''Backslashes in a raw docstring: m\\n'''
     >>> print(f.__doc__)
     Backslashes in a raw docstring: m\n

* The starting column doesn't matter:

     >>> assert "Easy!"
           >>> import math
               >>> math.floor(1.9)
               1

  and as many leading whitespace characters are stripped from the
  expected output as appeared in the initial "'>>> '" line that
  started the example.


What's the Execution Context?
-----------------------------

By default, each time "doctest" finds a docstring to test, it uses a
*shallow copy* of "M"'s globals, so that running tests doesn't change
the module's real globals, and so that one test in "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 "M",
and names defined earlier in the docstring being run. Examples cannot
see names defined in other docstrings.

You can force use of your own dict as the execution context by passing
"globs=your_dict" to "testmod()" or "testfile()" instead.


What About Exceptions?
----------------------

No problem, provided that the traceback is the only output produced by
the example:  just paste in the traceback. [1] Since tracebacks
contain details that are likely to change rapidly (for example, exact
file paths and line numbers), this is one case where doctest works
hard to be flexible in what it accepts.

Simple example:

   >>> [1, 2, 3].remove(42)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   ValueError: list.remove(x): x not in list

That doctest succeeds if "ValueError" is raised, with the
"list.remove(x): x not in list" detail as shown.

The expected output for an exception must start with a traceback
header, which may be either of the following two lines, indented the
same as the first line of the example:

   Traceback (most recent call last):
   Traceback (innermost last):

The traceback header is followed by an optional traceback stack, whose
contents are ignored by doctest.  The traceback stack is typically
omitted, or copied verbatim from an interactive session.

The traceback stack is followed by the most interesting part: the
line(s) containing the exception type and detail.  This is usually the
last line of a traceback, but can extend across multiple lines if the
exception has a multi-line detail:

   >>> raise ValueError('multi\n    line\ndetail')
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   ValueError: multi
       line
   detail

The last three lines (starting with "ValueError") are compared against
the exception's type and detail, and the rest are ignored.

Best practice is to omit the traceback stack, unless it adds
significant documentation value to the example.  So the last example
is probably better as:

   >>> raise ValueError('multi\n    line\ndetail')
   Traceback (most recent call last):
       ...
   ValueError: multi
       line
   detail

Note that tracebacks are treated very specially.  In particular, in
the rewritten example, the use of "..." is independent of doctest's
"ELLIPSIS" option.  The ellipsis in that example could be left out, or
could just as well be three (or three hundred) commas or digits, or an
indented transcript of a Monty Python skit.

Some details you should read once, but won't need to remember:

* Doctest can't guess whether your expected output came from an
  exception traceback or from ordinary printing.  So, e.g., an example
  that expects "ValueError: 42 is prime" will pass whether
  "ValueError" is actually raised or if the example merely prints that
  traceback text.  In practice, ordinary output rarely begins with a
  traceback header line, so this doesn't create real problems.

* Each line of the traceback stack (if present) must be indented
  further than the first line of the example, *or* start with a non-
  alphanumeric character. The first line following the traceback
  header indented the same and starting with an alphanumeric is taken
  to be the start of the exception detail.  Of course this does the
  right thing for genuine tracebacks.

* When the "IGNORE_EXCEPTION_DETAIL" doctest option is specified,
  everything following the leftmost colon and any module information
  in the exception name is ignored.

* The interactive shell omits the traceback header line for some
  "SyntaxError"s.  But doctest uses the traceback header line to
  distinguish exceptions from non-exceptions.  So in the rare case
  where you need to test a "SyntaxError" that omits the traceback
  header, you will need to manually add the traceback header line to
  your test example.

* For some "SyntaxError"s, Python displays the character position of
  the syntax error, using a "^" marker:

     >>> 1 1
       File "<stdin>", line 1
         1 1
           ^
     SyntaxError: invalid syntax

  Since the lines showing the position of the error come before the
  exception type and detail, they are not checked by doctest.  For
  example, the following test would pass, even though it puts the "^"
  marker in the wrong location:

     >>> 1 1
     Traceback (most recent call last):
       File "<stdin>", line 1
         1 1
         ^
     SyntaxError: invalid syntax


Option Flags
------------

A number of option flags control various aspects of doctest's
behavior. Symbolic names for the flags are supplied as module
constants, which can be bitwise ORed together and passed to various
functions. The names can also be used in doctest directives, and may
be passed to the doctest command line interface via the "-o" option.

New in version 3.4: The "-o" command line option.

The first group of options define test semantics, controlling aspects
of how doctest decides whether actual output matches an example's
expected output:

doctest.DONT_ACCEPT_TRUE_FOR_1

   By default, if an expected output block contains just "1", an
   actual output block containing just "1" or 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.  The default behavior caters to that Python changed the
   return type of many functions from integer to boolean; doctests
   expecting "little integer" output still work in these cases.  This
   option will probably go away, but not for several years.

doctest.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.  Because a genuinely blank line delimits
   the expected output, this is the only way to communicate that a
   blank line is expected.  When "DONT_ACCEPT_BLANKLINE" is specified,
   this substitution is not allowed.

doctest.NORMALIZE_WHITESPACE

   When specified, all sequences of whitespace (blanks and newlines)
   are treated as equal.  Any sequence of whitespace within the
   expected output will match any sequence of whitespace within the
   actual output. By default, whitespace must match exactly.
   "NORMALIZE_WHITESPACE" is especially useful when a line of expected
   output is very long, and you want to wrap it across multiple lines
   in your source.

doctest.ELLIPSIS

   When specified, an ellipsis marker ("...") in the expected output
   can match any substring in the actual output.  This includes
   substrings that span line boundaries, and empty substrings, so it's
   best to keep usage of this simple. Complicated uses can lead to the
   same kinds of "oops, it matched too much!" surprises that ".*" is
   prone to in regular expressions.

doctest.IGNORE_EXCEPTION_DETAIL

   When specified, an example that expects an exception passes if an
   exception of the expected type is raised, even if the exception
   detail does not match.  For example, an example expecting
   "ValueError: 42" will pass if the actual exception raised is
   "ValueError: 3*14", but will fail, e.g., if "TypeError" is raised.

   It will also ignore the module name used in Python 3 doctest
   reports. Hence both of these variations will work with the flag
   specified, regardless of whether the test is run under Python 2.7
   or Python 3.2 (or later versions):

      >>> raise CustomError('message')
      Traceback (most recent call last):
      CustomError: message

      >>> raise CustomError('message')
      Traceback (most recent call last):
      my_module.CustomError: message

   Note that "ELLIPSIS" can also be used to ignore the details of the
   exception message, but such a test may still fail based on whether
   or not the module details are printed as part of the exception
   name. Using "IGNORE_EXCEPTION_DETAIL" and the details from Python
   2.3 is also the only clear way to write a doctest that doesn't care
   about the exception detail yet continues to pass under Python 2.3
   or earlier (those releases do not support doctest directives and
   ignore them as irrelevant comments). For example:

      >>> (1, 2)[3] = 'moo'
      Traceback (most recent call last):
        File "<stdin>", line 1, in <module>
      TypeError: object doesn't support item assignment

   passes under Python 2.3 and later Python versions with the flag
   specified, even though the detail changed in Python 2.4 to say
   "does not" instead of "doesn't".

   Changed in version 3.2: "IGNORE_EXCEPTION_DETAIL" now also ignores
   any information relating to the module containing the exception
   under test.

doctest.SKIP

   When specified, do not run the example at all.  This can be useful
   in contexts where doctest examples serve as both documentation and
   test cases, and an example should be included for documentation
   purposes, but should not be checked.  E.g., the example's output
   might be random; or the example might depend on resources which
   would be unavailable to the test driver.

   The SKIP flag can also be used for temporarily "commenting out"
   examples.

doctest.COMPARISON_FLAGS

   A bitmask or'ing together all the comparison flags above.

The second group of options controls how test failures are reported:

doctest.REPORT_UDIFF

   When specified, failures that involve multi-line expected and
   actual outputs are displayed using a unified diff.

doctest.REPORT_CDIFF

   When specified, failures that involve multi-line expected and
   actual outputs will be displayed using a context diff.

doctest.REPORT_NDIFF

   When specified, differences are computed by "difflib.Differ", using
   the same algorithm as the popular "ndiff.py" utility. This is the
   only method that marks differences within lines as well as across
   lines.  For example, if a line of expected output contains digit
   "1" where actual output contains letter "l", a line is inserted
   with a caret marking the mismatching column positions.

doctest.REPORT_ONLY_FIRST_FAILURE

   When specified, display the first failing example in each doctest,
   but suppress output for all remaining examples.  This will prevent
   doctest from reporting correct examples that break because of
   earlier failures; but it might also hide incorrect examples that
   fail independently of the first failure.  When
   "REPORT_ONLY_FIRST_FAILURE" is specified, the remaining examples
   are still run, and still count towards the total number of failures
   reported; only the output is suppressed.

doctest.FAIL_FAST

   When specified, exit after the first failing example and don't
   attempt to run the remaining examples. Thus, the number of failures
   reported will be at most 1.  This flag may be useful during
   debugging, since examples after the first failure won't even
   produce debugging output.

   The doctest command line accepts the option "-f" as a shorthand for
   "-o FAIL_FAST".

   New in version 3.4.

doctest.REPORTING_FLAGS

   A bitmask or'ing together all the reporting flags above.

There is also a way to register new option flag names, though this
isn't useful unless you intend to extend "doctest" internals via
subclassing:

doctest.register_optionflag(name)

   Create a new option flag with a given name, and return the new
   flag's integer value.  "register_optionflag()" can be used when
   subclassing "OutputChecker" or "DocTestRunner" to create new
   options that are supported by your subclasses.
   "register_optionflag()" should always be called using the following
   idiom:

      MY_FLAG = register_optionflag('MY_FLAG')


Directives
----------

Doctest directives may be used to modify the option flags for an
individual example.  Doctest directives are special Python comments
following an example's source code:

   directive             ::= "#" "doctest:" directive_options
   directive_options     ::= directive_option ("," directive_option)\*
   directive_option      ::= on_or_off directive_option_name
   on_or_off             ::= "+" \| "-"
   directive_option_name ::= "DONT_ACCEPT_BLANKLINE" \| "NORMALIZE_WHITESPACE" \| ...

Whitespace is not allowed between the "+" or "-" and the directive
option name.  The directive option name can be any of the option flag
names explained above.

An example's doctest directives modify doctest's behavior for that
single example.  Use "+" to enable the named behavior, or "-" to
disable it.

For example, this test passes:

   >>> print(list(range(20))) 
   [0,   1,  2,  3,  4,  5,  6,  7,  8,  9,
   10,  11, 12, 13, 14, 15, 16, 17, 18, 19]

Without the directive it would fail, both because the actual output
doesn't have two blanks before the single-digit list elements, and
because the actual output is on a single line.  This test also passes,
and also requires a directive to do so:

   >>> print(list(range(20))) 
   [0, 1, ..., 18, 19]

Multiple directives can be used on a single physical line, separated
by commas:

   >>> print(list(range(20))) 
   [0,    1, ...,   18,    19]

If multiple directive comments are used for a single example, then
they are combined:

   >>> print(list(range(20))) 
   ...                        
   [0,    1, ...,   18,    19]

As the previous example shows, you can add "..." lines to your example
containing only directives.  This can be useful when an example is too
long for a directive to comfortably fit on the same line:

   >>> print(list(range(5)) + list(range(10, 20)) + list(range(30, 40)))
   ... 
   [0, ..., 4, 10, ..., 19, 30, ..., 39]

Note that since all options are disabled by default, and directives
apply only to the example they appear in, enabling options (via "+" in
a directive) is usually the only meaningful choice.  However, option
flags can also be passed to functions that run doctests, establishing
different defaults.  In such cases, disabling an option via "-" in a
directive can be useful.


Warnings
--------

"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
set, Python doesn't guarantee that the element is printed in any
particular order, so a test like

   >>> foo()
   {"Hermione", "Harry"}

is vulnerable!  One workaround is to do

   >>> foo() == {"Hermione", "Harry"}
   True

instead.  Another is to do

   >>> d = sorted(foo())
   >>> d
   ['Harry', 'Hermione']

Note: Before Python 3.6, when printing a dict, Python did not
  guarantee that the key-value pairs was printed in any particular
  order.

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
   7948648
   >>> class C: pass
   >>> C()   # the default repr() for instances embeds an address
   <__main__.C instance at 0x00AC18F0>

The "ELLIPSIS" directive gives a nice approach for the last example:

   >>> C() 
   <__main__.C instance at 0x...>

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
   0.14285714285714285
   >>> print(1./7) # safer
   0.142857142857
   >>> print(round(1./7, 6)) # much safer
   0.142857

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
   0.75

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


Basic API
=========

The functions "testmod()" and "testfile()" provide a simple interface
to doctest that should be sufficient for most basic uses.  For a less
formal introduction to these two functions, see sections Simple Usage:
Checking Examples in Docstrings and Simple Usage: Checking Examples in
a Text File.

doctest.testfile(filename, module_relative=True, name=None, package=None, globs=None, verbose=None, report=True, optionflags=0, extraglobs=None, raise_on_error=False, parser=DocTestParser(), encoding=None)

   All arguments except *filename* are optional, and should be
   specified in keyword form.

   Test examples in the file named *filename*.  Return
   "(failure_count, test_count)".

   Optional argument *module_relative* specifies how the filename
   should be interpreted:

   * If *module_relative* is "True" (the default), then *filename*
     specifies an OS-independent module-relative path.  By default,
     this path is relative to the calling module's directory; but if
     the *package* argument is specified, then it is relative to that
     package.  To ensure OS-independence, *filename* should use "/"
     characters to separate path segments, and may not be an absolute
     path (i.e., it may not begin with "/").

   * If *module_relative* is "False", then *filename* specifies an
     OS- specific path.  The path may be absolute or relative;
     relative paths are resolved with respect to the current working
     directory.

   Optional argument *name* gives the name of the test; by default, or
   if "None", "os.path.basename(filename)" is used.

   Optional argument *package* is a Python package or the name of a
   Python package whose directory should be used as the base directory
   for a module-relative filename.  If no package is specified, then
   the calling module's directory is used as the base directory for
   module-relative filenames.  It is an error to specify *package* if
   *module_relative* is "False".

   Optional argument *globs* gives a dict to be used as the globals
   when executing examples.  A new shallow copy of this dict is
   created for the doctest, so its examples start with a clean slate.
   By default, or if "None", a new empty dict is used.

   Optional argument *extraglobs* gives a dict merged into the globals
   used to execute examples.  This works like "dict.update()":  if
   *globs* and *extraglobs* have a common key, the associated value in
   *extraglobs* appears in the combined dict.  By default, or if
   "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
   *extraglobs* dict mapping the generic name to the subclass to be
   tested.

   Optional argument *verbose* prints lots of stuff if true, and
   prints only failures if false; by default, or if "None", it's true
   if and only if "'-v'" is in "sys.argv".

   Optional argument *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 *optionflags* (default value 0) takes the bitwise
   OR of option flags. See section Option Flags.

   Optional argument *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 *parser* specifies a "DocTestParser" (or
   subclass) that should be used to extract tests from the files.  It
   defaults to a normal parser (i.e., "DocTestParser()").

   Optional argument *encoding* specifies an encoding that should be
   used to convert the file to unicode.

doctest.testmod(m=None, name=None, globs=None, verbose=None, report=True, optionflags=0, extraglobs=None, raise_on_error=False, exclude_empty=False)

   All arguments are optional, and all except for *m* should be
   specified in keyword form.

   Test examples in docstrings in functions and classes reachable from
   module *m* (or module "__main__" if *m* is not supplied or is
   "None"), starting with "m.__doc__".

   Also test examples reachable from dict "m.__test__", if it exists
   and is not "None".  "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 *m* are
   searched.

   Return "(failure_count, test_count)".

   Optional argument *name* gives the name of the module; by default,
   or if "None", "m.__name__" is used.

   Optional argument *exclude_empty* defaults to false.  If true,
   objects for which no doctests are found are excluded from
   consideration. The default is a backward compatibility hack, so
   that code still using "doctest.master.summarize()" in conjunction
   with "testmod()" continues to get output for objects with no tests.
   The *exclude_empty* argument to the newer "DocTestFinder"
   constructor defaults to true.

   Optional arguments *extraglobs*, *verbose*, *report*,
   *optionflags*, *raise_on_error*, and *globs* are the same as for
   function "testfile()" above, except that *globs* defaults to
   "m.__dict__".

doctest.run_docstring_examples(f, globs, verbose=False, name="NoName", compileflags=None, optionflags=0)

   Test examples associated with object *f*; for example, *f* may be a
   string, a module, a function, or a class object.

   A shallow copy of dictionary argument *globs* is used for the
   execution context.

   Optional argument *name* is used in failure messages, and defaults
   to ""NoName"".

   If optional argument *verbose* is true, output is generated even if
   there are no failures.  By default, output is generated only in
   case of an example failure.

   Optional argument *compileflags* gives the set of flags that should
   be used by the Python compiler when running the examples.  By
   default, or if "None", flags are deduced corresponding to the set
   of future features found in *globs*.

   Optional argument *optionflags* works as for function "testfile()"
   above.


Unittest API
============

As your collection of doctest'ed modules grows, you'll want a way to
run all their doctests systematically.  "doctest" provides two
functions that can be used to create "unittest" test suites from
modules and text files containing doctests.  To integrate with
"unittest" test discovery, include a "load_tests()" function in your
test module:

   import unittest
   import doctest
   import my_module_with_doctests

   def load_tests(loader, tests, ignore):
       tests.addTests(doctest.DocTestSuite(my_module_with_doctests))
       return tests

There are two main functions for creating "unittest.TestSuite"
instances from text files and modules with doctests:

doctest.DocFileSuite(*paths, module_relative=True, package=None, setUp=None, tearDown=None, globs=None, optionflags=0, parser=DocTestParser(), encoding=None)

   Convert doctest tests from one or more text files to a
   "unittest.TestSuite".

   The returned "unittest.TestSuite" is to be run by the unittest
   framework and runs the interactive examples in each file.  If an
   example in any file fails, then the synthesized unit test fails,
   and a "failureException" exception is raised showing the name of
   the file containing the test and a (sometimes approximate) line
   number.

   Pass one or more paths (as strings) to text files to be examined.

   Options may be provided as keyword arguments:

   Optional argument *module_relative* specifies how the filenames in
   *paths* should be interpreted:

   * If *module_relative* is "True" (the default), then each
     filename in *paths* specifies an OS-independent module-relative
     path.  By default, this path is relative to the calling module's
     directory; but if the *package* argument is specified, then it is
     relative to that package.  To ensure OS-independence, each
     filename should use "/" characters to separate path segments, and
     may not be an absolute path (i.e., it may not begin with "/").

   * If *module_relative* is "False", then each filename in *paths*
     specifies an OS-specific path.  The path may be absolute or
     relative; relative paths are resolved with respect to the current
     working directory.

   Optional argument *package* is a Python package or the name of a
   Python package whose directory should be used as the base directory
   for module-relative filenames in *paths*.  If no package is
   specified, then the calling module's directory is used as the base
   directory for module-relative filenames.  It is an error to specify
   *package* if *module_relative* is "False".

   Optional argument *setUp* specifies a set-up function for the test
   suite. This is called before running the tests in each file.  The
   *setUp* function will be passed a "DocTest" object.  The setUp
   function can access the test globals as the *globs* attribute of
   the test passed.

   Optional argument *tearDown* specifies a tear-down function for the
   test suite.  This is called after running the tests in each file.
   The *tearDown* function will be passed a "DocTest" object.  The
   setUp function can access the test globals as the *globs* attribute
   of the test passed.

   Optional argument *globs* is a dictionary containing the initial
   global variables for the tests.  A new copy of this dictionary is
   created for each test.  By default, *globs* is a new empty
   dictionary.

   Optional argument *optionflags* specifies the default doctest
   options for the tests, created by or-ing together individual option
   flags.  See section Option Flags. See function
   "set_unittest_reportflags()" below for a better way to set
   reporting options.

   Optional argument *parser* specifies a "DocTestParser" (or
   subclass) that should be used to extract tests from the files.  It
   defaults to a normal parser (i.e., "DocTestParser()").

   Optional argument *encoding* specifies an encoding that should be
   used to convert the file to unicode.

   The global "__file__" is added to the globals provided to doctests
   loaded from a text file using "DocFileSuite()".

doctest.DocTestSuite(module=None, globs=None, extraglobs=None, test_finder=None, setUp=None, tearDown=None, checker=None)

   Convert doctest tests for a module to a "unittest.TestSuite".

   The returned "unittest.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
   "failureException" exception is raised showing the name of the file
   containing the test and a (sometimes approximate) line number.

   Optional argument *module* 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.

   Optional argument *globs* is a dictionary containing the initial
   global variables for the tests.  A new copy of this dictionary is
   created for each test.  By default, *globs* is a new empty
   dictionary.

   Optional argument *extraglobs* specifies an extra set of global
   variables, which is merged into *globs*.  By default, no extra
   globals are used.

   Optional argument *test_finder* is the "DocTestFinder" object (or a
   drop-in replacement) that is used to extract doctests from the
   module.

   Optional arguments *setUp*, *tearDown*, and *optionflags* are the
   same as for function "DocFileSuite()" above.

   This function uses the same search technique as "testmod()".

   Changed in version 3.5: "DocTestSuite()" returns an empty
   "unittest.TestSuite" if *module* contains no docstrings instead of
   raising "ValueError".

Under the covers, "DocTestSuite()" creates a "unittest.TestSuite" out
of "doctest.DocTestCase" instances, and "DocTestCase" is a subclass of
"unittest.TestCase". "DocTestCase" isn't documented here (it's an
internal detail), but studying its code can answer questions about the
exact details of "unittest" integration.

Similarly, "DocFileSuite()" creates a "unittest.TestSuite" out of
"doctest.DocFileCase" instances, and "DocFileCase" is a subclass of
"DocTestCase".

So both ways of creating a "unittest.TestSuite" run instances of
"DocTestCase".  This is important for a subtle reason: when you run
"doctest" functions yourself, you can control the "doctest" options in
use directly, by passing option flags to "doctest" functions.
However, if you're writing a "unittest" framework, "unittest"
ultimately controls when and how tests get run.  The framework author
typically wants to control "doctest" reporting options (perhaps, e.g.,
specified by command line options), but there's no way to pass options
through "unittest" to "doctest" test runners.

For this reason, "doctest" also supports a notion of "doctest"
reporting flags specific to "unittest" support, via this function:

doctest.set_unittest_reportflags(flags)

   Set the "doctest" reporting flags to use.

   Argument *flags* takes the bitwise OR of option flags.  See section
   Option Flags.  Only "reporting flags" can be used.

   This is a module-global setting, and affects all future doctests
   run by module "unittest":  the "runTest()" method of "DocTestCase"
   looks at the option flags specified for the test case when the
   "DocTestCase" instance was constructed.  If no reporting flags were
   specified (which is the typical and expected case), "doctest"'s
   "unittest" reporting flags are bitwise ORed into the option flags,
   and the option flags so augmented are passed to the "DocTestRunner"
   instance created to run the doctest.  If any reporting flags were
   specified when the "DocTestCase" instance was constructed,
   "doctest"'s "unittest" reporting flags are ignored.

   The value of the "unittest" reporting flags in effect before the
   function was called is returned by the function.


Advanced API
============

The basic API is a simple wrapper that's intended to make doctest easy
to use. It is fairly flexible, and should meet most users' needs;
however, if you require more fine-grained control over testing, or
wish to extend doctest's capabilities, then you should use the
advanced API.

The advanced API revolves around two container classes, which are used
to store the interactive examples extracted from doctest cases:

* "Example": A single Python *statement*, paired with its expected
  output.

* "DocTest": A collection of "Example"s, typically extracted from a
  single docstring or text file.

Additional processing classes are defined to find, parse, and run, and
check doctest examples:

* "DocTestFinder": Finds all docstrings in a given module, and uses
  a "DocTestParser" to create a "DocTest" from every docstring that
  contains interactive examples.

* "DocTestParser": Creates a "DocTest" object from a string (such as
  an object's docstring).

* "DocTestRunner": Executes the examples in a "DocTest", and uses an
  "OutputChecker" to verify their output.

* "OutputChecker": Compares the actual output from a doctest example
  with the expected output, and decides whether they match.

The relationships among these processing classes are summarized in the
following diagram:

                               list of:
   +------+                   +---------+
   |module| --DocTestFinder-> | DocTest | --DocTestRunner-> results
   +------+    |        ^     +---------+     |       ^    (printed)
               |        |     | Example |     |       |
               v        |     |   ...   |     v       |
              DocTestParser   | Example |   OutputChecker
                              +---------+


DocTest Objects
---------------

class doctest.DocTest(examples, globs, name, filename, lineno, docstring)

   A collection of doctest examples that should be run in a single
   namespace.  The constructor arguments are used to initialize the
   attributes of the same names.

   "DocTest" defines the following attributes.  They are initialized
   by the constructor, and should not be modified directly.

   examples

      A list of "Example" objects encoding the individual interactive
      Python examples that should be run by this test.

   globs

      The namespace (aka globals) that the examples should be run in.
      This is a dictionary mapping names to values.  Any changes to
      the namespace made by the examples (such as binding new
      variables) will be reflected in "globs" after the test is run.

   name

      A string name identifying the "DocTest".  Typically, this is the
      name of the object or file that the test was extracted from.

   filename

      The name of the file that this "DocTest" was extracted from; or
      "None" if the filename is unknown, or if the "DocTest" was not
      extracted from a file.

   lineno

      The line number within "filename" where this "DocTest" begins,
      or "None" if the line number is unavailable.  This line number
      is zero-based with respect to the beginning of the file.

   docstring

      The string that the test was extracted from, or "None" if the
      string is unavailable, or if the test was not extracted from a
      string.


Example Objects
---------------

class doctest.Example(source, want, exc_msg=None, lineno=0, indent=0, options=None)

   A single interactive example, consisting of a Python statement and
   its expected output.  The constructor arguments are used to
   initialize the attributes of the same names.

   "Example" defines the following attributes.  They are initialized
   by the constructor, and should not be modified directly.

   source

      A string containing the example's source code.  This source code
      consists of a single Python statement, and always ends with a
      newline; the constructor adds a newline when necessary.

   want

      The expected output from running the example's source code
      (either from stdout, or a traceback in case of exception).
      "want" ends with a newline unless no output is expected, in
      which case it's an empty string.  The constructor adds a newline
      when necessary.

   exc_msg

      The exception message generated by the example, if the example
      is expected to generate an exception; or "None" if it is not
      expected to generate an exception.  This exception message is
      compared against the return value of
      "traceback.format_exception_only()".  "exc_msg" ends with a
      newline unless it's "None".  The constructor adds a newline if
      needed.

   lineno

      The line number within the string containing this example where
      the example begins.  This line number is zero-based with respect
      to the beginning of the containing string.

   indent

      The example's indentation in the containing string, i.e., the
      number of space characters that precede the example's first
      prompt.

   options

      A dictionary mapping from option flags to "True" or "False",
      which is used to override default options for this example.  Any
      option flags not contained in this dictionary are left at their
      default value (as specified by the "DocTestRunner"'s
      "optionflags"). By default, no options are set.


DocTestFinder objects
---------------------

class doctest.DocTestFinder(verbose=False, parser=DocTestParser(), recurse=True, exclude_empty=True)

   A processing class used to extract the "DocTest"s that are relevant
   to a given object, from its docstring and the docstrings of its
   contained objects. "DocTest"s can be extracted from modules,
   classes, functions, methods, staticmethods, classmethods, and
   properties.

   The optional argument *verbose* can be used to display the objects
   searched by the finder.  It defaults to "False" (no output).

   The optional argument *parser* specifies the "DocTestParser" object
   (or a drop-in replacement) that is used to extract doctests from
   docstrings.

   If the optional argument *recurse* is false, then
   "DocTestFinder.find()" will only examine the given object, and not
   any contained objects.

   If the optional argument *exclude_empty* is false, then
   "DocTestFinder.find()" will include tests for objects with empty
   docstrings.

   "DocTestFinder" defines the following method:

   find(obj[, name][, module][, globs][, extraglobs])

      Return a list of the "DocTest"s that are defined by *obj*'s
      docstring, or by any of its contained objects' docstrings.

      The optional argument *name* specifies the object's name; this
      name will be used to construct names for the returned
      "DocTest"s.  If *name* is not specified, then "obj.__name__" is
      used.

      The optional parameter *module* is the module that contains the
      given object. If the module is not specified or is "None", then
      the test finder will attempt to automatically determine the
      correct module.  The object's module is used:

      * As a default namespace, if *globs* is not specified.

      * To prevent the DocTestFinder from extracting DocTests from
        objects that are imported from other modules.  (Contained
        objects with modules other than *module* are ignored.)

      * To find the name of the file containing the object.

      * To help find the line number of the object within its file.

      If *module* is "False", no attempt to find the module will be
      made.  This is obscure, of use mostly in testing doctest itself:
      if *module* is "False", or is "None" but cannot be found
      automatically, then all objects are considered to belong to the
      (non-existent) module, so all contained objects will
      (recursively) be searched for doctests.

      The globals for each "DocTest" is formed by combining *globs*
      and *extraglobs* (bindings in *extraglobs* override bindings in
      *globs*).  A new shallow copy of the globals dictionary is
      created for each "DocTest". If *globs* is not specified, then it
      defaults to the module's *__dict__*, if specified, or "{}"
      otherwise.  If *extraglobs* is not specified, then it defaults
      to "{}".


DocTestParser objects
---------------------

class doctest.DocTestParser

   A processing class used to extract interactive examples from a
   string, and use them to create a "DocTest" object.

   "DocTestParser" defines the following methods:

   get_doctest(string, globs, name, filename, lineno)

      Extract all doctest examples from the given string, and collect
      them into a "DocTest" object.

      *globs*, *name*, *filename*, and *lineno* are attributes for the
      new "DocTest" object.  See the documentation for "DocTest" for
      more information.

   get_examples(string, name='<string>')

      Extract all doctest examples from the given string, and return
      them as a list of "Example" objects.  Line numbers are 0-based.
      The optional argument *name* is a name identifying this string,
      and is only used for error messages.

   parse(string, name='<string>')

      Divide the given string into examples and intervening text, and
      return them as a list of alternating "Example"s and strings.
      Line numbers for the "Example"s are 0-based.  The optional
      argument *name* is a name identifying this string, and is only
      used for error messages.


DocTestRunner objects
---------------------

class doctest.DocTestRunner(checker=None, verbose=None, optionflags=0)

   A processing class used to execute and verify the interactive
   examples in a "DocTest".

   The comparison between expected outputs and actual outputs is done
   by an "OutputChecker".  This comparison may be customized with a
   number of option flags; see section Option Flags for more
   information.  If the option flags are insufficient, then the
   comparison may also be customized by passing a subclass of
   "OutputChecker" to the constructor.

   The test runner's display output can be controlled in two ways.
   First, an output function can be passed to "TestRunner.run()"; this
   function will be called with strings that should be displayed.  It
   defaults to "sys.stdout.write".  If capturing the output is not
   sufficient, then the display output can be also customized by
   subclassing DocTestRunner, and overriding the methods
   "report_start()", "report_success()",
   "report_unexpected_exception()", and "report_failure()".

   The optional keyword argument *checker* specifies the
   "OutputChecker" object (or drop-in replacement) that should be used
   to compare the expected outputs to the actual outputs of doctest
   examples.

   The optional keyword argument *verbose* controls the
   "DocTestRunner"'s verbosity.  If *verbose* is "True", then
   information is printed about each example, as it is run.  If
   *verbose* is "False", then only failures are printed.  If *verbose*
   is unspecified, or "None", then verbose output is used iff the
   command-line switch "-v" is used.

   The optional keyword argument *optionflags* can be used to control
   how the test runner compares expected output to actual output, and
   how it displays failures. For more information, see section Option
   Flags.

   "DocTestParser" defines the following methods:

   report_start(out, test, example)

      Report that the test runner is about to process the given
      example. This method is provided to allow subclasses of
      "DocTestRunner" to customize their output; it should not be
      called directly.

      *example* is the example about to be processed.  *test* is the
      test *containing example*.  *out* is the output function that
      was passed to "DocTestRunner.run()".

   report_success(out, test, example, got)

      Report that the given example ran successfully.  This method is
      provided to allow subclasses of "DocTestRunner" to customize
      their output; it should not be called directly.

      *example* is the example about to be processed.  *got* is the
      actual output from the example.  *test* is the test containing
      *example*.  *out* is the output function that was passed to
      "DocTestRunner.run()".

   report_failure(out, test, example, got)

      Report that the given example failed.  This method is provided
      to allow subclasses of "DocTestRunner" to customize their
      output; it should not be called directly.

      *example* is the example about to be processed.  *got* is the
      actual output from the example.  *test* is the test containing
      *example*.  *out* is the output function that was passed to
      "DocTestRunner.run()".

   report_unexpected_exception(out, test, example, exc_info)

      Report that the given example raised an unexpected exception.
      This method is provided to allow subclasses of "DocTestRunner"
      to customize their output; it should not be called directly.

      *example* is the example about to be processed. *exc_info* is a
      tuple containing information about the unexpected exception (as
      returned by "sys.exc_info()"). *test* is the test containing
      *example*.  *out* is the output function that was passed to
      "DocTestRunner.run()".

   run(test, compileflags=None, out=None, clear_globs=True)

      Run the examples in *test* (a "DocTest" object), and display the
      results using the writer function *out*.

      The examples are run in the namespace "test.globs".  If
      *clear_globs* is true (the default), then this namespace will be
      cleared after the test runs, to help with garbage collection. If
      you would like to examine the namespace after the test
      completes, then use *clear_globs=False*.

      *compileflags* gives the set of flags that should be used by the
      Python compiler when running the examples.  If not specified,
      then it will default to the set of future-import flags that
      apply to *globs*.

      The output of each example is checked using the
      "DocTestRunner"'s output checker, and the results are formatted
      by the "DocTestRunner.report_*()" methods.

   summarize(verbose=None)

      Print a summary of all the test cases that have been run by this
      DocTestRunner, and return a *named tuple* "TestResults(failed,
      attempted)".

      The optional *verbose* argument controls how detailed the
      summary is.  If the verbosity is not specified, then the
      "DocTestRunner"'s verbosity is used.


OutputChecker objects
---------------------

class doctest.OutputChecker

   A class used to check the whether the actual output from a doctest
   example matches the expected output.  "OutputChecker" defines two
   methods: "check_output()", which compares a given pair of outputs,
   and returns "True" if they match; and "output_difference()", which
   returns a string describing the differences between two outputs.

   "OutputChecker" defines the following methods:

   check_output(want, got, optionflags)

      Return "True" iff the actual output from an example (*got*)
      matches the expected output (*want*).  These strings are always
      considered to match if they are identical; but depending on what
      option flags the test runner is using, several non-exact match
      types are also possible.  See section Option Flags for more
      information about option flags.

   output_difference(example, got, optionflags)

      Return a string describing the differences between the expected
      output for a given example (*example*) and the actual output
      (*got*).  *optionflags* is the set of option flags used to
      compare *want* and *got*.


Debugging
=========

Doctest provides several mechanisms for debugging doctest examples:

* Several functions convert doctests to executable Python programs,
  which can be run under the Python debugger, "pdb".

* The "DebugRunner" class is a subclass of "DocTestRunner" that
  raises an exception for the first failing example, containing
  information about that example. This information can be used to
  perform post- mortem debugging on the example.

* The "unittest" cases generated by "DocTestSuite()" support the
  "debug()" method defined by "unittest.TestCase".

* You can add a call to "pdb.set_trace()" in a doctest example, and
  you'll drop into the Python debugger when that line is executed.
  Then you can inspect current values of variables, and so on.  For
  example, suppose "a.py" contains just this module docstring:

     """
     >>> def f(x):
     ...     g(x*2)
     >>> def g(x):
     ...     print(x+3)
     ...     import pdb; pdb.set_trace()
     >>> f(3)
     9
     """

  Then an interactive Python session may look like this:

     >>> import a, doctest
     >>> doctest.testmod(a)
     --Return--
     > <doctest a[1]>(3)g()->None
     -> import pdb; pdb.set_trace()
     (Pdb) list
       1     def g(x):
       2         print(x+3)
       3  ->     import pdb; pdb.set_trace()
     [EOF]
     (Pdb) p x
     6
     (Pdb) step
     --Return--
     > <doctest a[0]>(2)f()->None
     -> g(x*2)
     (Pdb) list
       1     def f(x):
       2  ->     g(x*2)
     [EOF]
     (Pdb) p x
     3
     (Pdb) step
     --Return--
     > <doctest a[2]>(1)?()->None
     -> f(3)
     (Pdb) cont
     (0, 3)
     >>>

Functions that convert doctests to Python code, and possibly run the
synthesized code under the debugger:

doctest.script_from_examples(s)

   Convert text with examples to a script.

   Argument *s* is a string containing doctest examples.  The string
   is converted to a Python script, where doctest examples in *s* are
   converted to regular code, and everything else is converted to
   Python comments.  The generated script is returned as a string. For
   example,

      import doctest
      print(doctest.script_from_examples(r"""
          Set x and y to 1 and 2.
          >>> x, y = 1, 2

          Print their sum:
          >>> print(x+y)
          3
      """))

   displays:

      # Set x and y to 1 and 2.
      x, y = 1, 2
      #
      # Print their sum:
      print(x+y)
      # Expected:
      ## 3

   This function is used internally by other functions (see below),
   but can also be useful when you want to transform an interactive
   Python session into a Python script.

doctest.testsource(module, name)

   Convert the doctest for an object to a script.

   Argument *module* is a module object, or dotted name of a module,
   containing the object whose doctests are of interest.  Argument
   *name* is the name (within the module) of the object with the
   doctests of interest.  The result is a string, containing the
   object's docstring converted to a Python script, as described for
   "script_from_examples()" above.  For example, if module "a.py"
   contains a top-level function "f()", then

      import a, doctest
      print(doctest.testsource(a, "a.f"))

   prints a script version of function "f()"'s docstring, with
   doctests converted to code, and the rest placed in comments.

doctest.debug(module, name, pm=False)

   Debug the doctests for an object.

   The *module* and *name* arguments are the same as for function
   "testsource()" above.  The synthesized Python script for the named
   object's docstring is written to a temporary file, and then that
   file is run under the control of the Python debugger, "pdb".

   A shallow copy of "module.__dict__" is used for both local and
   global execution context.

   Optional argument *pm* controls whether post-mortem debugging is
   used.  If *pm* has a true value, the script file is run directly,
   and the debugger gets involved only if the script terminates via
   raising an unhandled exception.  If it does, then post-mortem
   debugging is invoked, via "pdb.post_mortem()", passing the
   traceback object from the unhandled exception.  If *pm* is not
   specified, or is false, the script is run under the debugger from
   the start, via passing an appropriate "exec()" call to "pdb.run()".

doctest.debug_src(src, pm=False, globs=None)

   Debug the doctests in a string.

   This is like function "debug()" above, except that a string
   containing doctest examples is specified directly, via the *src*
   argument.

   Optional argument *pm* has the same meaning as in function
   "debug()" above.

   Optional argument *globs* gives a dictionary to use as both local
   and global execution context.  If not specified, or "None", an
   empty dictionary is used. If specified, a shallow copy of the
   dictionary is used.

The "DebugRunner" class, and the special exceptions it may raise, are
of most interest to testing framework authors, and will only be
sketched here.  See the source code, and especially "DebugRunner"'s
docstring (which is a doctest!) for more details:

class doctest.DebugRunner(checker=None, verbose=None, optionflags=0)

   A subclass of "DocTestRunner" that raises an exception as soon as a
   failure is encountered.  If an unexpected exception occurs, an
   "UnexpectedException" exception is raised, containing the test, the
   example, and the original exception.  If the output doesn't match,
   then a "DocTestFailure" exception is raised, containing the test,
   the example, and the actual output.

   For information about the constructor parameters and methods, see
   the documentation for "DocTestRunner" in section Advanced API.

There are two exceptions that may be raised by "DebugRunner"
instances:

exception doctest.DocTestFailure(test, example, got)

   An exception raised by "DocTestRunner" to signal that a doctest
   example's actual output did not match its expected output. The
   constructor arguments are used to initialize the attributes of the
   same names.

"DocTestFailure" defines the following attributes:

DocTestFailure.test

   The "DocTest" object that was being run when the example failed.

DocTestFailure.example

   The "Example" that failed.

DocTestFailure.got

   The example's actual output.

exception doctest.UnexpectedException(test, example, exc_info)

   An exception raised by "DocTestRunner" to signal that a doctest
   example raised an unexpected exception.  The constructor arguments
   are used to initialize the attributes of the same names.

"UnexpectedException" defines the following attributes:

UnexpectedException.test

   The "DocTest" object that was being run when the example failed.

UnexpectedException.example

   The "Example" that failed.

UnexpectedException.exc_info

   A tuple containing information about the unexpected exception, as
   returned by "sys.exc_info()".


Soapbox
=======

As mentioned in the introduction, "doctest" has grown to have three
primary uses:

1. Checking examples in docstrings.

2. Regression testing.

3. Executable documentation / literate testing.

These uses have different requirements, and it is important to
distinguish them. In particular, filling your docstrings with obscure
test cases makes for bad documentation.

When writing a docstring, 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 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 "doctest" examples
stops working after a "harmless" change.

Doctest also makes an excellent tool for regression testing,
especially if you don't skimp on explanatory text.  By interleaving
prose and examples, it becomes much easier to keep track of what's
actually being tested, and why.  When a test fails, good prose can
make it much easier to figure out what the problem is, and how it
should be fixed.  It's true that you could write extensive comments in
code-based testing, but few programmers do. Many have found that using
doctest approaches instead leads to much clearer tests.  Perhaps this
is simply because doctest makes writing prose a little easier than
writing code, while writing comments in code is a little harder.  I
think it goes deeper than just that: the natural attitude when writing
a doctest-based test is that you want to explain the fine points of
your software, and illustrate them with examples. This in turn
naturally leads to test files that start with the simplest features,
and logically progress to complications and edge cases.  A coherent
narrative is the result, instead of a collection of isolated functions
that test isolated bits of functionality seemingly at random.  It's a
different attitude, and produces different results, blurring the
distinction between testing and explaining.

Regression testing is best confined to dedicated objects or files.
There are several options for organizing tests:

* Write text files containing test cases as interactive examples,
  and test the files using "testfile()" or "DocFileSuite()".  This is
  recommended, although is easiest to do for new projects, designed
  from the start to use doctest.

* Define functions named "_regrtest_topic" that consist of single
  docstrings, containing test cases for the named topics.  These
  functions can be included in the same file as the module, or
  separated out into a separate test file.

* Define a "__test__" dictionary mapping from regression test topics
  to docstrings containing test cases.

When you have placed your tests in a module, the module can itself be
the test runner.  When a test fails, you can arrange for your test
runner to re-run only the failing doctest while you debug the problem.
Here is a minimal example of such a test runner:

   if __name__ == '__main__':
       import doctest
       flags = doctest.REPORT_NDIFF|doctest.FAIL_FAST
       if len(sys.argv) > 1:
           name = sys.argv[1]
           if name in globals():
               obj = globals()[name]
           else:
               obj = __test__[name]
           doctest.run_docstring_examples(obj, globals(), name=name,
                                          optionflags=flags)
       else:
           fail, total = doctest.testmod(optionflags=flags)
           print("{} failures out of {} tests".format(fail, total))

-[ Footnotes ]-

[1] Examples containing both expected output and an exception are
    not supported. Trying to guess where one ends and the other begins
    is too error-prone, and that also makes for a confusing test.
