23.2.3.5 Option Flags and Directives

Python 2.5

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23.2.3.5 Option Flags and Directives

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 or'ed together and passed to various functions. The names can also be used in doctest directives (see below).

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

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.

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.

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.

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.

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.

Note that a similar effect can be obtained using ELLIPSIS, and IGNORE_EXCEPTION_DETAIL may go away when Python releases prior to 2.4 become uninteresting. Until then, IGNORE_EXCEPTION_DETAIL is the only clear way to write a doctest that doesn't care about the exception detail yet continues to pass under Python releases prior to 2.4 (doctest directives appear to be comments to them). For example,

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

passes under Python 2.4 and Python 2.3. The detail changed in 2.4, to say "does not" instead of "doesn't".

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.

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

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

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

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

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.

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.

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

"Doctest directives" may be used to modify the option flags for individual examples. Doctest directives are expressed as a special Python comment 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 range(20) #doctest: +NORMALIZE_WHITESPACE
[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 range(20) # doctest:+ELLIPSIS
[0, 1, ..., 18, 19]

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

>>> print range(20) # doctest: +ELLIPSIS, +NORMALIZE_WHITESPACE
[0,    1, ...,   18,    19]

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

>>> print range(20) # doctest: +ELLIPSIS
...                 # doctest: +NORMALIZE_WHITESPACE
[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 range(5) + range(10,20) + range(30,40) + range(50,60)
... # doctest: +ELLIPSIS
[0, ..., 4, 10, ..., 19, 30, ..., 39, 50, ..., 59]

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.

Changed in version 2.4: Constants DONT_ACCEPT_BLANKLINE, NORMALIZE_WHITESPACE, ELLIPSIS, IGNORE_EXCEPTION_DETAIL, REPORT_UDIFF, REPORT_CDIFF, REPORT_NDIFF, REPORT_ONLY_FIRST_FAILURE, COMPARISON_FLAGS and REPORTING_FLAGS were added; by default <BLANKLINE> in expected output matches an empty line in actual output; and doctest directives were added. Changed in version 2.5: Constant SKIP was added.

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

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')

New in version 2.4.

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