Rationale
=========
argparse performs a complex formatting of the usage for argument grouping
and for line wrapping to fit the terminal width. This formatting has been
a constant source of bugs for at least 10 years (see linked issues below)
where defensive assertion errors are triggered or brackets and paranthesis
are not properly handeled.
Problem
=======
The current implementation of argparse usage formatting relies on regular
expressions to group arguments usage only to separate them again later
with another set of regular expressions. This is a complex and error prone
approach that caused all the issues linked below. Special casing certain
argument formats has not solved the problem. The following are some of
the most common issues:
- empty `metavar`
- mutually exclusive groups with `SUPPRESS`ed arguments
- metavars with whitespace
- metavars with brackets or paranthesis
Solution
========
The following two comments summarize the solution:
- https://github.com/python/cpython/issues/82091#issuecomment-1093832187
- https://github.com/python/cpython/issues/77048#issuecomment-1093776995
Mainly, the solution is to rewrite the usage formatting to avoid the
group-then-separate approach. Instead, the usage parts are kept separate
and only joined together at the end. This allows for a much simpler
implementation that is easier to understand and maintain. It avoids the
regular expressions approach and fixes the corresponding issues.
This closes the following GitHub issues:
- #62090
- #62549
- #77048
- #82091
- #89743
- #96310
- #98666
These PRs become obsolete:
- #15372
- #96311
When parsing positional vs optional arguments, the use of min with a
list comprehension inside of a loop results in quadratic time based
on the number of optional arguments given. When combined with use of
prefix based argument files and a large number of optional flags, this
can result in extremely slow parsing behavior.
This replaces the min call with a simple loop with a short circuit to
break at the next optional argument.
Co-authored-by: Zsolt Dollenstein <zsol.zsol@gmail.com>
Reproducer depends on terminal size - the traceback occurs when there's
an option long enough so the usage line doesn't fit the terminal width.
Option order is also important for reproducibility.
Excluding empty groups (with all options suppressed) from inserts
fixes the problem.
If the option with argument has short and long names,
output argument only once, after the long name:
-o, --option ARG description
instead of
-o ARG, --option ARG description
This removes the unused `name` variable in the block where `ArgumentTypeError` is handled.
`ArgumentTypeError` errors are handled by showing just the string of the exception; unlike `ValueError`, the name (`__name__`) of the function is not included in the error message.
Fixes#96548
Help for other actions omit the default value if default is SUPPRESS or
already contains the special format string '%(default)'. Add those
special cases to BooleanOptionalAction's help formatting too.
Fixes https://bugs.python.org/issue44587 so that default=SUPPRESS is not
emitted.
Fixes https://bugs.python.org/issue38956 as this code will detect
whether '%(default)s' has already been specified in the help string.
Signed-off-by: Micky Yun Chan (michiboo): <chanmickyyun@gmail.com>
Co-authored-by: Micky Yun Chan <michan@redhat.com>
Raise an ArgumentError when the same subparser name is added twice to an
ArgumentParser. This is consistent with the (default) behavior when the
same option string is added twice to an ArgumentParser.
(Support for `conflict_handler="resolve"` could be considered as a
followup feature, although real use cases seem even rarer than
"resolve"ing option-strings.)
Automerge-Triggered-By: GH:rhettinger
# Adding 'required' to names in Lib.argparse.Action
gh-91832:
Added 'required' to the list `names` in `Lib.argparse.Action`.
Changed constant strings that test the Action object.
Automerge-Triggered-By: GH:merwok
Also made modes containing 'a' or 'x' act the same as a mode containing 'w' when argument is '-'
(so 'a'/'x' return sys.stdout like 'w', and 'ab'/'xb' return sys.stdout.buffer like 'wb').
Fix an uncaught exception during help text generation when
argparse.BooleanOptionalAction is used with default=argparse.SUPPRESS
and help is specified.
When `allow_abbrev` was first added, disabling the abbreviation of
long options broke the grouping of short flags ([bpo-26967](https://bugs.python.org/issue26967)). As a fix,
b1e4d1b603 (contained in v3.8) ignores `allow_abbrev=False` for a
given argument string if the string does _not_ start with "--"
(i.e. it doesn't look like a long option).
This fix, however, doesn't take into account that long options can
start with alternative characters specified via `prefix_chars`,
introducing a regression: `allow_abbrev=False` has no effect on long
options that start with an alternative prefix character.
The most minimal fix would be to replace the "starts with --" check
with a "starts with two prefix_chars characters". But
`_get_option_tuples` already distinguishes between long and short
options, so let's instead piggyback off of that check by moving the
`allow_abbrev` condition into `_get_option_tuples`.
https://bugs.python.org/issue39546
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