[issue39106] Add suggestions to argparse error message output for unrecognized arguments

[paul j3]

paul j3  added the comment:

In the subparser example, it's the `build` subparser that puts '--overwritte' 
in the unrecognized list.  That is then passed back to the main parser, which 
then issues the 'unrecognized' error, along with its own usage.

The subparser is called with `parse_known_args` while the proposed patch is run 
in the `parse_args` method of the main parser.  It doesn't have access to the 
subparser's arguments.  So implementing the proposed matching will be much 
harder.

For some types of error, such as type or choices, the subparser itself raises 
the error, with the appropriate usage.  

===

https://bugs.python.org/issue42297
[argparse] Bad error message formatting when using custom usage text

is another case where error messages produced by the subparser differ from 
messages produced by the main. In this case the unrecognized error usage 
message is clearer since it is produced by the main parser.

===

I didn't close this issue, but it does feel like an enhancement that's too big 
for the bug/issues forum.  The proposed patch could be developed as a separate 
'parser.parse_args_with_hints' method, and distributed as a pypi addition.  
During development and testing, the regular 'parser.parse_args()' does not need 
to be touched.

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[issue39106] Add suggestions to argparse error message output for unrecognized arguments

[conchylicultor]

conchylicultor  added the comment:

I don't think this should have been closed.

[1]  If the user is using sub_parser, the options are not even displayed. For 
example in our https://github.com/tensorflow/datasets project:

```
$ tfds build mnist --overwritte
usage: tfds [-h] [--helpfull] [--version] {build,new} ...
tfds: error: unrecognized arguments: --overwritte
```

[2] For some programs, there can be 20+ options and having to scroll through 
the list is not user friendly at all.

[3] Other CLI, like Google absl.flags has this option too and it is very 
convenient.

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nosy: +conchylicultor

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[issue39106] Add suggestions to argparse error message output for unrecognized arguments

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

Thanks for the feedback. Closing it as rejected.

--
resolution:  -> rejected
stage:  -> resolved
status: open -> closed

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[issue39106] Add suggestions to argparse error message output for unrecognized arguments

[hai shi]

hai shi  added the comment:

I checked some other common clis and it is show all right available options 
too. So I thought the argparse's help function is good enough too ;)

```
$ ps -etest
error: TTY could not be found

Usage:
 ps [options]

 Try 'ps --help '
  or 'ps --help '
 for additional help text.

For more details see ps(1).
```

```
$ top test
top: unknown option 't'
Usage:
  top -hv | -bcHiOSs -d secs -n max -u|U user -p pid(s) -o field -w [cols]
```

--
nosy: +shihai1991

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[issue39106] Add suggestions to argparse error message output for unrecognized arguments

[Raymond Hettinger]

Raymond Hettinger  added the comment:

-1  Given an unknown argument, we really can't know what the user intended.  
The usage string already lists all available options and -h --help gives more 
detail when requested.

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[issue39106] Add suggestions to argparse error message output for unrecognized arguments

[Karthikeyan Singaravelan]

New submission from Karthikeyan Singaravelan :

I came across this idea while working on error messages for click at 
https://github.com/pallets/click/issues/1446. Currently for unknown arguments 
which could in some case be typos argparse throws an error but doesn't make any 
suggestions. It could do some heuristic to suggest matches. The unrecognized 
argument error prints all unrecognized arguments so in that case it will be 
less useful to mix match suggestions. It can be helpful for single argument 
usages. argparse is performance sensitive since it's used in cli environments 
so I feel the tradeoff to do simple match to make suggestions as a good user 
experience.

# ssl_helper.py

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--include-ssl', action='store_true')
namespace = parser.parse_args()

No suggestions are included currently

$ python3.8 ssl_helper.py --include-ssll
usage: ssl_helper.py [-h] [--include-ssl]
ssl_helper.py: error: unrecognized arguments: --include-ssll

Include suggestions based when one of the option starts with the argument 
supplied similar to click

$ ./python.exe ssl_helper.py --include-ssll
usage: ssl_helper.py [-h] [--include-ssl]
ssl_helper.py: error: unrecognized argument: --include-ssll . Did you mean 
--include-ssl?

difflib.get_close_matches could also provide better suggestions in some cases 
but comes at import cost and could be imported only during error messages as 
proposed in the click issue

./python.exe ssl_helper.py --exclude-ssl
usage: ssl_helper.py [-h] [--include-ssl]
ssl_helper.py: error: unrecognized argument: --exclude-ssl . Did you mean 
--include-ssl?


Attached is a simple patch of the implementation with startswith which is more 
simple and difflib.get_close_matches

diff --git Lib/argparse.py Lib/argparse.py
index 5d3ce2ad70..e10a4f0c9b 100644
--- Lib/argparse.py
+++ Lib/argparse.py
@@ -1818,8 +1818,29 @@ class ArgumentParser(_AttributeHolder, 
_ActionsContainer):
 def parse_args(self, args=None, namespace=None):
 args, argv = self.parse_known_args(args, namespace)
 if argv:
-msg = _('unrecognized arguments: %s')
-self.error(msg % ' '.join(argv))
+suggestion = None
+if len(argv) == 1:
+argument = argv[0]
+
+# simple startswith
+for option in self._option_string_actions:
+if argument.startswith(option):
+suggestion = option
+break
+
+# difflib impl
+import difflib
+try:
+suggestion = difflib.get_close_matches(argv[0], 
self._option_string_actions, n=1)[0]
+except IndexError:
+pass
+
+if suggestion:
+msg = _('unrecognized argument: %s . Did you mean %s?')
+self.error(msg % (' '.join(argv), suggestion))
+else:
+msg = _('unrecognized arguments: %s')
+self.error(msg % ' '.join(argv))
 return args
 
 def parse_known_args(self, args=None, namespace=None):

--
components: Library (Lib)
messages: 358699
nosy: paul.j3, rhettinger, xtreak
priority: normal
severity: normal
status: open
title: Add suggestions to argparse error message output for unrecognized 
arguments
type: enhancement
versions: Python 3.9

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