fix: capture mypy stderr output on fatal options error instead of generic message

[sbrunner]

Description

Example of new output:

************* Module .
.:0:0: fatal-options-error(mypy): usage: mypy [-h] [-v] [-V] [more options; see below]
            [-m MODULE] [-p PACKAGE] [-c PROGRAM_TEXT] [files ...]
mypy: error: argument --python-version: Python 3.9 is not supported (must be 3.10 or higher)

Check Information
=================
         Started: 2026-07-02 11:47:24.052039
        Finished: 2026-07-02 11:47:24.214779
      Time Taken: 0.16 seconds
       Formatter: pylint
        Profiles: .prospector.yaml, utils:base, utils:base-all, strictness_none, member_warnings, utils:no-design-checks, utils:fix, duplicated, duplicated:ruff, duplicated:black, duplicated:isort, no_doc_warnings, no_test_warnings
      Strictness: from profile
  Libraries Used: 
       Tools Run: mypy
  Messages Found: 1

Related Issue

Probably fix: #825 I don't have the exact same issue.

Motivation and Context

How Has This Been Tested?

Run in a project that has a mypy error

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change requires a change to the dependencies
• I have updated the dependencies accordingly
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[carlio]

This looks like it's repeating some things already done with the stdout/stderr capturing - https://github.com/prospector-dev/prospector/blob/master/prospector/run.py#L110

Am I missing something? What does this PR add that isn't already available via the --die-on-tool-error or --direct-tool-stdout options?

Is there a way to capture the stderr using the exising mechanism instead of repeating it?

[sbrunner]

Yes your right it's not so good, do you like like this, to get:

.: failure(mypy): Tool mypy failed to run , with stderr:
usage: mypy [-h] [-v] [-V] [more options; see below]
            [-m MODULE] [-p PACKAGE] [-c PROGRAM_TEXT] [files ...]
mypy: error: argument --python-version: Python 3.0 is not supported (must be 3.10 or higher)
(exception was raised, re-run prospector with -X to see the stacktrace)

Or should we propose to use the option --die-on-tool-error to get:

usage: mypy [-h] [-v] [-V] [more options; see below]
           [-m MODULE] [-p PACKAGE] [-c PROGRAM_TEXT] [files ...]
mypy: error: argument --python-version: Python 3.0 is not supported (must be 3.10 or higher)
************* Module .
.: failure(mypy): Tool mypy failed to run (exception was raised, re-run prospector with -X to see the stacktrace)

[carlio]

I asked the reporter for the output from --die-on-tool-error, let's see if they reply, but it might help understand what the issue is - the current version you have looks good I think but let's wait to get extra information from the person who can reproduce it. Unless they never reply then we just merge anyway and hope :)

[sbrunner]

The --die-on-tool-error returns that (if we removes the try in mypy):

Traceback (most recent call last):
 File "/home/sbrunner/.pyenv/versions/3.13.9/lib/python3.13/argparse.py", line 2551, in _get_value
   result = type_func(arg_string)
 File "mypy/config_parser.py", line 53, in parse_version
mypy.config_parser.VersionTypeError: Python 3.0 is not supported (must be 3.10 or higher)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
 File "/home/sbrunner/.pyenv/versions/3.13.9/lib/python3.13/argparse.py", line 1937, in _parse_known_args2
   namespace, args = self._parse_known_args(args, namespace, intermixed)
                     ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/home/sbrunner/.pyenv/versions/3.13.9/lib/python3.13/argparse.py", line 2188, in _parse_known_args
   start_index = consume_optional(start_index)
 File "/home/sbrunner/.pyenv/versions/3.13.9/lib/python3.13/argparse.py", line 2112, in consume_optional
   take_action(action, args, option_string)
   ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/home/sbrunner/.pyenv/versions/3.13.9/lib/python3.13/argparse.py", line 1998, in take_action
   argument_values = self._get_values(action, argument_strings)
 File "/home/sbrunner/.pyenv/versions/3.13.9/lib/python3.13/argparse.py", line 2518, in _get_values
   value = self._get_value(action, arg_string)
 File "/home/sbrunner/.pyenv/versions/3.13.9/lib/python3.13/argparse.py", line 2556, in _get_value
   raise ArgumentError(action, msg)
argparse.ArgumentError: argument --python-version: Python 3.0 is not supported (must be 3.10 or higher)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
 File "/home/sbrunner/workspace-ai/prospector/prospector/run.py", line 94, in execute
   messages += tool.run(found_files)
               ~~~~~~~~^^^^^^^^^^^^^
 File "/home/sbrunner/workspace-ai/prospector/prospector/tools/mypy/__init__.py", line 125, in run
   return self._run_std(args)
          ~~~~~~~~~~~~~^^^^^^
 File "/home/sbrunner/workspace-ai/prospector/prospector/tools/mypy/__init__.py", line 129, in _run_std
   sources, options = mypy.main.process_options(args, fscache=self.fscache)
                      ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "mypy/main.py", line 1436, in process_options
 File "/home/sbrunner/.pyenv/versions/3.13.9/lib/python3.13/argparse.py", line 1898, in parse_args
   args, argv = self.parse_known_args(args, namespace)
                ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
 File "/home/sbrunner/.pyenv/versions/3.13.9/lib/python3.13/argparse.py", line 1908, in parse_known_args
   return self._parse_known_args2(args, namespace, intermixed=False)
          ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/home/sbrunner/.pyenv/versions/3.13.9/lib/python3.13/argparse.py", line 1939, in _parse_known_args2
   self.error(str(err))
   ~~~~~~~~~~^^^^^^^^^^
 File "mypy/main.py", line 450, in error
 File "mypy/main.py", line 437, in exit
SystemExit: 2

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
 File "/home/sbrunner/.pyenv/versions/3.13.9/bin/prospector", line 7, in <module>
   sys.exit(main())
            ~~~~^^
 File "/home/sbrunner/workspace-ai/prospector/prospector/run.py", line 207, in main
   prospector.execute()
   ~~~~~~~~~~~~~~~~~~^^
 File "/home/sbrunner/workspace-ai/prospector/prospector/run.py", line 112, in execute
   raise FatalProspectorException(f"Tool {toolname} failed to run.") from ex
prospector.exceptions.FatalProspectorException: Tool mypy failed to run.

It's useful to debug but its hart to read and we don't get the error message!

PS: I edit my last message because I write --die-on-tool-error in place of --direct-tool-stdout!

[mdabrowski1990]

I am facing the same issue in my repo - 100% reproducible on python 3.14
Feel free to share potential fixes for testing or simply clone https://github.com/mdabrowski1990/uds (dependencies are in pyproject.toml)