show help for no-arg processor calls, fix #562, fix #274

[kba]

This is a surprisingly tricky-to-implement problem: We want processor calls without any arguments to print the help screen and exit with code 1, i.e. if sys.argv[2:] == [].

It really isn't, it was just hard to test because sys.argv contained the arguments to pytest, not to the CLI. Turns out, sys.argv can be overridden at runtime for the tests and checking not sys.argv[1:] was enough

However, I could not find a way to get the un-procesed command line arguments to click. `get_current_context()` does have an `args` property but that only contains the remaining positional arguments after parsing.

If anyone knows a better way to get the click equivalent of sys.argv[2:], I'd be interested!

For now, I've disabled the defaults for input_file_grp/output_file_grp (as proposed in #274) and check for all relevant CLI options one-by-one:

    if not (dump_json or help or version or overwrite) and \
            mets == 'mets.xml' and \
            not (kwargs['input_file_grp'] or kwargs['output_file_grp']):
        processorClass(workspace=None, show_help=True)
        sys.exit(1)

As I said before, I would prefer a more robust solution. The clause above will also trigger for processors being called with --mets mets.xml but neither input nor output file group (such as the ocrd-sanitize processor...).

[codecov-commenter]

Codecov Report

Merging #586 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #586   +/-   ##
=======================================
  Coverage   84.87%   84.87%           
=======================================
  Files          52       52           
  Lines        2890     2890           
  Branches      564      564           
=======================================
  Hits         2453     2453           
  Misses        328      328           
  Partials      109      109           

Impacted Files	Coverage Δ
ocrd/ocrd/decorators/__init__.py	88.57% <100.00%> (ø)

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[bertsky]

Energize!

[mikegerber]

This sometimes breaks running a test using click.testing.CliRunner, e.g.:

   # Run through the OCR-D interface                                                                                   
    with working_directory(str(test_workspace_dir)):                                                                    
        runner = CliRunner()                                                                                            
        result = runner.invoke(ocrd_dinglehopper, [                                                                                                          
            '-m', 'mets.xml',                                                                                           
            '-I', 'OCR-D-GT-PAGE,OCR-D-OCR-CALAMARI',                                                                   
            '-O', 'OCR-D-OCR-CALAMARI-EVAL'                                                                             
        ])                                                                                                              
    print(result.output, file=sys.stderr)                                                                               
    assert result.exit_code == 0

The call/invoke prints help and exits with 1. (Funny, it's also a Heisenbug, it doesn't break when I run the test alone, but does when I run all the tests.)

Edit: I worked around it by setting sys.argv[1:] manually.