Present a clearer error message when the user misses a space before ; in marker

[pradyunsg]

Currently, if you try to use:

tomli @ https://github.com/hukkin/tomli/archive/master.zip; python_version < '3.11'

The error you get is:

>>> Requirement("tomli @ https://github.com/hukkin/tomli/archive/master.zip; python_version < '3.11'")
Traceback (most recent call last):
  File "/Users/pradyunsg/Developer/github/packaging/.venv/lib/python3.11/site-packages/packaging/requirements.py", line 102, in __init__
    req = REQUIREMENT.parseString(requirement_string)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/pradyunsg/Developer/github/packaging/.venv/lib/python3.11/site-packages/pyparsing/core.py", line 1141, in parse_string
    raise exc.with_traceback(None)
pyparsing.exceptions.ParseException: Expected string_end, found 'python'  (at char 60), (line:1, col:61)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/pradyunsg/Developer/github/packaging/.venv/lib/python3.11/site-packages/packaging/requirements.py", line 104, in __init__
    raise InvalidRequirement(
packaging.requirements.InvalidRequirement: Parse error at "'python_v'": Expected string_end

It would be good if it presented an error that suggested adding a (space) before the semicolon.

[Jackenmen]

I think this was supposed to refer to packaging.requirements as:

tomli @ https://github.com/hukkin/tomli/archive/master.zip ; python_version < '3.11'

is not a valid marker string.
Assuming that's true, I made #592 to fix the issue.

[pradyunsg]

In #624, these cases get an error like:

packaging.requirements.InvalidRequirement: Expected end or semicolon (after URL and whitespace)
    package @ https://example.com/; python_version <= '3.2'
              ~~~~~~~~~~~~~~~~~~~~~~^

[Jackenmen]

Could the arrow's position be adjusted so that it points at the semicolon's position (since that's where the space is missing)? I think that it pointing at the first letter of the marker is confusing.

// #592 also hints directly that it might be the space between URL and semicolon that's missing while in your error it IMO seems a bit less clear. But I guess users will probably be able to figure it out just fine anyway so I wouldn't say it's a big deal.

[pradyunsg]

Yea, I flip-flopped on those aspects (I think that might actually be visible in the force-push history), and ended up on this style since it was consistent with how "bad" versions specifiers ended up being handled:

packaging.requirements.InvalidRequirement: Expected end or semicolon (after version specifier)
     name ==0.this.is.a.bad.version ; python_version == "1.0"
          ~~~^

[pradyunsg]

Granted all of the options we have at hand are improvements over the current variant. :)

packaging.requirements.InvalidRequirement: Parse error at "'python_v'": Expected string_end