Permit local version string to be empty (e.g. "1.0+" equals "1.0")

[paulproteus]

This permits "2.7.15+" to be a valid Python version for pythonfinder. For details, see #149. It includes a test. I think that this is a safe change -- 2.7.15+foo would be a valid version, as would 2.7.15, so permitting 2.7.15+ to equal 2.7.15 seems OK to me.

Steps to reproduce (copy-pasting from #149)

• Create any Pipfile and pipenv environment
• Run pipenv install
• Get a Python binary whose version number is 2.7.15+, as on Ubuntu 18.10
• See a crash

I've tested that this fixes the issue for me.

Close #149

Before

Trying to run pyfinder --findall crashes.

  asheesh@obscura:/tmp/banana$ bin/pyfinder --findall
Traceback (most recent call last):
  File "bin/pyfinder", line 11, in <module>
    load_entry_point('pythonfinder', 'console_scripts', 'pyfinder')()
  File "/tmp/banana/local/lib/python2.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/tmp/banana/local/lib/python2.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/tmp/banana/local/lib/python2.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/tmp/banana/local/lib/python2.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/tmp/banana/local/lib/python2.7/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/tmp/banana/pythonfinder/src/pythonfinder/cli.py", line 33, in cli
    versions = [v for v in finder.find_all_python_versions()]
  File "/tmp/banana/local/lib/python2.7/site-packages/backports/functools_lru_cache.py", line 137, in wrapper
    result = user_function(*args, **kwds)
  File "/tmp/banana/pythonfinder/src/pythonfinder/pythonfinder.py", line 136, in find_all_python_versions
    major=major, minor=minor, patch=patch, pre=pre, dev=dev, arch=arch, name=name
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/path.py", line 328, in find_all_python_versions
    values = list(self.get_pythons(sub_finder))
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/path.py", line 279, in get_pythons
    reverse=True
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/path.py", line 277, in <genexpr>
    (p for p in self._filter_paths(finder) if p.is_python),
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/path.py", line 264, in <genexpr>
    pth for pth in unnest(finder(p) for p in self.path_entries if p is not None)
  File "/tmp/banana/pythonfinder/src/pythonfinder/utils.py", line 138, in unnest
    for el in target:
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/path.py", line 264, in <genexpr>
    pth for pth in unnest(finder(p) for p in self.path_entries if p is not None)
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/mixins.py", line 76, in find_all_python_versions
    path_filter = filter(None, (sub_finder(p) for p in self.children.values()))
  File "/tmp/banana/local/lib/python2.7/site-packages/cached_property.py", line 35, in __get__
    value = obj.__dict__[self.func.__name__] = self.func(obj)
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/path.py", line 494, in children
    for child_key, child_val in self._gen_children():
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/path.py", line 487, in _gen_children
    yield (child.as_posix(), PathEntry.create(path=child, **pass_args))
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/path.py", line 567, in create
    _new = cls(**creation_args)
  File "<attrs generated init b5a14ef20d54b43281ba439a6be98c24578e233d>", line 16, in __init__
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/path.py", line 510, in get_py_version
    py_version = PythonVersion.from_path(path=self, name=self.name)
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/python.py", line 418, in from_path
    instance_dict = cls.parse(py_version.strip())
  File "/tmp/banana/pythonfinder/src/pythonfinder/models/python.py", line 366, in parse
    raise ValueError("Not a valid python version: %r" % version)
ValueError: Not a valid python version: <LegacyVersion('2.7.15+')>

After

Running pyfinder --findall treats 2.7.15+ as 2.7.15.

asheesh@obscura:/tmp/banana$ bin/pyfinder --findall
Found python at the following locations:
python3.6: 3.6.6 (None) @ /usr/bin/python3.6
python3.6m: 3.6.6 (None) @ /usr/bin/python3.6m
python2.7: 2.7.15 (None) @ /usr/bin/python2.7

[di]

A version like 2.7.15+ is not a valid version identifier, per PEP 440:

Local version labels MUST start and end with an ASCII letter or digit.

[paulproteus]

Thanks @di for the feedback -- good point!

This is resolved to my satisfaction within pythonfinder -- see sarugaku/pythonfinder#42 -- so I'll close this now. Cheers!

[svenpanne]

A version like 2.7.15+ is not a valid version identifier, per PEP 440:

Local version labels MUST start and end with an ASCII letter or digit.

As correct as this statement may be, it is highly irrelevant: Ubuntu is one of the most widely used distros (depending on the survey even the most widely used one), and the current cosmic release ships with a Python claiming a version of 2.7.15+, so the PyPA tools should better deal with that fact. Perhaps an issue can be opened for the Ubuntu package maintainers, but this won't change the current state of affairs.

[di]

This API is used to parse versions of Python packages, not versions of Python itself.

It was being misused downstream, but has been fixed, as @paulproteus mentioned.

[svenpanne]

Hmmm, then I misunderstood things, sorry. But note that sarugaku/pythonfinder#42 still misuses the API then, it just add more confusing code around that, trying to handle more stuff, but that doesn't work: "2.7.15+" is parsed as a LegacyVersion, so the attempted fix is not executed at all...