Fix issue with prerelease git versions where the dash before the version number disappears

[smarie]

Consider some random python module that is in a git-versioned folder, tagged as 1.0.0-rc1 (release candidate. See semantic versioning: https://semver.org/spec/v2.0.0-rc.1.html)

The following code does not retrieve the right version string: the dash before the pre-release info is removed

from getversion import get_module_version

# import whatever local folder or file is available in this git folder
import my_module

# get the version and details. it will fallback to using the setuptools_scm one
ver, details = get_module_version(my_module)
print(details)  # SUCCESS on the setuptools_scm one (the last one)
print(ver)       # prints "1.0.0rc1" instead of "1.0.0-rc1" 

This has for example as consequence that the resulting version string is not compliant with semver while the initial git tag was:

from semver import parse_version_info
parse_version_info("1.0.0-rc1")  # this is ok
parse_version_info("1.0.0rc1")  # this fails

raises ValueError: 1.0.0rc1 is not valid SemVer string

[smarie]

Digging a little bit, this is what happens

• getversion delegates to its plugin_setuptools_scm.py that calls setuptools_scm.get_version(<root_folder>)

• setuptools_scm.get_version :

  • executes git describe --dirty --tags --long --match *.* to get the correct version number 1.0.0-rc1
  • uses pkg_resources.parse_version(version_string) to get a Version object with the correct information, and stores it in it own wrapping object's version.tag attribute.
  • uses the string representation (__str__) of this Version object in the various "version scheme" string formatting methods

The issue is that since pkg_resources version 0.6a9 the string representation of a Version removes the dash !!

from pkg_resources import parse_version
ver_str = parse_version("1.0.0-rc1")
print(ver_str)  # yields 1.0.0rc1 !!!

There are several ways we can fix this:

• to secure the usage of the output of get_version for use for example with semver, I will add today a fix that will use a custom version scheme.

• this could be fixed in setuptools_scm. It would be my preferred option, since the default versioning scheme should probably closely match the tag actually visible in github, without removing dashes.

• this could be fixed in pkg_resources. I am not sure that this should/could happen for retrocompatibility issues, and since this is not at all tied to git scm