Compatible release clause behaviour differs from PEP440 for versions with prerelease

[timdiels]

PEP440 gives the example that ~= 1.4.5a4 should be equivalent to >= 1.4.5a4, == 1.4.*, however:

>>> from packaging.specifiers import SpecifierSet
>>> '1.4.6' in SpecifierSet('~=1.4.5a4', prereleases=True)  # incorrect
False
>>> '1.4.6' in SpecifierSet('>=1.4.5a4, ==1.4.*', prereleases=True)  # correct
True

It appears there has been a misinterpretation, treating ~=1.4.5a4 as >= 1.4.5a4, == 1.4.5.*. Compare this to PEP440: "If a pre-release, post-release or developmental release is named in a compatible release clause as V.N.suffix , then the suffix is ignored when determining the required prefix match".

My environment:

(venv) $ pip freeze
appdirs==1.4.2
attrs==16.3.0
chicken-turtle-util==4.0.1
lxml==3.7.3
numpy==1.12.0
packaging==16.8
patool==1.12
pkginfo==1.4.1
py==1.4.32
pypandoc==1.3.3
pyparsing==2.2.0
pytest==3.0.6
pytest-capturelog==0.7
requests==2.13.0
six==1.10.0
(venv) $ python --version
Python 3.5.2

[di]

It seems like that wording in the PEP 440 is indeed at odds with the actual implementation, which explicitly treats prereleases differently:

packaging/packaging/specifiers.py

Lines 452 to 462 in fc86d7e

	# We want everything but the last item in the version, but we want to
	# ignore post and dev releases and we want to treat the pre-release as
	# it's own separate segment.
	prefix = ".".join(
	list(
	itertools.takewhile(
	lambda x: (not x.startswith("post") and not x.startswith("dev")),
	_version_split(spec),
	)
	)[:-1]
	)

Currently prereleases are not ignored, but treated as it's own segment, so you get this instead:

~=1.4.5a4 as
>=1.4.5a4, ==1.4.5.*

Given that @dstufft was the one to originally implement it like this, perhaps he has an idea about why?

[jiasli]

This bug becomes more outstanding as pip's new resolver is more strict and fails due to incompatible dependencies.