nab-markersets

A PEP 508 marker read as the set of environments it selects. packaging answers "does this marker hold here"; this answers "can these two ever both hold", "does one imply the other", and "is this a contradiction".

Experimental. Anything here can change in any release, so pin an exact version. The intent is to land this algebra in packaging itself, and the shape below is what that proposal is being tried against; the discussion is pypa/packaging#448.

>>> from nab_markersets.markersets import MarkerSet
>>> old = MarkerSet.from_marker('python_version < "3.11"')
>>> new = MarkerSet.from_marker('python_version >= "3.12"')
>>> old.is_disjoint(new)
True

A set also holds what a marker string cannot: the full set of an absent marker, the empty set of a contradiction, and complements the grammar cannot spell. witness returns a point in the set, which is what separates two markers that look like one constraint.

>>> minor = MarkerSet.from_marker('python_version >= "3.11"')
>>> exact = MarkerSet.from_marker('python_full_version >= "3.11.0"')
>>> minor.equivalent(exact)
False
>>> (minor & ~exact).witness()["python_full_version"]
'3.11.0.dev0'

Installing

The engine runs on packaging's parse tree and its single-atom evaluator, and two copies of packaging exist: the released one, and the fork nab vendors. An extra picks which.

pip install "nab-markersets[packaging]"
pip install "nab-markersets[nab-vendored-packaging]"

The first copy at packaging>=26.3 is bound, so with both installed the fork wins and a Marker built inside nab keeps the class the algebra tests against. A Marker from the other copy is accepted too. With nothing that new, importing nab_markersets.markersets fails and says what it found.

When to use it

Whether two lock entries can both apply, whether a dependency is reachable inside your requires-python, or what a marker still says once you fix the platform. The guide walks through those, and through what the decisions do not decide: https://nab.readthedocs.io/en/stable/how-to/reason-about-markers.html

The public API

The supported API is the module paths below. Everything else in the package is internal and may be renamed or relocated in any release.

nab_markersets.errors       IntractableMarkerSet, UnserializableMarkerSet
nab_markersets.markersets   DecisionStore, MarkerSet, variable_names

The package root binds no names, so importing nab_markersets pulls in no submodules.

Two things before you call it. A MarkerSet comes from from_marker, full or empty, so pickle.loads fails on one: the constructor it reaches for refuses. And == is structural, over the tree the set was built from, where equivalent is the semantic test.

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Author: Damian Shaw

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MIT
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Python >=3.10

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nab-vendored-packaging packaging

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