pep508: add MarkerTree::negate #4996
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It's unclear to me whether this was intentional or not, but I realized that converting a MarkerExpression to a string treated EqualStar and NotEqualStar as Equal and NotEqual, respectively. I tweaked this to match the Display impl for VersionSpecifier. (Negation tests in the next commit cover this change.)
It does what you think it does, for the most part.
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This does pretty much what you might expect: it takes an existing
MarkerTreeand returns a new one that evaluates to the opposite of themarker given for all possible marker environments. For the most part.
There are two main hitches here.
The first is that some
MarkerTreevalues are nonsense. For example,"foo" == "bar"andsys_platform ~= "linux". Both such expressionsalways return false today. Technically, a logical negation would imply
that they would always return true in the result. But this would imply
that negation turns nonsense into sense. We shouldn't do that. So the
negation of nonsense remains nonsense.
The second is that negating compatible version constraints like
python_version ~= "3.6"cannot be done in a single marker expression.Instead, the negation is itself a disjunction of negations. In this
case,
python_version < 3.6 or python_version != 3.6.*.I've added some basic tests to cover this new operation.
The main motivation for adding a negation operation is in service of
fixing bugs #4732, #4687, #4640 and #4992.