markers: recognize empty python_version markers

[radoering]

Without this fix, the intersection of python_version > "3.8" and python_version < "3.9" is not detected as empty marker.

Related to: python-poetry/poetry#10249

• Added tests for changed code.
• Updated documentation for changed code.

Summary by Sourcery

Tests:

• Add tests to verify that the intersection of mutually exclusive markers is correctly identified as empty.

[sourcery-ai]

Reviewer's Guide by Sourcery

This pull request fixes an issue where the intersection of certain python_version markers (e.g., python_version > "3.8" and python_version < "3.9") was not being correctly detected as an empty marker. The fix involves modifying the _merge_single_markers function to properly identify these cases and return an EmptyMarker. New tests have been added to verify the fix, and existing tests have been updated to reflect the changes.

Updated class diagram for markers

classDiagram
    class Marker {
        <<Abstract>>
        +name: str
        +value: str
    }
    class EmptyMarker {
        +EmptyMarker()
    }

    Marker <|-- EmptyMarker

    class _merge_single_markers {
        +marker1: Marker
        +marker2: Marker
        +merge_class: type
        +result_constraint: Constraint
        +result_marker: Marker
    }

    note for _merge_single_markers "Modified to detect empty python_version intersections"

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File-Level Changes

Change	Details	Files
Added tests to verify that the intersection of certain python_version markers is correctly detected as empty.	Added a new test case test_single_marker_intersect_is_empty with several scenarios where the intersection of two python_version markers should result in an empty marker.	tests/version/test_markers.py
Modified the _merge_single_markers function to handle cases where the intersection of python_version markers results in an empty marker.	Added logic to detect when the intersection of two python_version markers results in an empty constraint. When an empty constraint is detected, the function now returns an EmptyMarker.	src/poetry/core/version/markers.py
Updated existing tests to reflect changes in marker intersection logic.	Modified test_intersection_no_endless_recursion to assert the correct marker string representation after intersection. Removed a redundant test case from test_multi_marker_union_multi_is_multi.	tests/version/test_markers.py

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[sourcery-ai]

Hey @radoering - I've reviewed your changes - here's some feedback:

Overall Comments:

• The added tests cover a good range of scenarios for empty marker intersections.

Here's what I looked at during the review

• 🟢 General issues: all looks good
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

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[radoering]

The last test is the relevant one but since I did not find such a basic test function that tests for empty single marker intersections, I just added some additional tests.

[radoering]

The test does not make sense because both markers are empty. Similar (but correct) tests are above and below.

[radoering]

The original marker is actually empty and does not trigger a potential endless recursion anymore. I checked that the changed marker triggers the recursion error without the fix from #832.