Fix flip to handle negative axis correctly

[Copilot]

Fix pt.flip to handle negative axis correctly using normalize_axis_tuple

Summary

Fixed the issue where pt.flip ignored negative axis values by using NumPy's normalize_axis_tuple function, as suggested in the issue.

Root Cause

The old implementation checked if i in axis where axis could contain negative values like -1, but i ranged from 0 to arr.ndim-1. Since -1 is never in [0, arr.ndim-1), the condition never matched, causing negative axes to be ignored.

Solution

Replaced manual axis conversion with normalize_axis_tuple(axis, arr.ndim) which:

• Converts single integers to tuples automatically
• Normalizes negative indices to positive (e.g., -1 → ndim-1)
• Validates axis bounds
• Matches NumPy's behavior exactly
• Follows the pattern already used in moveaxis() and roll()

Changes Made

1. pytensor/tensor/subtensor.py (3 lines changed)

• Added normalize_axis_tuple to imports
• Use normalized_axis = normalize_axis_tuple(axis, arr.ndim) to avoid mypy type error
• This handles both integers and tuples, and normalizes negative indices
• Applied ruff formatting to comply with code style

2. tests/tensor/test_subtensor.py (21 lines added)

• Expanded existing test_flip() function to include negative axis tests
• Tests single negative axis values for all dimensions
• Tests tuples with multiple negative axes (e.g., (-1, -2))
• Tests mixed positive and negative axes (e.g., (0, -1))

Verification

✅ Syntax validated - Both files compile correctly
✅ Mypy validated - No type errors
✅ Ruff validated - Formatting and linting pass
✅ Logic verified - Manual testing confirms the fix works as expected
✅ Edge cases tested - All boundary conditions handled properly
✅ Compatibility checked - Existing uses in pad.py continue to work
✅ Style compliant - All lines under 88 chars per ruff config
✅ Pattern consistent - Uses same approach as other functions in codebase

Example

# Before fix:
pt.flip(x, axis=-1)  # Would NOT flip (ignored) ❌

# After fix:
pt.flip(x, axis=-1)  # Correctly flips last axis ✅

Impact

• Minimal changes: Only 3 lines modified in implementation
• Complete fix: Handles all negative axis cases correctly
• No breaking changes: Existing code continues to work
• Better API: Now matches NumPy's behavior exactly

Original prompt

---

This section details on the original issue you should resolve

<issue_title>pt.flip doesn't handle negative axis correctly</issue_title>
<issue_description>### Describe the issue:

It simply ignores them. We should use numpy normalize_axis_tuple, then we don't need the special line for integer axis either

Reproducable code example:

na

Error message:

na

PyTensor version information:

na

Context for the issue:

No response</issue_description>

<agent_instructions>Add a regression test</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #1627

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[ricardoV94]

This whole test is so overkill but predates the PR

[jessegrabowski]

I'm sure whoever wrote it was a really smart handsome guy who believed in making sure our code works great in every possible case

[jessegrabowski]

approved once mypy is happy :)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.67%. Comparing base (b05acfd) to head (a7a183a).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1628      +/-   ##
==========================================
- Coverage   81.67%   81.67%   -0.01%     
==========================================
  Files         232      232              
  Lines       53132    53131       -1     
  Branches     9410     9409       -1     
==========================================
- Hits        43396    43395       -1     
  Misses       7283     7283              
  Partials     2453     2453              

Files with missing lines	Coverage Δ
pytensor/tensor/subtensor.py	89.44% <100.00%> (-0.01%)	⬇️

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[ricardoV94]

@copilot mypy is unhappy about the subtensor file

[Copilot]

@copilot mypy is unhappy about the subtensor file

Fixed the mypy error by using a separate normalized_axis variable instead of reassigning to the typed parameter (commit d4acd33).

[jessegrabowski]

@copilot now pre-commit is failing

[Copilot]

@copilot now pre-commit is failing

Fixed ruff formatting issues (commit 1bf9ad6).