GH-51043: [Python] Reject null required Arrow objects

[1fanwang]

Rationale for this change

Passing null values for required Arrow objects can terminate Python or raise an unrelated AttributeError. Array buffer constructors should reject missing types and child arrays with TypeError.

Fixes #51043.

What changes are included in this PR?

Required schema, file-format, data-type, dictionary, fragment, and child-array arguments reject null before use. The base Array and RunEndEncodedArray buffer constructors require a data type. String constructors retain their existing buffer validation, including null offsets for empty arrays.

Are these changes tested?

Native Arrow 26.0.0-SNAPSHOT reproduced the remaining AttributeErrors before the fix and TypeErrors afterward. The focused buffer tests include dictionary construction and both string variants.

Raw logs

$ python -c 'import pyarrow as pa; pa.Array.from_buffers(type=None, length=0, buffers=[])'
Before:
AttributeError: 'NoneType' object has no attribute 'num_fields'
After:
TypeError: Argument 'type' has incorrect type (expected pyarrow.lib.DataType, got NoneType)

$ python -c 'import pyarrow as pa; pa.RunEndEncodedArray.from_buffers(type=None, length=0, buffers=[])'
Before:
AttributeError: 'NoneType' object has no attribute 'num_fields'
After:
TypeError: Argument 'type' has incorrect type (expected pyarrow.lib.DataType, got NoneType)

$ python -c 'import pyarrow as pa; pa.Array.from_buffers(type=pa.list_(pa.int8()), length=0, buffers=[None, pa.py_buffer(bytes(4))], children=[None])'
Before:
AttributeError: 'NoneType' object has no attribute 'ap'
After:
TypeError: Array child must not be None

$ cd python
$ python -m pytest pyarrow/tests/test_array.py -k from_buffers -q
.............                                                            [100%]
13 passed, 317 deselected, 1 warning in 0.45s

Are there any user-facing changes?

Missing required objects raise TypeError. Valid null buffers keep their existing behavior.

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• GitHub Issue: [Python] Several APIs segfault when required Arrow object arguments are None #51043

[github-actions]

⚠️ GitHub issue #51043 has been automatically assigned in GitHub to PR creator.

[Copilot]

🟡 Changes recommended

DictionaryArray.from_buffers() still allows dictionary=None while dereferencing it unconditionally, so a None argument can still trigger a crash.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR hardens several PyArrow Cython entry points by rejecting None for required Arrow extension objects, preventing null C++ pointer dereferences that can segfault the Python interpreter (GH-51043).

Changes:

• Mark required Schema, FileFormat, and DataType parameters as non-nullable (not None) at the Cython boundary to raise TypeError instead of crashing.
• Add an explicit None check for FileSystemDataset fragments before unwrapping.
• Add focused regression tests covering the newly rejected None arguments.

File summaries

File	Description
python/pyarrow/_dataset.pyx	Reject None fragments and make schema/format non-nullable in FileSystemDataset.
python/pyarrow/_parquet.pyx	Make SortingColumn conversion helpers reject schema=None at the boundary.
python/pyarrow/array.pxi	Make DictionaryArray.from_buffers reject type=None at the boundary.
python/pyarrow/tests/test_dataset.py	Add regression assertions for FileSystemDataset(..., schema=None/format=None) and [None] fragments.
python/pyarrow/tests/parquet/test_metadata.py	Add regression assertions for SortingColumn.* with schema=None.
python/pyarrow/tests/test_array.py	Add regression assertion for DictionaryArray.from_buffers(type=None, ...).

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 1
• Review effort level: Lite

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[github-actions]

⚠️ GitHub issue #51043 has been automatically assigned in GitHub to PR creator.

[Copilot]

🟢 Approval recommended

The changes directly address the null-dereference crash at the Cython boundary and are covered by targeted regression tests for the reported cases.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0 new
• Review effort level: Lite

[AlenkaF]

Are there any other instances of this pattern anywhere else in the Cython bindings? They might produce other errors (like AttributeError) but would still benefit from similar change.

[AlenkaF]

Would it make sense to also update other from_buffer() methods (base Array class too)?

[1fanwang]

Done in e031131.

[AlenkaF]

Are there any other instances of this pattern anywhere else in the Cython bindings?

Ah, ok, there is another PR aiming at other files in PyArrow: #51163

For future work we could try keeping the number of PRs down and tackle similar issues in one.

[Copilot]

🟢 Approval recommended

No unresolved issues were identified, and focused regression tests cover the changes.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0 new
• Review effort level: Lite

[github-actions]

⚠️ GitHub issue #51043 has been automatically assigned in GitHub to PR creator.