[Mohit-Ak]

Description

The line_idx column of an empty record DataFrame (e.g. ANISOU for a file without anisotropic records, or HETATM for a protein-only file) comes out as object, while non-empty frames get int64. _construct_df casts every column listed in pdb_records, but line_idx is appended separately and was never cast. This casts it to int64 after the existing dtype loop, as suggested in the issue.

Tested: the new test_line_idx_dtype_for_empty_records fails on main (dtype('O') for ANISOU in 3eiy.pdb) and passes with the change. Full pytest: 144 passed, 3 skipped, 14 xfailed. flake8 reports nothing new on the touched files.

Related issues or pull requests

Fixes #64

Pull Request Checklist

• Added a note to docs/CHANGELOG.md
• Added a unit test in tests/pdb/test_read_pdb.py
• Ran pytest and all tests pass
• Checked for style issues with flake8

[Copilot]

🟢 Approval recommended

The focused implementation correctly resolves the dtype inconsistency and includes appropriate regression coverage.

0 open findings

What changed in this PR

Ensures line_idx consistently uses int64, including empty PDB record DataFrames.

Changes:

• Casts line_idx to np.int64 during DataFrame construction.
• Adds regression coverage for empty records.
• Documents the fix for issue #64.

File	Description
biopandas/​pdb/​pandas_pdb.py	Applies the consistent line_idx dtype.
tests/​pdb/​test_read_pdb.py	Verifies dtypes across record frames.
docs/​CHANGELOG.md	Records the bug fix.

🧠 Review effort: Balanced

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