Make to_string match pandas (drop repr-style truncation/footer)

[galipremsagar]

Description

Makes Series.to_string() / DataFrame.to_string() match pandas, fixing the cudf.pandas failures in tests/io/formats/test_to_string.py (16 cases).

Previously cuDF's to_string was just str(self) (i.e. repr), so it took no arguments, truncated based on display.max_rows/display.max_columns, rendered nulls as <NA>, and appended a dtype/name/length footer. pandas' to_string does none of these by default and accepts a rich set of formatting arguments.

IndexedFrame.to_string now accepts the pandas signature and delegates to pandas' formatter (self.to_pandas().to_string(*args, **kwargs)), so cuDF prints identically to pandas — as the docstring already promised.

Behavior change

to_string() no longer truncates based on the display.max_rows/display.max_columns options (use the max_rows/max_cols arguments, matching pandas) and no longer appends the dtype/name/length footer or renders nulls as <NA> by default. repr / __repr__ are unchanged, so interactive display still truncates and shows the footer as before.

cuDF tests that encoded the old repr-style to_string output are updated to the pandas-matching behavior (passing max_rows/max_cols explicitly where truncation is intended), and two tests are added to lock in the new contract (test_to_string_ignores_display_options, test_series_to_string_no_dtype_footer). The now-passing test_to_string.py xfail entries are removed from the cudf.pandas plugin.

Verification

• pandas test_to_string.py under cudf.pandas (with plugin): 93 passed, 1 xfailed, 0 failed, 0 XPASS.
• cuDF to_string + repr unit suites: pass (repr unchanged).
• Broad regression across all dataframe/series methods + other to_string callers: 12496 passed, 0 failed.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📥 Commits

Reviewing files that changed from the base of the PR and between e6e682a and 3acf2cc.

📒 Files selected for processing (2)

• python/cudf/cudf/core/dataframe.py
• python/cudf/cudf/core/series.py

🚧 Files skipped from review as they are similar to previous changes (2)

• python/cudf/cudf/core/series.py
• python/cudf/cudf/core/dataframe.py

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📝 Walkthrough

Summary by CodeRabbit

• New Features
  • Added pandas-compatible to_string() to both DataFrame and Series, including support for common formatting and display-control options (and correct buf behavior).
• Tests
  • Updated and refactored to_string() expectations to be generated from pandas, added coverage that cudf.to_string() ignores pandas display limits, verified Series output has no dtype footer by default, and refreshed IO/compression expected-failure annotations.
• Chores
  • Updated SPDX copyright headers.

Walkthrough

IndexedFrame.to_string is removed; DataFrame.to_string and Series.to_string are added with full pandas parameter support and pandas delegation. Tests are refactored to pass truncation arguments explicitly and compute expected output from pandas; expected-failure entries are updated.

Changes

DataFrame and Series to_string pandas delegation

Layer / File(s)	Summary
DataFrame and Series to_string implementations python/cudf/cudf/core/indexed_frame.py, python/cudf/cudf/core/dataframe.py, python/cudf/cudf/core/series.py	IndexedFrame.to_string base implementation is removed. DataFrame.to_string is added with full pandas parameter set (formatting, truncation, layout options) and delegates to self.to_pandas().to_string(). Series.to_string is added with pandas Series parameters and same delegation pattern. Copyright headers updated.
Test refactor and behavioral validation python/cudf/cudf/tests/dataframe/methods/test_to_string.py, python/cudf/cudf/tests/series/methods/test_to_string.py	Existing DataFrame and Series to_string tests are refactored to pass max_rows/max_cols directly to cudf and compute expected output via to_pandas().to_string(). Two new DataFrame tests are added asserting display-option independence and dtype footer absence. Series init-None test switches from repr() to to_string() for output comparison. Copyright headers updated.
Expected-failure metadata updates python/cudf/cudf/pandas/scripts/pandas-testing-plugin.py	Removes AssertionError-based expected-failure entries for test_to_string.py cases that no longer apply. Adds three test_to_csv.py::TestToCSV entries with TODO placeholder reasons. Removes test_to_string_index_with_nan expected-failure entry. Updates test_with_s3_url expected-failure reasons to reference moto server requirement at http://localhost:5000.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested labels

improvement

Suggested reviewers

• mroeschke
• vyasr
• rjzamora
• brandon-b-miller

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 52.94% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly summarizes the main change: making to_string match pandas behavior by dropping repr-style truncation and footer functionality.
Description check	✅ Passed	The description comprehensively explains the changes, behavioral impacts, rationale, and includes verification results that align with the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@python/cudf/cudf/core/series.py`:
- Around line 852-871: The docstring for the Series.to_string method is
incomplete and does not document the parameters (such as buf, max_rows, etc.) or
return value semantics. Add a Parameters section listing all accepted arguments
and their descriptions, including clarification that the buf parameter controls
whether the method returns a string or None. Add a Returns section documenting
the return type and behavior, explicitly noting that when buf is provided, the
method returns None, and when buf is None (the default), it returns a formatted
string. Ensure the docstring follows pandas-style documentation conventions to
maintain consistency with the codebase.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 90d8b2e4-dc39-4d69-b948-3206e0042ccf

📥 Commits

Reviewing files that changed from the base of the PR and between 656f672 and e6e682a.

📒 Files selected for processing (6)

• python/cudf/cudf/core/dataframe.py
• python/cudf/cudf/core/indexed_frame.py
• python/cudf/cudf/core/series.py
• python/cudf/cudf/pandas/scripts/pandas-testing-plugin.py
• python/cudf/cudf/tests/dataframe/methods/test_to_string.py
• python/cudf/cudf/tests/series/methods/test_to_string.py

💤 Files with no reviewable changes (1)

• python/cudf/cudf/core/indexed_frame.py

✅ Files skipped from review due to trivial changes (1)

• python/cudf/cudf/tests/series/methods/test_to_string.py

🚧 Files skipped from review as they are similar to previous changes (2)

• python/cudf/cudf/tests/dataframe/methods/test_to_string.py
• python/cudf/cudf/pandas/scripts/pandas-testing-plugin.py

[galipremsagar]

/merge