String match strategies (PREFIX, SUFFIX, CONTAINS) fail on pandas/narwhals backends

[discreteds]

Summary

The starts_with, ends_with, and contains expression methods fail on pandas-based backends when used with per-row column references. The pandas str accessor receives an Expr object where it expects a plain string.

Discovered during cross-backend performance benchmarking of mountainash-utils-rules.

Affected backends and strategies

Strategy	polars	pandas	narwhals-polars	narwhals-pandas	ibis-duckdb	ibis-sqlite
PREFIX	✅	❌	❌	❌	✅	✅
SUFFIX	✅	❌	❌	❌	✅	✅
CONTAINS	✅	❌	✅	❌	✅	✅

Error messages

pandas / narwhals-pandas:

TypeError: expected a string or tuple, not Expr
  .venv/.../pandas/core/strings/accessor.py:3172: TypeError

narwhals-polars:

TypeError: cannot create expression literal for value of type Expr.
Hint: Pass `allow_object=True` to accept any value and create a literal of type Object.

Root cause

The starts_with, ends_with, and contains compile methods produce column-reference expressions for per-row pattern matching (e.g., matching a context value against each rule's string pattern). The polars and ibis backends handle column references natively in string operations, but:

• pandas passes the Expr object directly to pd.Series.str.startswith() / .endswith() / .contains(), which expect scalar strings
• narwhals wraps pandas and inherits the same limitation; narwhals-polars fails on PREFIX/SUFFIX but not CONTAINS (suggesting partial support)

Reproduction

From mountainash-utils-rules (branch release/26.4.0):

hatch run test:test-perf-target "tests/test_benchmarks.py::TestStrategyIsolation" -v -k "pandas and PREFIX" --benchmark-only

Workaround

mountainash-utils-rules benchmarks skip these combinations:

_STRING_MATCH_STRATEGIES = {MatchStrategy.PREFIX, MatchStrategy.SUFFIX, MatchStrategy.CONTAINS}
_STRING_MATCH_BROKEN_BACKENDS = {"pandas", "narwhals-pandas", "narwhals-polars"}

Related

• ibis-polars backend missing WindowFunction translation — blocks with_row_index #78 — ibis-polars with_row_index (separate issue, same category of backend-specific gaps)

[discreteds]

Partial resolution: narwhals-polars PREFIX/SUFFIX now fixed upstream

narwhals-dev/narwhals#3601 (closing narwhals-dev/narwhals#3581) added expression argument support for str.starts_with and str.ends_with on the polars-backed narwhals path.

Updated status:

Strategy	polars	pandas	narwhals-polars	narwhals-pandas	ibis-duckdb	ibis-sqlite
PREFIX	✅	❌	✅ (fixed)	❌	✅	✅
SUFFIX	✅	❌	✅ (fixed)	❌	✅	✅
CONTAINS	✅	❌	✅	❌	✅	✅

mountainash changes (PR #122):

• Added STARTS_WITH and ENDS_WITH to the _NW_POLARS_FIXED allow-list in tests/cross_backend/argument_types/_test_template.py
• The KNOWN_EXPR_LIMITATIONS registry entries remain (they still apply to narwhals-pandas)

Remaining: pandas and narwhals-pandas backends still fail on all three string match strategies. This is a pandas str accessor limitation — it expects scalar strings, not expressions.