FIX: gate SQL_WVARCHAR output-converter fallback to string/binary columns (#691)

[jahnvi480]

Work Item / Issue Reference

AB#46685

GitHub Issue: #691

---

Summary

A SQL_WVARCHAR output converter was being applied as an unconditional
catch-all to non-string columns. Cursor._build_converter_map fell back to the
converter registered for SQL_WVARCHAR for any column that had no direct
type-keyed converter, so registering a single SQL_WVARCHAR converter mangled
INT / DECIMAL / DATE values (e.g. 42 came back converted).

This PR gates that legacy fallback on the column's mapped Python type being
str/bytes, mirroring the isinstance(value, (str, bytes)) guard already
present in Row._apply_output_converters. The optimized and fallback apply
paths now behave consistently.

Root cause

sql_type = desc[1] is the column's mapped Python type. The fallback ran
whenever no direct converter matched, regardless of the column type:

if converter is None:
    converter = self.connection.get_output_converter(SQL_WVARCHAR)

Row._apply_output_converters gates the same fallback on
isinstance(value, (str, bytes)), so the two paths disagreed.

Fix

if converter is None and sql_type in (str, bytes):
    converter = self.connection.get_output_converter(ddbc_sql_const.SQL_WVARCHAR.value)

bytes is intentionally included for parity with Row._apply_output_converters
(a SQL_WVARCHAR converter still applies to VARBINARY, which the new test
asserts). Also removed the now-redundant in-loop ConstantsDDBC import.

Test

tests/test_003_connection.py::test_output_converter_wvarchar_not_catchall_for_non_string_columns_gh691
registers only a SQL_WVARCHAR converter and selects INT, DECIMAL, DATE,
NVARCHAR, and VARBINARY columns. It asserts the non-string columns are
untouched, the string/binary columns are converted, and — via a spy — that the
converter is invoked exactly twice (only the str/bytes columns).

The call-count assertion matters: the optimized apply path swallows converter
exceptions, so a plain value.decode() converter raises AttributeError on
int/Decimal/date and is silently swallowed. A value-only assertion would
therefore pass with and without the fix. Verified (via git stash) that the
test fails on the unfixed code and passes once the fallback is gated.

Validation

• black --check --line-length=100 clean on both files.
• tests/test_003_connection.py: 151 passed, 2 skipped.
• tests/test_004_cursor.py: 517 passed, 4 skipped.
• New test proven to fail without the fix and pass with it.

[Copilot]

Pull request overview

This PR fixes an output-converter dispatch bug in the optimized fetch path: a converter registered for SQL_WVARCHAR was being treated as an unconditional fallback for any column without a direct converter, which could corrupt non-string types (e.g., INT, DECIMAL, DATE). The change gates the legacy SQL_WVARCHAR fallback so it only applies to columns whose mapped Python type is str or bytes, aligning the optimized converter map path with the existing runtime guard behavior.

Changes:

• Gate the SQL_WVARCHAR fallback in Cursor._build_converter_map() to only apply when desc[1] maps to str/bytes.
• Add a regression test covering mixed INT/DECIMAL/DATE with NVARCHAR and VARBINARY, including a converter invocation count assertion to detect the previously-masked behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
mssql_python/cursor.py	Restricts the legacy SQL_WVARCHAR fallback converter to string/binary-mapped columns during converter map construction.
tests/test_003_connection.py	Adds a regression test ensuring SQL_WVARCHAR converters do not act as a catch-all for non-string columns and asserts call count.

[github-actions]

📊 Code Coverage Report

🔥 Diff Coverage 100%

🎯 Overall Coverage 81%

📈 Total Lines Covered: 7067 out of 8675 📁 Project: mssql-python

---

Diff Coverage

Diff: main...HEAD, staged and unstaged changes

• mssql_python/cursor.py (100%)

Summary

• Total: 2 lines
• Missing: 0 lines
• Coverage: 100%

---

📋 Files Needing Attention

📉 Files with overall lowest coverage (click to expand)

mssql_python.pybind.logger_bridge.cpp: 59.2%
mssql_python.pybind.ddbc_bindings.h: 59.9%
mssql_python.pybind.logger_bridge.hpp: 70.8%
mssql_python.pybind.ddbc_bindings.cpp: 76.3%
mssql_python.__init__.py: 77.3%
mssql_python.row.py: 77.6%
mssql_python.ddbc_bindings.py: 79.6%
mssql_python.pybind.connection.connection_pool.cpp: 81.4%
mssql_python.pybind.connection.connection.cpp: 83.7%
mssql_python.connection.py: 84.7%

---

🔗 Quick Links

⚙️ Build Summary	📋 Coverage Details
View Azure DevOps Build	Browse Full Coverage Report