FIX: NULL parameter type mapping for VARBINARY columns (#458)

[gargsaumya]

Work Item / Issue Reference

AB#42894

GitHub Issue: #458

---

Summary

This pull request makes a targeted fix to the parameter type mapping logic for NULL values in the mssql_python/cursor.py file. The change improves compatibility and prevents implicit conversion errors when executing queries with NULL parameters.

Parameter mapping improvement:

• Changed the mapping for NULL parameters in the _map_sql_type function to use SQL_UNKNOWN_TYPE instead of SQL_VARCHAR, allowing the driver to correctly determine the column type and avoid conversion errors.

[Copilot]

Pull request overview

This PR fixes a bug (issue #458) where inserting a Python None into a VARBINARY column via a bound parameter would raise an implicit conversion error from varchar to varbinary. The root cause was that the _map_sql_type function mapped None parameters to SQL_VARCHAR, causing the ODBC driver to assume a varchar type. The fix changes this to SQL_UNKNOWN_TYPE (value 0), which triggers the C++ layer (in the SQL_C_DEFAULT handler) to call SQLDescribeParam to determine the actual target column type, allowing correct NULL binding for any column type.

Changes:

• Updated _map_sql_type in cursor.py to use SQL_UNKNOWN_TYPE instead of SQL_VARCHAR when binding a None (NULL) parameter, enabling the driver to infer the correct target column type via SQLDescribeParam.

Comments suppressed due to low confidence (1)

mssql_python/cursor.py:400

• No test covers the exact bug scenario from issue #458: calling execute('insert into bytestest values (?)', None) with None as a single bound parameter to a VARBINARY column. The existing test_varbinarymax_insert_fetch_null uses CAST(NULL AS VARBINARY(MAX)) as a SQL literal rather than a Python None parameter binding, and test_only_null_and_empty_binary uses executemany where the column type is inferred from non-null bytes values in the same column. A test that specifically exercises execute with a standalone None parameter against a VARBINARY column (the regression case from the bug report) should be added to prevent future regressions.

        if param is None:
            logger.debug("_map_sql_type: NULL parameter - index=%d", i)
            return (
                ddbc_sql_const.SQL_UNKNOWN_TYPE.value,
                ddbc_sql_const.SQL_C_DEFAULT.value,
                1,
                0,
                False,
            )

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[github-actions]

📊 Code Coverage Report

🔥 Diff Coverage 72%

🎯 Overall Coverage 77%

📈 Total Lines Covered: 5597 out of 7264 📁 Project: mssql-python

---

Diff Coverage

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

• mssql_python/cursor.py (100%)
• mssql_python/pybind/ddbc_bindings.cpp (62.5%): Missing lines 483-485

Summary

• Total: 11 lines
• Missing: 3 lines
• Coverage: 72%

mssql_python/pybind/ddbc_bindings.cpp

Lines 479-489

  479                         sqlType = SQL_VARCHAR;
  480                         columnSize = 1;
  481                         decimalDigits = 0;
  482                     } else {
! 483                         sqlType = describedType;
! 484                         columnSize = describedSize;
! 485                         decimalDigits = describedDigits;
  486                     }
  487                 }
  488                 dataPtr = nullptr;
  489                 strLenOrIndPtr = AllocateParamBuffer<SQLLEN>(paramBuffers);

---

---

📋 Files Needing Attention

📉 Files with overall lowest coverage (click to expand)

mssql_python.pybind.logger_bridge.hpp: 58.8%
mssql_python.pybind.logger_bridge.cpp: 59.2%
mssql_python.row.py: 66.2%
mssql_python.pybind.ddbc_bindings.h: 67.8%
mssql_python.pybind.connection.connection.cpp: 75.3%
mssql_python.ddbc_bindings.py: 79.6%
mssql_python.pybind.connection.connection_pool.cpp: 79.6%
mssql_python.__init__.py: 84.9%
mssql_python.cursor.py: 84.9%
mssql_python.connection.py: 85.1%

---

🔗 Quick Links

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