[chalmerlowe]

Problem / Context

BigQuery provides SAFE_CAST(expression AS typename), which behaves identically to CAST except that it returns NULL instead of raising a runtime error when a conversion fails (for example, casting a malformed string 'abc' to INT64). Previously, sqlalchemy-bigquery only compiled standard sqlalchemy.cast(...) calls to CAST(...) and had no built-in way to emit SAFE_CAST(...) or compile SQLAlchemy 2.0's sqlalchemy.try_cast(...).

Solution

• Added SafeCast and the safe_cast(expression, type_) helper function to sqlalchemy_bigquery (and exported both in sqlalchemy_bigquery.__all__) so users on both SQLAlchemy 1.4 and SQLAlchemy 2.x can construct SAFE_CAST(...) expressions that preserve target-type result processors and SQL compilation caching (inherit_cache = True).
• Implemented BigQueryCompiler.visit_try_cast so that both sqlalchemy_bigquery.safe_cast(...) and SQLAlchemy 2.0's standard sqlalchemy.try_cast(...) compile to SAFE_CAST(<expression> AS <type>).
• Added unit tests in tests/unit/test_compiler.py covering column expressions across multiple BigQuery types, literals, NULL expressions, nested safe_cast expressions, and sqlalchemy.try_cast.
• Documented safe_cast and sqlalchemy.try_cast usage in README.rst and docs/README.rst.

Testing

Unit test execution (covering both SQLAlchemy 1.4 and SQLAlchemy 2.x environments) and linting are verified in CI.

Fixes #15779

[gemini-code-assist]

Code Review

This pull request adds support for BigQuery's SAFE_CAST expression by introducing a SafeCast class and a safe_cast helper function, which compiles to SAFE_CAST in BigQuery. It also integrates with SQLAlchemy 2.0's try_cast to compile to SAFE_CAST. The changes include comprehensive unit tests covering various data types, nested casts, and literals, along with updated documentation in the README files. I have no feedback to provide.

[chalmerlowe]

Waiting on Blocker: #18956