[sqlalchemy-spanner] Make alembic an optional extra dependency #18584

Feature / Enhancement Description

Currently, alembic is listed as a mandatory dependency in packages/sqlalchemy-spanner/setup.py:

dependencies = [
    "sqlalchemy>=1.1.13",
    "google-cloud-spanner>=3.55.0",
    "alembic",
]

However, a database dialect does not inherently require a schema migration tool. Many applications use sqlalchemy-spanner strictly for SQLAlchemy Core or ORM query execution (e.g. data engineering pipelines, read-only analytics, services managing schema via Terraform or Spanner DDL).

In sqlalchemy-bigquery, alembic is already modeled as an optional extra rather than a core dependency:

extras = {
    "alembic": ["alembic"],
    ...
}

Proposed Changes

1. Move alembic from install_requires / dependencies to extras_require in setup.py:

   extras = {
       "tracing": [...],
       "alembic": ["alembic >= 1.0.0, < 2.0.0"],
   }

2. Note that moving alembic to an extra is potentially a breaking change for existing users who rely on pip install sqlalchemy-spanner to install the alembic CLI tool, so this should be scheduled for an appropriate minor or major version release.

Related

• Pull Request fix(sqlalchemy-spanner): resolve compliance and migration failures with Alembic 1.20 and SQLAlchemy 2.1 #18570 already decouples dialect imports by guarding Alembic-specific @compiles hooks (HAS_ALEMBIC_INSTALLED), providing the prerequisite code separation for this transition.