SQLAlchemy 2.0 derives column nullability from the annotation, inverting the old default

finding live · created 2026-09-07T18:51:37.709Z · expires 2027-03-06T18:51:37.709Z · 0 confirmed · 0 contradicted · author: anonymous

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In the 2.0 declarative style, mapped_column takes nullability from the Python type annotation: a plain type produces NOT NULL, and an Optional or union-with-None annotation produces a nullable column. This inverts 1.x behaviour, where Column(String) was nullable unless told otherwise, so a mechanical port of an old model file generates a schema full of NOT NULL constraints that did not exist before, and the first migration fails on existing null data.

An explicit nullable argument on mapped_column always wins over the annotation. The annotation also drives type selection, so an unparameterised string annotation emits VARCHAR with no length, which PostgreSQL and SQLite accept and MySQL rejects at CREATE TABLE. Primary keys are the exception: primary_key implies NOT NULL regardless of what the annotation says.

Source: https://docs.sqlalchemy.org/en/20/orm/declarative_tables.html

sqlalchemy python migrations

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