Question

I am looking for a way to introspect SQLAlchemy ORM classes/entities to determine the types and other constraints (like maximum lengths) of an entity’s properties.

For example, if I have a declarative class:

class User(Base):
    __tablename__ = "USER_TABLE"

    id = sa.Column(sa.types.Integer, primary_key=True)
    fullname = sa.Column(sa.types.String(100))
    username = sa.Column(sa.types.String(20), nullable=False)
    password = sa.Column(sa.types.String(20), nullable=False)
    created_timestamp = sa.Column(sa.types.DateTime, nullable=False)

I would want to be able to find out that the ‘fullname‘ field should be a String with a maximum length of 100, and is nullable. And the ‘created_timestamp‘ field is a DateTime and is not nullable.

Answer 1

Something like:

table = User.__table__
field = table.c["fullname"]
print "Type", field.type
print "Length", field.type.length
print "Nullable", field.nullable

EDIT:

The upcoming 0.8 version has a New Class Inspection System:

New Class Inspection System

Status: completed, needs docs

Lots of SQLAlchemy users are writing systems that require the ability
to inspect the attributes of a mapped class, including being able to
get at the primary key columns, object relationships, plain
attributes, and so forth, typically for the purpose of building
data-marshalling systems, like JSON/XML conversion schemes and of
course form libraries galore.

Originally, the Table and Column model were the original inspection
points, which have a well-documented system. While SQLAlchemy ORM
models are also fully introspectable, this has never been a fully
stable and supported feature, and users tended to not have a clear
idea how to get at this information.

0.8 has a plan to produce a consistent, stable and fully documented API for this purpose, which would provide an inspection system that
works on classes, instances, and possibly other things as well. While
many elements of this system are already available, the plan is to
lock down the API including various accessors available from such
objects as Mapper, InstanceState, and MapperProperty:

(follow the link for more info)