How to define Primary Composite Key in SQLAlchemy

When using fully declared table metadata,use the primary_key=True flag on those columns:

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class Dummy(Base):
    __tablename__ = "dummy"

    pk1 = Column(Integer, primary_key=True)
    pk2 = Column(String, primary_key=True)

This will create the following table (in postgres) :

testdb=> \d dummy;
                              Table "public.dummy"
Column |       Type        |                      Modifiers
-------+-------------------+-----------------------------------------------------
pk1    | integer           | not null default nextval('dummy_pk1_seq'::regclass)
pk2    | character varying | not null
Indexes:
   "dummy_pkey" PRIMARY KEY, btree (pk1, pk2)

See SQLAlchemy documentation