How to correctly add Foreign Key constraints to SQLite DB using SQLAlchemy

I'm very new to SQLAlchemy and I'm trying to figure it out.

Please have in mind the following test setup:

class Nine(Base):
    __tablename__ = 'nine'
    __table_args__ = (sqlalchemy.sql.schema.UniqueConstraint('nine_b', name='uq_nine_b'), )

    nine_a = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), primary_key=True, autoincrement=False, nullable=False)
    nine_b = sqlalchemy.Column(sqlalchemy.String(20), nullable=False)


class Seven(Base):
    __tablename__ = 'seven'
    __table_args__ = (sqlalchemy.sql.schema.PrimaryKeyConstraint('seven_a', 'seven_b'),
                      sqlalchemy.sql.schema.Index('fk_seven_c_nine_a_idx', 'seven_c'),)

    seven_a = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), nullable=False)
    seven_b = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), nullable=False)
    seven_c = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), sqlalchemy.ForeignKey('nine.nine_a'), nullable=False)
    seven_d = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), nullable=False)

    nine = sqlalchemy.orm.relationship(Nine, backref=sqlalchemy.orm.backref('seven'), uselist=False)


class Three(Base):
    __tablename__ = 'three'
    __table_args__ = (sqlalchemy.sql.schema.UniqueConstraint('three_b', 'three_c', name='uq_three_b_c'),
                      sqlalchemy.sql.schema.Index('fk_three_c_seven_a_idx', 'three_c'), )

    three_a = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), primary_key=True, autoincrement=True, nullable=False)
    three_b = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), nullable=False)
    three_c = sqlalchemy.Column(sqlalchemy.dialects.sqlite.INTEGER(), sqlalchemy.ForeignKey('seven.seven_a'), nullable=False)

    seven = sqlalchemy.orm.relationship(Seven, backref=sqlalchemy.orm.backref('three'), uselist=False)

That translates into the following DDLs:

CREATE TABLE nine (
    nine_a INTEGER NOT NULL, 
    nine_b VARCHAR(20) NOT NULL, 
    PRIMARY KEY (nine_a), 
    CONSTRAINT uq_nine_b UNIQUE (nine_b)
);

CREATE TABLE seven (
    seven_a INTEGER NOT NULL, 
    seven_b INTEGER NOT NULL, 
    seven_c INTEGER NOT NULL, 
    seven_d INTEGER NOT NULL, 
    PRIMARY KEY (seven_a, seven_b), 
    FOREIGN KEY(seven_c) REFERENCES nine (nine_a)
);

CREATE INDEX fk_seven_c_nine_a_idx ON seven (seven_c);

CREATE TABLE three (
    three_a INTEGER NOT NULL, 
    three_b INTEGER NOT NULL, 
    three_c INTEGER NOT NULL, 
    PRIMARY KEY (three_a), 
    CONSTRAINT uq_three_b_c UNIQUE (three_b, three_c), 
    FOREIGN KEY(three_c) REFERENCES seven (seven_a)
);

CREATE INDEX fk_three_c_seven_a_idx ON three (three_c);

All tables are empty. Then, the following code statements:

session.add(Nine(nine_a=1, nine_b='something'))
session.add(Nine(nine_a=2, nine_b='something else'))
session.commit()

session.add(Seven(seven_a=7, seven_b=7, seven_c=7, seven_d=7))
session.commit()

session.add(Three(three_a=3, three_b=3, three_c=3))
sessionDB.commit()

Can somebody please explain why is the above code snippet executing without errors? Should't the FK constraints stop from inserting a new row into seven or three? I assume there is something wrong with how the FKs are described in the classes themselves, but I don't know where the problem is (and how to fix it).

[Edit 1]

Adding __table_args__ for all classes (forgot to include them).

[Edit 2]

Adding DDLs for further reference.

via Chebli Mohamed