How to define a table without primary key with SQLAlchemy?

There is no proper solution for this but there are workarounds for it:

Workaround 1

Adding parameter primary_key to the existing column that is not having a primary key will work.

class SomeTable(Base):
    __table__ = 'some_table'
    some_other_already_existing_column = Column(..., primary_key=True) # just add primary key to it whether or not this column is having primary key or not

Workaround 2

Just declare a new dummy column on the ORM layer, not in actual DB. Just define in SQLalchemy model

class SomeTable(Base):
    __table__ = 'some_table'
    column_not_exist_in_db = Column(Integer, primary_key=True) # just add for sake of this error, dont add in db

There is only one way that I know of to circumvent the primary key constraint in SQL Alchemy - it's to map specific column or columns to your table as a primary keys, even if they aren't primary key themselves. http://docs.sqlalchemy.org/en/latest/faq/ormconfiguration.html#how-do-i-map-a-table-that-has-no-primary-key.