alembic – TypeError: Boolean value of this clause is not defined

When trying to add columns to a table using alembic, I ran into the following error:

File "/path/to/site-packages/sqlalchemy/sql/elements.py", line 460, in __bool__
    raise TypeError("Boolean value of this clause is not defined")

Here’s the code that I was using:

def upgrade():
   op.add_column('table', 
        sa.Column('field_one', sa.String(50), nullable=False),
        sa.Column('field_two', sa.String(50), nullable=True)
   )

I’d copied and pasted it from somewhere online and just added the second column as I wanted to add multiple columns. This is what broke SQLAlchemy.

Unfortunately, the error message isn’t very descriptive. It sent me on a wild goose chase through the SQLAlchemy docs before I finally decided just to delete code until it worked. After realising that the second item was causing issues, I tried using op.add_columns rather than op.add_column but was greeted by the following error:

AttributeError: 'module' object has no attribute 'add_columns'

To get it working, you have to add the columns one at a time:

def upgrade():
    op.add_column('table', sa.Column('field_one', sa.String(50), nullable=False))
    op.add_column('table', sa.Column('field_two', sa.String(50), nullable=True))

It’s worth noting that the same applies to dropping columns too:

def downgrade():
    op.drop_column("table", "field_one")
    op.drop_column("table", "field_two")

Michael is a polyglot software engineer, committed to reducing complexity in systems and making them more predictable. Working with a variety of languages and tools, he shares his technical expertise to audiences all around the world at user groups and conferences. You can follow @mheap on Twitter

Thoughts on this post

jake 2014-07-18

after saying “add them one at a time:” you still use the `add_columns` method

michael 2014-08-04

Thanks, updated the post 🙂

agodber 2014-07-25

Good tip, spared me some needless digging.

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