alembic autogeneration "false-positively" detects change to comments from the empty string to None

[fmv1992]

Describe the bug Setting column comment to "" causes autogenerate to create changes even though no changes were made.

Expected behavior I expected Column("mycol", sa.types.String(), comment="") to be treated as it is, and that sequential autogenerate commands would generate empty migrations.

To Reproduce Please try to provide a Minimal, Complete, and Verifiable example, with the migration script and/or the SQLAlchemy tables or models involved. See also Reporting Bugs on the website.

• Code:

  class MyTable(Base):
      __tablename__ = "my_table"
      mycol = ColumnNonNull(
          sa.types.String(),
          primary_key=True,
          comment="""
  """.strip(),
      )
  

• Table generation ok as expected:

  def upgrade() -> None:
      # ### commands auto generated by Alembic - please adjust! ###
      op.create_table(
          "my_table",
          sa.Column("mycol", sa.String(), nullable=False),
          sa.PrimaryKeyConstraint("mycol"),
          schema="public",
      )
      # ### end Alembic commands ###
  
  
  def downgrade() -> None:
      # ### commands auto generated by Alembic - please adjust! ###
      op.drop_table("my_table", schema="public")
      # ### end Alembic commands ###
  

• Table generation not ok:

  def upgrade() -> None:
      # ### commands auto generated by Alembic - please adjust! ###
      op.alter_column(
          "my_table",
          "mycol",
          existing_type=sa.VARCHAR(),
          comment="",
          existing_nullable=False,
      )
      # ### end Alembic commands ###
  
  
  def downgrade() -> None:
      # ### commands auto generated by Alembic - please adjust! ###
      op.alter_column(
          "my_table",
          "mycol",
          existing_type=sa.VARCHAR(),
          comment=None,
          existing_comment="",
          existing_nullable=False,
      )
      # ### end Alembic commands ###
  

  • Expected:

    def upgrade() -> None:
        # ### commands auto generated by Alembic - please adjust! ###
        pass
        # ### end Alembic commands ###
    
    
    def downgrade() -> None:
        # ### commands auto generated by Alembic - please adjust! ###
        pass
        # ### end Alembic commands ###
    

Error

No errors per se.

Fix If I set the column to anything other than "" it then starts to work as expected.

Versions.

• OS: Ubuntu 20.04.5 LTS
• Python: 3.8
• Alembic: alembic==1.8.1
• Database: PostgreSQL

[CaselIT]

Hi,

this is probably because somewhere there is something like an if comment and somewhere else there is a if commnet is not None. Thanks for reporting, but I'm not sure this warrants a fix, since the workaround is trivial