In addition to Umar Draz answer, here you can find the documentation:

https://docs.sqlalchemy.org/en/13/core/metadata.html#sqlalchemy.schema.Column.params.server_default

for enum you have to use parenthesis:

server_default=("")

I had the same issue, I resolved it changine 'default' by 'server_default', as following:

alias = db.Column(db.Enum('N','Y'), nullable=False, server_default="N")

Problem

I had a similar issue with Enum and default value. I had the code below which gave me the error:

sqlalchemy.exc.DataError: (psycopg2.errors.InvalidTextRepresentation) invalid input value for enum taskcategory: "todo"
LINE 8:  category taskcategory DEFAULT 'todo' NOT NULL

The code that gave me this error looked roughly like this:

from enum import Enum

from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
from src.sql_models.BaseModel import Base
import enum

import sqlalchemy


class Base(DeclarativeBase):
    __abstract__ = True
    type_annotation_map = {
        enum.Enum: sqlalchemy.Enum(enum.Enum),
    }


class TaskCategory(Enum):
    TODO = "todo"
    DOING = "doing"
    FINISHED = "finished"


class Task(Base):
    __tablename__ = "tasks"

    title: Mapped[str] = mapped_column(nullable=False)
    category: Mapped[TaskCategory] = mapped_column(
        server_default=TaskCategory.TODO.value
    )

Observation

When sqlalchemy was trying to build the tables using

Base.metadata.create_all(database_engine)

I noticed the following amongst all the SQL logs:

sqlalchemy.engine.Engine CREATE TYPE taskcategory AS ENUM ('TODO', 'DOING', 'FINISHED')

So it seems like the enum values that get created in SQLAlchemy are uppercase.

My solution

Simply changing my Enum to using uppercase values solved the issue:

class TaskCategory(Enum):
    TODO = "TODO"
    DOING = "DOING"
    FINISHED = "FINISHED"

Hope this helps anyone.