What is the purpose of the commit() method in SQLAlchemy?

In SQLAlchemy, the commit() method is used to commit the current transaction to the database. SQLAlchemy operates within a transactional model, where database operations are grouped into transactions. The commit() method is responsible for finalizing any changes made within the current transaction, ensuring that they are persisted in the database.

Here’s a basic outline of how it works:

1. Begin a transaction.
2. Perform database operations (e.g., insert, update, delete).
3. Call commit() to commit the transaction, making the changes permanent.
4. Optionally, handle any exceptions that may occur during the transaction and rollback changes if necessary.

By explicitly calling commit(), you ensure that changes are applied atomically; that is, either all changes are committed successfully, or none of them are applied (in case of an error or explicit rollback).

Here’s a simplified example of using commit() in SQLAlchemy:

from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

# Create an engine
engine = create_engine('sqlite:///example.db')

# Define a base class for declarative
Base = declarative_base()

# Define a model
class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String)

# Create the schema
Base.metadata.create_all(engine)

# Create a session
Session = sessionmaker(bind=engine)
session = Session()

# Create a new user
new_user = User(name='John Doe')
session.add(new_user)

# Commit the transaction to persist the changes
session.commit()

# Close the session
session.close()

In this example, session.commit() is called after adding a new user to the session. This commits the transaction and persists the new user record in the database.