When do I need to use sqlalchemy back_populates?

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When I try SQLAlchemy Relation Example following this guide: Basic Relationship Patterns

I have this code

#!/usr/bin/env python
# encoding: utf-8
from sqlalchemy import create_engine
from sqlalchemy import Table, Column, Integer, ForeignKey
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine("sqlite:///:memory:", echo=True)
Session = sessionmaker(bind=engine)
session = Session()
Base = declarative_base(bind=engine)

class Parent(Base):
    __tablename__ = "parent"
    id = Column(Integer, primary_key=True)
    children = relationship("Child")

class Child(Base):
    __tablename__ = "child"
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey("parent.id"))
    parent = relationship("Parent")

Base.metadata.create_all()

p = Parent()
session.add(p)
session.commit()
c = Child(parent_id=p.id)
session.add(c)
session.commit()
print "children: {}".format(p.children[0].id)
print "parent: {}".format(c.parent.id)

It works well, but in the guide, it says the model should be:

class Parent(Base):
    __tablename__ = "parent"
    id = Column(Integer, primary_key=True)
    **children = relationship("Child", back_populates="parent")**

class Child(Base):
    __tablename__ = "child"
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey("parent.id"))
    **parent = relationship("Parent", back_populates="children")**

Why don"t I need back_populates or backref in my example? When should I use one or the other?

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When do I need to use sqlalchemy back_populates? __del__: Questions

import time
time.sleep(5)   # Delays for 5 seconds. You can also use a float value.

import time
while True:
    print("This prints once a minute.")
    time.sleep(60) # Delay for 1 minute (60 seconds).

from time import sleep
sleep(0.1) # Time in seconds

When do I need to use sqlalchemy back_populates? __del__: Questions

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