python文件操作seek()偏移量,读取指正到指定位置操作

#encoding: utf-8

from sqlalchemy import create_engine,Column,Integer,String,Float,func,and_,or_,\
  DateTime
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
from random import randint
from datetime import datetime

HOSTNAME = '127.0.0.1'
PORT = 3306
DATABASE = 'first_sqlalchemy'
USERNAME = 'root'
PASSWORD = '123456'

#dialect+driver://username:password@host:port/database
DB_URI = "mysql+pymysql://{username}:{password}@{host}:{port}/" \
     "{db}?charset=utf8".format(username=USERNAME,password=PASSWORD,host=HOSTNAME,port=PORT,db=DATABASE)

engine = create_engine(DB_URI)
Base = declarative_base(engine)

# Session = sessionmaker(engine)
# session = Session()
session = sessionmaker(engine)() #Session(**local_kw)

class Article(Base):
  __tablename__ = 'article'
  id = Column(Integer,primary_key=True,autoincrement=True)
  title = Column(String(50),nullable=False)
  create_time = Column(DateTime,default=datetime.now)

  def __repr__(self):
    return '<article:{title}>'.format(title=self.title)

# Base.metadata.drop_all()
#
# Base.metadata.create_all()
#
#
# for x in range(0,100):
#   article = Article(title = 'title%s'%x)
#   session.add(article)
# session.commit()

#第一limit的用法，限制查询多少数据
article = session.query(Article).limit(10).all()#用limit限制只查询10个数据
print(article)

#第二个参数offset的用法，本意是偏移量，在这里就是从多少开始查询
article_offset = session.query(Article).offset(10).all()
print(article_offset)

#offset和limit联合起来用，就相当于python 的字符串和列表、元祖的切片操作
article_offset_limit = session.query(Article).offset(10).limit(5).all()
print(article_offset_limit)

#如果查询最新的10篇文章，就可以用order_by 和 limit 一起用
article_order_by_limit = session.query(Article).order_by(Article.id.desc()).limit(10).all()
print(article_order_by_limit)

#slice,本身就是切片的意思
article_order_by_slice = session.query(Article).order_by(Article.id.desc()).slice(0,10).all()
print(article_order_by_slice)

#还有一个更简单的方法，就想python的列表切片操作

article_list_slice = session.query(Article).order_by(Article.id.desc())[0:10]
print(article_list_slice)