https://docs.djangoproject.com/en/2.2/topics/pagination/

Paginator objects

The Paginator class has this constructor:

class Paginator(object_listper_pageorphans=0allow_empty_first_page=True)[source]

Required arguments

object_list

A list, tuple, QuerySet, or other sliceable object with a count() or __len__() method. For consistent pagination, QuerySets should be ordered, e.g. with an order_by() clause or with a default ordering on the model.

Performance issues paginating large QuerySets

If you’re using a QuerySet with a very large number of items, requesting high page numbers might be slow on some databases, because the resulting LIMIT/OFFSET query needs to count the number of OFFSET records which takes longer as the page number gets higher.

per_page
The maximum number of items to include on a page, not including orphans (see the orphans optional argument below).

Optional arguments

orphans
Use this when you don’t want to have a last page with very few items. If the last page would normally have a number of items less than or equal to orphans, then those items will be added to the previous page (which becomes the last page) instead of leaving the items on a page by themselves. For example, with 23 items, per_page=10, and orphans=3, there will be two pages; the first page with 10 items and the second (and last) page with 13 items. orphans defaults to zero, which means pages are never combined and the last page may have one item.
allow_empty_first_page
Whether or not the first page is allowed to be empty. If False and object_list is empty, then an EmptyPage error will be raised.

Methods

Paginator.get_page(number)[source]

Returns a Page object with the given 1-based index, while also handling out of range and invalid page numbers.

If the page isn’t a number, it returns the first page. If the page number is negative or greater than the number of pages, it returns the last page.

It raises an exception (EmptyPage) only if you specify Paginator(..., allow_empty_first_page=False) and the object_list is empty.

Paginator.page(number)[source]

Returns a Page object with the given 1-based index. Raises InvalidPage if the given page number doesn’t exist.

Attributes

Paginator.count

The total number of objects, across all pages.

Note

When determining the number of objects contained in object_listPaginator will first try calling object_list.count(). If object_list has no count() method, then Paginator will fallback to using len(object_list). This allows objects, such as Django’s QuerySet, to use a more efficient count() method when available.

Paginator.num_pages

The total number of pages.

Paginator.page_range

A 1-based range iterator of page numbers, e.g. yielding [1, 2, 3, 4].

InvalidPage exceptions

exception InvalidPage[source]

A base class for exceptions raised when a paginator is passed an invalid page number.

The Paginator.page() method raises an exception if the requested page is invalid (i.e., not an integer) or contains no objects. Generally, it’s enough to catch the InvalidPage exception, but if you’d like more granularity, you can catch either of the following exceptions:

exception PageNotAnInteger[source]

Raised when page() is given a value that isn’t an integer.

exception EmptyPage[source]

Raised when page() is given a valid value but no objects exist on that page.

Both of the exceptions are subclasses of InvalidPage, so you can handle them both with a simple except InvalidPage.

Page objects

You usually won’t construct Page objects by hand – you’ll get them using Paginator.page().

class Page(object_listnumberpaginator)[source]

A page acts like a sequence of Page.object_list when using len() or iterating it directly.

Methods

Page.has_next()[source]

Returns True if there’s a next page.

Page.has_previous()[source]

Returns True if there’s a previous page.

Page.has_other_pages()[source]

Returns True if there’s a next or previous page.

Page.next_page_number()[source]

Returns the next page number. Raises InvalidPage if next page doesn’t exist.

Page.previous_page_number()[source]

Returns the previous page number. Raises InvalidPage if previous page doesn’t exist.

Page.start_index()[source]

Returns the 1-based index of the first object on the page, relative to all of the objects in the paginator’s list. For example, when paginating a list of 5 objects with 2 objects per page, the second page’s start_index() would return 3.

Page.end_index()[source]

Returns the 1-based index of the last object on the page, relative to all of the objects in the paginator’s list. For example, when paginating a list of 5 objects with 2 objects per page, the second page’s end_index() would return 4.

Attributes

Page.object_list

The list of objects on this page.

Page.number

The 1-based page number for this page.

Page.paginator

The associated Paginator object.

https://docs.djangoproject.com/en/2.2/ref/models/querysets/

https://docs.djangoproject.com/en/2.2/topics/db/queries/#querysets-are-lazy

QuerySets are lazy

QuerySets are lazy – the act of creating a QuerySet doesn’t involve any database activity. You can stack filters together all day long, and Django won’t actually run the query until the QuerySet is evaluated. Take a look at this example:

>>> q = Entry.objects.filter(headline__startswith="What")
>>> q = q.filter(pub_date__lte=datetime.date.today())
>>> q = q.exclude(body_text__icontains="food")
>>> print(q)

Though this looks like three database hits, in fact it hits the database only once, at the last line (print(q)). In general, the results of aQuerySet aren’t fetched from the database until you “ask” for them. When you do, the QuerySet is evaluated by accessing the database. For more details on exactly when evaluation takes place, see When QuerySets are evaluated.

When QuerySets are evaluated

Internally, a QuerySet can be constructed, filtered, sliced, and generally passed around without actually hitting the database. No database activity actually occurs until you do something to evaluate the queryset.

You can evaluate a QuerySet in the following ways:

  • Iteration. A QuerySet is iterable, and it executes its database query the first time you iterate over it. For example, this will print the headline of all entries in the database:

    for e in Entry.objects.all():
    print(e.headline)

    Note: Don’t use this if all you want to do is determine if at least one result exists. It’s more efficient to use exists().

  • Slicing. As explained in Limiting QuerySets, a QuerySet can be sliced, using Python’s array-slicing syntax. Slicing an unevaluatedQuerySet usually returns another unevaluated QuerySet, but Django will execute the database query if you use the “step” parameter of slice syntax, and will return a list. Slicing a QuerySet that has been evaluated also returns a list.

    Also note that even though slicing an unevaluated QuerySet returns another unevaluated QuerySet, modifying it further (e.g., adding more filters, or modifying ordering) is not allowed, since that does not translate well into SQL and it would not have a clear meaning either.

  • Pickling/Caching. See the following section for details of what is involved when pickling QuerySets. The important thing for the purposes of this section is that the results are read from the database.

  • repr(). A QuerySet is evaluated when you call repr() on it. This is for convenience in the Python interactive interpreter, so you can immediately see your results when using the API interactively.

  • len(). A QuerySet is evaluated when you call len() on it. This, as you might expect, returns the length of the result list.

    Note: If you only need to determine the number of records in the set (and don’t need the actual objects), it’s much more efficient to handle a count at the database level using SQL’s SELECT COUNT(*). Django provides a count() method for precisely this reason.

  • list(). Force evaluation of a QuerySet by calling list() on it. For example:

    entry_list = list(Entry.objects.all())
    
  • bool(). Testing a QuerySet in a boolean context, such as using bool()orand or an if statement, will cause the query to be executed. If there is at least one result, the QuerySet is True, otherwise False. For example:

    if Entry.objects.filter(headline="Test"):
    print("There is at least one Entry with the headline Test")

    Note: If you only want to determine if at least one result exists (and don’t need the actual objects), it’s more efficient to use exists().

Paginator Django 分页 When QuerySets are evaluated QuerySets 执行原理 QuerySets are lazy 惰性执行 访问db取数据的时机的更多相关文章

  1. Django——分页功能Paginator

    Django分页功能----Paginator Paginator所需参数: Paginator(object_list,per_page) Paginator常用属性: per_page: 每页显示 ...

  2. django orm 分页(paginator)取数据出现警告manage.py:1: UnorderedObjectListWarning: Pagination may yield inconsistent results with an unordered object_list: <class 'sign.models.Guest'> QuerySet.

    使用django的orm做分页(Paginator)时出现了下面的警告 In [19]: p=Paginator(guest_list,2) manage.py:1: UnorderedObjectL ...

  3. 2 django系列之django分页与templatetags

    preface 当页面出现的条目多的时候,我们就需要使用分页功能了.Django作为一个知名的web框架,自然也提供了分页功能,下面说说它. Python-shell 练练手 在python下入手 先 ...

  4. Django 分页功能

    Django 分页功能比较强大,这边是结合官网的内容写的可以参考 https://docs.djangoproject.com/en/1.9/topics/pagination/ 分页命令行练习案列 ...

  5. 原生的 django 分页

    原始的 django 分页 # 基本 写法 class Paginator(object): def __init__(self, object_list, per_page, orphans=0, ...

  6. django 分页组件

      一.仿django分页功能自己实现 urls.py 1 2 3 4 5 6 7 8 9 from django.conf.urls import url from django.contrib i ...

  7. Django分页(一)

    Django分页(一) 手动实现简单分页 HTML <!DOCTYPE html> <html lang="en"> <head> <me ...

  8. 2019.03.20 mvt,Django分页

    MVT模式   MVT各部分的功能:   M全拼为Model,与MVC中的M功能相同,负责和数据库交互,进行数据处理.       V全拼为View,与MVC中的C功能相同,接收请求,进行业务处理,返 ...

  9. Django 分页查询并返回jsons数据,中文乱码解决方法

    Django 分页查询并返回jsons数据,中文乱码解决方法 一.引子 Django 分页查询并返回 json ,需要将返回的 queryset 序列化, demo 如下: # coding=UTF- ...

随机推荐

  1. spring的事物传递

    Propagation.REQUIRED:默认也是常用的事物级别,在当前事物中执行,不存在事物,则创建新事物执行. Propagation.SUPPORTS:支持使用当前事物,当前事物不存爱,则不使用 ...

  2. sql中筛选条件为空值

    <select id="getEmployeeBasicInformationList" resultType="org.springblade.entity.Al ...

  3. Oracle中除数为0的两种解决办法(decode与nullif)

    Oracle中Decode函数,语句DECODE(tag,''ZCGS'',0,1)=decode(''@corp-No@'',''6010'',1,0) decode(字段或字段的运算,值1,值2, ...

  4. Windows下不同版本的JDK共存

    1.安装jdk7,将C:\Windows\System32目录下的java.exe.javaw.exe.javac.exe删除. 2.安装jdk8,将系统环境变量path中的C:\ProgramDat ...

  5. Unity Package Manager

    (注:Unity 2018.1及以后的版本才可以使用Package Manager.) 一个package是一个容器,里面放的是Assets, Shaders, Textures, plug-ins, ...

  6. vue项目中的字符串每隔4位一个空格

    项目中遇到现实银行卡号的需求所以需要这个方法 我们这里运用 JavaScript replace()方法 replace() 方法用于在字符串中用一些字符替换另一些字符,或替换一个与正则表达式匹配的子 ...

  7. Hbase性能调优(二)

    一.HBase关键参数配置指导 如果同时存在读和写的操作,这两种操作的性能会相互影响.如果写入导致的flush和Compaction操作频繁发生,会占用大量的磁盘IO操作,从而影响读取的性能.如果写入 ...

  8. ceph对接k8s storage class

    简介 对接ceph的rbd和cephfs到k8s中提供持久化存储 环境 主机名 IP role 操作系统 ceph-01 172.16.31.11 mon osd CentOS7.8 ceph-02 ...

  9. 深入理解MySQL索引(上)

    简单来说,索引的出现就是为了提高数据查询的效率,就像字典的目录一样.如果你想快速找一个不认识的字,在不借助目录的情况下,那我估计你的找好长时间.索引其实就相当于目录. 几种常见的索引模型 索引的出现是 ...

  10. Docker学习笔记之Dockerfile

    Dockerfile的编写格式为<命令><形式参数>,命令不区分大小写,但一般使用大写字母.Docker会依据Dockerfile文件中编写的命令顺序依次执行命令.Docker ...