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

 

 

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

Paginator objects¶

The Paginator class has this constructor:

class Paginator(object_list, per_page, orphans=0, allow_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_list, Paginator 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_list, number, paginator)[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(), or, and 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().