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:
Required arguments¶
object_list
-
A list, tuple,
QuerySet
, or other sliceable object with acount()
or__len__()
method. For consistent pagination,QuerySet
s should be ordered, e.g. with anorder_by()
clause or with a defaultordering
on the model.Performance issues paginating large
QuerySet
sIf you’re using a
QuerySet
with a very large number of items, requesting high page numbers might be slow on some databases, because the resultingLIMIT
/OFFSET
query needs to count the number ofOFFSET
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
, andorphans=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
andobject_list
is empty, then anEmptyPage
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 specifyPaginator(..., allow_empty_first_page=False)
and theobject_list
is empty.
Paginator.
page
(number)[source]¶-
Returns a
Page
object with the given 1-based index. RaisesInvalidPage
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 callingobject_list.count()
. Ifobject_list
has nocount()
method, thenPaginator
will fallback to usinglen(object_list)
. This allows objects, such as Django’sQuerySet
, to use a more efficientcount()
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
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 usinglen()
or iterating it directly.
Methods¶
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 return3
.
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 return4
.
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
QuerySet
s are lazy¶QuerySets
are lazy – the act of creating aQuerySet
doesn’t involve any database activity. You can stack filters together all day long, and Django won’t actually run the query until theQuerySet
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, theQuerySet
is evaluated by accessing the database. For more details on exactly when evaluation takes place, see When QuerySets are evaluated.When
QuerySet
s 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 unevaluatedQuerySet
, but Django will execute the database query if you use the “step” parameter of slice syntax, and will return a list. Slicing aQuerySet
that has been evaluated also returns a list.Also note that even though slicing an unevaluated
QuerySet
returns another unevaluatedQuerySet
, 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 callrepr()
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 calllen()
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 acount()
method for precisely this reason.list(). Force evaluation of a
QuerySet
by callinglist()
on it. For example:entry_list = list(Entry.objects.all())
bool(). Testing a
QuerySet
in a boolean context, such as usingbool()
,or
,and
or anif
statement, will cause the query to be executed. If there is at least one result, theQuerySet
isTrue
, otherwiseFalse
. 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()
.
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