Changed "Don't overuse count() or exists()" example to Python.
@@ -260,47 +260,39 @@ Don't overuse ``count()`` and ``exists()``
If you are going to need other data from the QuerySet, evaluate it immediately.
-For example, assuming an Email model that has a ``body`` attribute and a
-many-to-many relation to User, the following template code is optimal:
-
-.. code-block:: html+django
-
- {% if display_inbox %}
- {% with emails=user.emails.all %}
- {% if emails %}
- <p>You have {{ emails|length }} email(s)</p>
- {% for email in emails %}
- <p>{{ email.body }}</p>
- {% endfor %}
- {% else %}
- <p>No messages today.</p>
- {% endif %}
- {% endwith %}
- {% endif %}
-
+For example, assuming an Email model that has a ``subject`` attribute and a
+many-to-many relation to User, the following code is optimal::
+
+ if display_emails:
+ emails = user.emails.all()
+ if emails:
+ print('You have', len(emails), 'emails:')
+ for email in emails:
+ print(email.subject)
+ else:
+ print('You do not have any emails.')
It is optimal because:
-#. Since QuerySets are lazy, this does no database queries if 'display_inbox'
- is False.
+#. Since QuerySets are lazy, this does no database queries if
+ ``display_emails`` is ``False``.
-#. Use of :ttag:`with` means that we store ``user.emails.all`` in a variable
- for later use, allowing its cache to be re-used.
+#. Storing ``user.emails.all()`` in the ``emails`` variable allows its result
+ cache to be re-used.
-#. The line ``{% if emails %}`` causes ``QuerySet.__bool__()`` to be called,
- which causes the ``user.emails.all()`` query to be run on the database, and
- at the least the first line to be turned into an ORM object. If there aren't
- any results, it will return False, otherwise True.
+#. The line ``if emails`` causes ``QuerySet.__bool__()`` to be called, which
+ causes the ``user.emails.all()`` query to be run on the database. If there
+ aren't any results, it will return ``False``, otherwise ``True``.
-#. The use of ``{{ emails|length }}`` calls ``QuerySet.__len__()``, filling
- out the rest of the cache without doing another query.
+#. The use of ``len(emails)`` calls ``QuerySet.__len__()``, reusing the result
+ cache.
-#. The :ttag:`for` loop iterates over the already filled cache.
+#. The ``for`` loop iterates over the already filled cache.
In total, this code does either one or zero database queries. The only
-deliberate optimization performed is the use of the :ttag:`with` tag. Using
-``QuerySet.exists()`` or ``QuerySet.count()`` at any point would cause
-additional queries.
+deliberate optimization performed is using the ``emails`` variable. Using
+``QuerySet.exists()`` for the ``if`` or ``QuerySet.count()`` for the count
+would each cause additional queries.
Use ``QuerySet.update()`` and ``delete()``
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