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Code Issues 32 Releases Wiki Activity

Removed distracting note from tutorial 4.

Browse Source 
The note on a possible race condition is inappropriate in this
tutorial setting. To quote Diátaxis:

> Your job is to guide the learner to a successful conclusion. There
> may be many interesting diversions along the way … - ignore them.

Co-Authored-By: Ryan Hiebert <ryan@ryanhiebert.com>
This commit is contained in:
Carlton Gibson 2024-02-20 16:26:21 +01:00 committed by Mariusz Felisiak
parent 98e6f2396c
commit 0a646c8e08
1 changed files with 5 additions and 14 deletions
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19
docs/intro/tutorial04.txt
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@ -74,6 +74,7 @@ create a real version. Add the following to ``polls/views.py``:
.. code-block:: python .. code-block:: python
:caption: ``polls/views.py`` :caption: ``polls/views.py``
from django.db.models import F
from django.http import HttpResponse, HttpResponseRedirect from django.http import HttpResponse, HttpResponseRedirect
from django.shortcuts import get_object_or_404, render from django.shortcuts import get_object_or_404, render
from django.urls import reverse from django.urls import reverse
@ -97,7 +98,7 @@ create a real version. Add the following to ``polls/views.py``:
}, },
) )
else: else:
selected_choice.votes += 1 selected_choice.votes = F("votes") + 1
selected_choice.save() selected_choice.save()
# Always return an HttpResponseRedirect after successfully dealing # Always return an HttpResponseRedirect after successfully dealing
# with POST data. This prevents data from being posted twice if a # with POST data. This prevents data from being posted twice if a
@ -123,6 +124,9 @@ This code includes a few things we haven't covered yet in this tutorial:
:exc:`KeyError` and redisplays the question form with an error :exc:`KeyError` and redisplays the question form with an error
message if ``choice`` isn't given. message if ``choice`` isn't given.
* ``F("votes") + 1`` :ref:`instructs the database
<avoiding-race-conditions-using-f>` to increase the vote count by 1.
* After incrementing the choice count, the code returns an * After incrementing the choice count, the code returns an
:class:`~django.http.HttpResponseRedirect` rather than a normal :class:`~django.http.HttpResponseRedirect` rather than a normal
:class:`~django.http.HttpResponse`. :class:`~django.http.HttpResponse`.
@ -190,19 +194,6 @@ Now, go to ``/polls/1/`` in your browser and vote in the question. You should se
results page that gets updated each time you vote. If you submit the form results page that gets updated each time you vote. If you submit the form
without having chosen a choice, you should see the error message. without having chosen a choice, you should see the error message.
.. note::
The code for our ``vote()`` view does have a small problem. It first gets
the ``selected_choice`` object from the database, then computes the new
value of ``votes``, and then saves it back to the database. If two users of
your website try to vote at *exactly the same time*, this might go wrong:
The same value, let's say 42, will be retrieved for ``votes``. Then, for
both users the new value of 43 is computed and saved, but 44 would be the
expected value.
This is called a *race condition*. If you are interested, you can read
:ref:`avoiding-race-conditions-using-f` to learn how you can solve this
issue.
Use generic views: Less code is better Use generic views: Less code is better
====================================== ======================================