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Django for ISchoolers: Difference between revisions
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TODO: Actually write my own dang installation instructions.
On Debian Wheezy: aptitude install python git python-virtualenv python-django-south python-django python-pip
Prior to starting this tutorial, please install Python, git, pip, virtualenv, Django, and South on your computer. There's instructions for all of those at [http://pystar.org/setup_machine.html http://pystar.org/setup_machine.html].
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Think of branches in git as alternate timelines--you "branch" off of the main timeline of changes to the code into a separate timeline. You can try out a new feature or code setup in a branch, without overwriting or getting in the way of yourself or anybody else working on the code as-is. Sometimes a new idea doesn't work out, and you delete the branch or just let it sit around. Sometimes the new feature works, though, and if so you can merge it into the "real" timeline. You can make as many branches as you want, and branches of branches. You can even share branches with others (though we won't be doing that today). Git's branching system (and its non-centralized architecture generally) make it easy to try out new ideas in code without having to ask permission of everyone else on the project first.
To make your first branch, be sure you've changed directories to the django-for-ischoolers directory and then type <source lang="python">$ git branch my-chunk-1</source> to make your new branch, and <source lang="python">$ git checkout my-chunk-1</source> to switch into that branch. Type <source lang="python">$ git branch</source> to confirm that you're in your new branch (and that you only have two branches so far).
If you're not following this tutorial in class, just run <source lang="python">$ git init</source>.
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)</source>
Suppose a visitor goes to http:// 127.0.0.1:8000/questions/23/.
# which regex pattern is tripped?
# what function is then called?
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Ah. There’s no template yet. Let’s make one!
* Make a qandabear/templates/qandabear directory where templates will live
<source lang="python">
mkdir -p qandabear/templates/qandabear
</source>
Edit qandabear/templates/qandabear/index.html to contain:
<source lang="python">
{% if latest_qs %}
<ul>
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A reference to Django template filters and tags can be found here: [https://docs.djangoproject.com/en/dev/ref/templates/builtins/ https://docs.djangoproject.com/en/dev/ref/templates/builtins/]
Note: One important thing to keep in mind is that the Django templating language is ''NOT'' as powerful as Python or Django proper! There are many functions that Python has that the templating language does not support. For instance, you can't add or subtract within the templating language. Similarly, there are limits to what data the templating language
=== Template inheritance ===
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* Commit your work.
== Oh, CRUD! <!---(plus, ModelForms)--> ==
So, we've got a basic web app, with some templates and dynamic data. But right now the only way we have to edit that data -- add, edit, or delete questions or answers -- is from the command line. Our web app's users can't do it at all! Let's fix that.
A common acronym for these sorts of basic data operations is CRUD -- Create, Read, Update, and Delete. We've already got 'Read' covered. Time to do the rest!
===Create the form===
Recall that the prototype spec allows users to add answers to questions. We are going to use a form for that functionality. As an alternative, we could have used AJAX Requests or some other mechanism.
* Update our question detail template (qandabear/detail.html) to contain an HTML <form> element:
<source lang="html4strict">
{% extends "base.html" %}
{% block content %}
<h1>{{ question.text }}</h1>
<ul>
{% for answer in question.answer_set.all %}
<li>{{ answer.text }} -- <em>Answered at {{ answer.pub_date }} -- {{ answer.votes }} votes</em></li>
{% endfor %}
</ul>
<h2>Add new answer<h2>
<form action="/questions/{{ question.id }}/answer/" method="post">
{% csrf_token %}
<textarea name="answer"></textarea>
<input type="submit" value="Enter" />
</form>
{% endblock content %}
</source>
There is a lot going on there. A quick rundown:
* The above template displays a textarea below the list of previously-submitted answers, with an 'Enter' button.
* We set the form’s action to '/questions/{{ question.id }}/answer/ method="post"'. Normal web pages are requested using GET, but the standards for HTTP indicate that if you are changing data on the server, you must use the POST method. (Whenever you create a form that alters data server-side, use method="post". This tip isn’t specific to Django; it’s just good Web development practice!)
* Since we’re creating a POST form (which can have the effect of modifying data), we need to worry about Cross Site Request Forgeries. Thankfully, you don’t have to worry too hard, because Django comes with a very easy-to-use system for protecting against it. In short, all POST forms that are targeted at internal URLs should have the {% csrf_token %} template tag inside the form.
* The {% csrf_token %} tag requires information from the request object, which is not normally accessible from within the template context. To fix this, a small adjustment needs to be made to the detail view in the views.py file.
* Fix views.py to protect against CSRF hacking:
<source lang="python">
from django.template import RequestContext
from django.shortcuts import get_object_or_404, render_to_response
# ...
def detail(request, question_id):
q = get_object_or_404(Question, pk=question_id)
return render_to_response('qandabear/detail.html', {'question': q}, context_instance=RequestContext(request))
</source>
Notice we also added a function that checks if a 404 is returned for us. This is a common pattern, so there is a pre-built shortcut function for it so we can use fewer lines of code! The details of how the RequestContext works are explained in the documentation for RequestContext.
* Review your work at [http://127.0.0.1:8000/questions/1/ http://127.0.0.1:8000/questions/1/] .
* Save and commit.
===Process the form===
* Add <source lang="python">(r'^questions/(?P<question_id>\d+)/answer/$', 'add_answer'),</source> to urls.py
* Add add_answer() function to views.py:
<source lang="python">
from django.shortcuts import get_object_or_404, render_to_response
from django.http import HttpResponseRedirect, HttpResponse
from django.core.urlresolvers import reverse
from django.template import RequestContext
from qandabear.models import Answer, Question
# ...
def add_answer(request, question_id):
q = get_object_or_404(Question, pk=question_id)
answer_text = q.answer_set.get(pk=request.POST['answer'])
a = Answer(text=answer_text, question=q)
a.save()
# Always return an HttpResponseRedirect after successfully dealing
# with POST data. This prevents data from being posted twice if a
# user hits the Back button.
return HttpResponseRedirect(reverse('qandabear.views.detail', args=(q.id,)))
</source>
This code includes a few things we haven’t covered yet in this tutorial:
* request.POST is a dictionary-like object that lets you access submitted data by key name. In this case, request.POST['answer'] returns the contents of our textarea (named 'answer'), as a string. request.POST values are always strings.
* Note that Django also provides request.GET for accessing GET data in the same way – but we’re explicitly using request.POST in our code, to ensure that data is only altered via a POST call.
* After making and saving our new Answer, the code returns an HttpResponseRedirect rather than a normal HttpResponse. HttpResponseRedirect takes a single argument: the URL to which the user will be redirected (see the following point for how we construct the URL in this case).
* As the Python comment above points out, you should always return an HttpResponseRedirect after successfully dealing with POST data. This tip isn’t specific to Django; it’s just good Web development practice. That way, if the web surfer hits reload, they get the success page again, rather than re-doing the action.
* We are using the reverse() function in the HttpResponseRedirect constructor in this example. This function helps avoid having to hardcode a URL in the view function. It is given the name of the view that we want to pass control to and the variable portion of the URL pattern that points to that view. In this case, using the URLconf we set up in urls.py, this reverse() call will return a string like <source lang="python">'/questions/3/'</source> where the 3 is the value of q.id. This redirected URL will then call the results view to display the final page. Note that you need to use the full name of the view here (including the prefix).
* Restart your dev server.
* Navigate to [http://127.0.0.1:8000/questions/1/ http://127.0.0.1:8000/questions/1/] in your browser and answer the question. You should see your new answer appear in the list of answers.
* Save and commit.
=== Exercises for the reader ===
* We just made it so users could add answers. Make another form, on the index page, to let users add questions!
* Add an 'edit' link to each answer on the detail page, which when clicked takes you to another page where you can edit the answer using a form. When the edit form is filled out, it should take you back to the detail page for the question you came from.
* Add a 'delete' link to each answer on the detail page that, when clicked, deletes the answer and takes you back to that detail page. Huzzah -- we now have full CRUD functionality for Answers!
* Add a try/except to your view to check to see if the answer textarea is empty. If so, instead of making a new answer, return the user to the detail page right away with a warning message that their answer was invalid.
* With more complex models, [https://docs.djangoproject.com/en/dev/topics/forms/modelforms/ ModelForms] are super helpful in generating a form that will fill out your model without having to match up all the fields from scratch yourself. Rewrite QandaBear to use ModelForms!
= Bonus points =
== Static files (aka: Django for designers) ==
TODO: Write this!
[https://docs.djangoproject.com/en/1.3/howto/static-files/ https://docs.djangoproject.com/en/1.3/howto/static-files/]
== AJAX and Django ==
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== Authentication and Users (and reusing apps) ==
[https://docs.djangoproject.com/en/dev/topics/auth/ https://docs.djangoproject.com/en/dev/topics/auth/]
== Deploying to a real live server ==
TODO
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