Newbie mistakes

Please feel free to share the things that tripped you up when you started with Django. We'll try to improve Django's error handling to catch such mistakes in the future.

POST to views loses POST data

Symptom

You've got a form that does a POST to some view, and the view doesn't get the POST data from the form.

Probable cause

You might be missing the / at the end of the form action. If that's the case, the CommonMiddleware (check your MIDDLEWARE_CLASSES redirects to the exact name of the view - and that always includes a trailing slash. Because it does so by using the standard HTTP redirection through Location header, it can't pass on the POST data - and that's why it gets lost.

Solution

Always make sure that your form actions end with a slash, like this:

<form action="/registration/login/" method="POST">
<input name="username">
<input name="password" type="password">
</form>

Note that Django now attempts to catch this problem, as of [3109].

URLconf include() misbehaving

Symptom

You're trying to get your URLconf files to work, but getting cryptic errors about how there is no module 'index' (where 'index' is some function you are trying to assign as a view), or module 'foo' has no attribute 'urlpatterns'.

Probable cause

You may have tried to load your view files using include() in the URLconf file (in tutorial 3, this is myproject/settings/urls/main.py). The include() call assumes that the file it's loading is also a URLconf file.

Solution

Remove the include(). Just give the module and function name (e.g., 'myproject.apps.polls.views.polls.index') as a string, with no include() around it.

Blank object names

Symptom

The automatic admin interface is showing nothing (or a single  ) in the "Select [object_type] to change" view.

Probable cause

You may have forgotten to create a __str__() function for your model. Django calls __str__() to find out how to display objects in the admin interface. An alternate cause is the string you return from __str__() includes brackets (an therefore looks like an html tag), and is cleaned up by the strip_tags template filter, resulting in blank entries.

Solution

Add a correct __str__() function (without brackets in the output) to all your models. Make it a habit so it becomes automatic.

Integer & NULLS

Problem

When you have a Field: current_zip = meta.IntegerField(maxlength=5,blank=True)

django will create a not nullable field in the DB. However leaving the field blank (in admin/web) django will try and insert a NULL value in the DB.

Solution

Add null=True:

current_zip = meta.IntegerField(maxlength=5,null=True,blank=True)

Date & NULLS

Same problem as Integer & NULLS with the same solution.

Appending to a list in session doesn't work

Problem

If you have a list in your session, append operations don't get saved to the object.

Solution

Copy the list out of the session object, append to it, then copy it back in:

sessionlist = request.session['my_list']
sessionlist.append(new_object)
request.session['my_list'] = sessionlist

Errors about undefined attributes with one-char names

Problem

You get an AttributeError with some weird attribute name that's only one char long. You don't have that attribute name anywhere in your code.

Solution

Search your model and code for situations where you have to pass a tuple of values and want to pass a tuple with one element - and that element is a string like in this sample:

class META:     
    ...
    admin = meta.Admin(
        list_display = ('total_price'),
        ...
    )

You are just missing a comma in the list_display assignment like this:

class META:     
    ...
    admin = meta.Admin(
        list_display = ('total_price',),
        ...
    )

Remember, in python:

>>> a = (1) ## This causes problems
1
>>> a = (1,) ## These are fine.
(1,)
>>> a = [1]
[1]
>>> a = [1,]
[1]

Since a tuple is expected but a string provided, the code will merrily iterate over the characters of the string instead of the tuple elements - and that's where the single-char attribute names come from. If the commas are consistently causing you problems, try using brackets [] instead of parentheses.

I'm using formfields.FormWrapper and none of my form fields show up

Problem

You are using code similar to that documented ​here, but when you put {{ form.field_name }}, you get nothing.

Solution

Make sure when you create your form object, you are passing in empty dictionaries, not tuples. For example:

    manip = things.AddManipulator()
    form = formfields.FormWrapper(manip, {}, {})

Not:

    manip = things.AddManipulator()
    form = formfields.FormWrapper(manip, (), ())

If you pass in empty tuples for data & errors, it will silently fail to insert your form fields.

Django says "Unable to Open Database File" when using SQLite3

Problem

You're using SQLite3, your DATABASE_NAME is set to the database file's full path, the database file is writeable by Apache, but you still get the above error.

Solution

Make sure Apache can also write to the parent directory of the database. SQLite needs to be able to write to this directory.

QuerySets aren't Lists

Problem

You have been playing with the database API and noticed that a returned query set looks a lot like a list:

>>> from mysite.polls.models import Poll,Choice
>>> Poll.objects.all()
[<Poll: What is up?>, <Poll: What is your favourite colour?>]  ## looks a lot like a list to me

But, it doesn't behave like a list in some cases.

Solution

Here are a couple of cases where the behaviour is not list-like and their solution.

In Python this is how we can test for an empty list:

>>> b=[]
>>> b==[]
True

This doesn't work with a QuerySet. You might try the following but it will fail:

>>> from mysite.polls.models import Poll,Choice
>>> p = Poll.objects.filter(question__startswith='ZZZZZZZZZZZZ')
>>> p
[]
>>> p==[]
False

The way to do it is test for the length of p:

>>> from mysite.polls.models import Poll,Choice
>>> p = Poll.objects.filter(question__startswith='ZZZZZZZZZZZZ')
>>> p
[]
>>> len(p) == 0
True

Another case occurs when you want to retrieve the last member of a QuerySet:

>>> from mysite.polls.models import Poll,Choice
>>> p = Poll.objects.all()
>>> p
[<Poll: What is up?>, <Poll: What is your favourite colour?>]
>>> p[-1]
Traceback (most recent call last):
  File "<console>", line 1, in ?
  File "c:\python24\lib\site-packages\django-0.95-py2.4.egg\django\db\models\query.py", line 98, in
__getitem__
    assert (not isinstance(k, slice) and (k >= 0)) \
AssertionError: Negative indexing is not supported.

The way I get the last member is:

>>> p[len(p)-1]
<Poll: What is your favourite colour?>

Using reserved name on application breaks admin

Symptom

After creating a new app, with a model that validates, trying to log on to the admin causes an error similar to:

ImproperlyConfigured: Error importing middleware django.middleware.common: "No module named ... "

Possible cause

Check to see that you didn't use a reserved name in naming your application, i.e. "email", "date" and "time" are common application names that would validate when the server starts but will break Django's admin.

Solution

Rename your application directory using a non-reserved name, i.e., "email_app" instead of "email". Go into the INSTALLED_APPS section of settings.py and change the name there too. Also, don't forget to do a syncdb to create the newly renamed app in your database. You may also want to go in to your database and drop the old "mis-named" table.