In late 2015 I found myself working with Python, Django and py.test. I was trying to apply some practices that I had been applying for a very long time with different tools, but Django resisted, so here’s the survival kit I had while I was struggling not to compare Django with more modern frameworks. I’m sure a more experienced Django engineer would have found more elegant solutions but these actually did the trick for me.

1. HTML arrays in POST requests

Coming from a background that HTML arrays (or Hashes) are posted in forms can cause some pain in Django. If for example you’re coming from a Ruby on Rails or PHP background, you may have found easy to access HTML arrays by simply checking the request object in Rails or $_POST array in PHP. So, for example if you need to post the following form:

<input type="text" name="person[name]" value="John">

In Rails for example, by accessing request[:person][:name] you would get the expected result, while in Django this is not the case. In order to do that, the following snippet is what you need

import re
def get_dict_array(post, key):
    """
    Get an entry from an HTML array eg:
    <input type="text" name="person[name]" value="John">
    Usage:
    get_dict_array(request.POST, "person")
    """
    result = {}
    if post:
        patt = re.compile('^([a-zA-Z_]\w+)\[([a-zA-Z_\-0-9][\w\-]*)\]$')
        for post_name, value in post.items():
            value = post[post_name]
            match = patt.match(post_name)
            if not match or not value:
                continue
            name = match.group(1)
            if name == key:
                k = match.group(2)
                result.update({k:value})
    return result

2. Error message overloading for Unique composite keys

Let’s say in your database, there is a table with a composite key, for example in a table of users’ Portfolio Items, the fields user and url should be unique together. In case you need to customise the error message for when a user enters an item which already exists, then you’re in for a surprise: You need to overload the model’s unique_error_message method. Sounds dangerous? Probably because it is…

    def unique_error_message(self, model_class, unique_check):
        if model_class == type(self) and unique_check == ('user', 'url'):
            return _("There already is a portfolio item with the specific url")
        else:
            return super(MyModel, self).unique_error_message(model_class, unique_check)

3. Not able to chain scopes (use QuerySet instead of ModelManager and objects = QuerySet.as_manager)

If you have created a model in django and you want to set a few scopes for it, you might need to use a ModelManager, right? Well, sort of… You see, chaining ModelManager objects can be painful, so if for example you need to have

Users.objects.active().social_user().all()

you might get a few not-so-clear error.

The most solid approach I’ve found, is the following:

• Declare a QuerySet instead of ModelManager
• In your model, you can set objects = QuerySet.as_manager() to use your custom objects manager.

Example:

from django.db import models

class AccountQuerySet(models.QuerySet):
    # ... other scopes here ...

    def active():
      """Filter accounts by active status"""
      return self.filter(is_active=True)

class Account(models.Model):
    # ... fields go here ...
    
    objects = AccountQuerySet.as_manager()

4. Converting QueryDict to plain dict

Casting a QueryDict to a plain dict might sound like a trivial thing, but you need to be aware of the following caveat: QueryDict.dict() and dict(QueryDict...) return different things, as shown in the output below

In [3]: QueryDict("utm_source=email_campaign").dict()
Out[3]: {u'utm_source': u'email_campaign'}

In [4]: dict(QueryDict("utm_source=email_campaign"))
Out[4]: {u'utm_source': [u'email_campaign']}

5. Package seems missing even though you have just installed

You have just installed a package, for example boto and you want to run a command, for example fab. If you receive the following error:

ImportError: No module named boto

all you have to do is:

export PYTHONPATH=/usr/local/lib/python2.7/site-packages

or in order to make the change permanent, you need to add this line to your shell’s rc file like .bashrc or .zshrc.

6. Uninstall all python packages

Want to start fresh or for some reason remove every python package in your system? There you go:

pip freeze | xargs pip uninstall -y