Making your code compatible with both Python 2.7 and Python 3

Since version 1.7, Django can be used with Python 2.7 and Python 3. In this recipe, we will take a look at the operations to make your code compatible with both the Python versions.

Getting ready

When creating a new Django project or upgrading an old existing project, consider following the rules given in this recipe.

How to do it…

Making your code compatible with both Python versions consists of the following steps:

1. At the top of each module, add from __future__ import unicode_literals and then use usual quotes without a u prefix for Unicode strings and a b prefix for bytestrings.
2. To ensure that a value is bytestring, use the django.utils.encoding.smart_bytes function. To ensure that a value is Unicode, use the django.utils.encoding.smart_text or django.utils.encoding.force_text function.
3. For your models, instead of the __unicode__ method, use the __str__ method and add the python_2_unicode_compatible decorator, as follows:

   # models.py
   # -*- coding: UTF-8 -*-
   from __future__ import unicode_literals
   from django.db import models
   from django.utils.translation import ugettext_lazy as _
   from django.utils.encoding import \
       python_2_unicode_compatible
   
   @python_2_unicode_compatible
   class NewsArticle(models.Model):
       title = models.CharField(_("Title"), max_length=200)
       content = models.TextField(_("Content"))
   
       def __str__(self):
           return self.title
   
       class Meta:
           verbose_name = _("News Article")
           verbose_name_plural = _("News Articles")

4. To iterate through dictionaries, use iteritems(), iterkeys(), and itervalues() from django.utils.six. Take a look at the following:

   from django.utils.six import iteritems
   d = {"imported": 25, "skipped": 12, "deleted": 3}
   for k, v in iteritems(d):
       print("{0}: {1}".format(k, v))

5. When you capture exceptions, use the as keyword, as follows:

   try:
       article = NewsArticle.objects.get(slug="hello-world")
   except NewsArticle.DoesNotExist as exc:
       pass
   except NewsArticle.MultipleObjectsReturned as exc:
       pass

6. To check the type of a value, use django.utils.six, as shown in the following:

   from django.utils import six
   isinstance(val, six.string_types) # previously basestring
   isinstance(val, six.text_type) # previously unicode
   isinstance(val, bytes) # previously str
   isinstance(val, six.integer_types) # previously (int, long)

7. Instead of xrange, use range from django.utils.six.moves, as follows:

   from django.utils.six.moves import range
   for i in range(1, 11):
       print(i)

8. To check whether the current version is Python 2 or Python 3, you can use the following conditions:

   from django.utils import six
   if six.PY2:
       print("This is Python 2")
   if six.PY3:
       print("This is Python 3")

How it works…

All strings in Django projects should be considered as Unicode strings. Only the input of HttpRequest and output of HttpResponse is usually in the UTF-8 encoded bytestring.

Many functions and methods in Python 3 now return the iterators instead of lists, which make the language more efficient. To make the code compatible with both the Python versions, you can use the six library that is bundled in Django.

Read more about writing compatible code in the official Django documentation at https://docs.djangoproject.com/en/1.8/topics/python3/.

Tip

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