Django translations and gettext: The deprecation of the % (string interpolation) operator

Question

Although Django Django does not yet support Python 3, it eventually will, so I want to keep my code the more "future-proof" possible.

Since Python 2.7 the string interpolation operator (%) is being deprecated. And I realized that every string that needs to be translated is using the % interpolation syntax. And in the Django docs there is no mention of the new str.format method (the "new" official way of string formatting)...

Maybe there is a limitation of the gettext library, but I don't think so, since the string appears identical in the .PO files.

The question is if I can use the new string format method for translation.

The old way:

class Post(models.Model):
    title = models.CharField(max_length=50)
    date = models.DateField()
    # ...
    def __unicode__(self):
        return _('%(title)s (%(date)s)') % {
            'title': self.title,
            'date': self.date,
        }

The "new" way:

class Post(models.Model):
    title = models.CharField(max_length=50)
    date = models.DateField()
    # ...
    def __unicode__(self):
        return _('{title} ({date})').format(
            title=self.title,
            date=self.date,
        )

Also, ugettext_lazy does not really return strings, but Promises, objects that are evaluated only when needed.

Answer 1

You could use it safely. For example

ugettext_lazy('{foo}').format(foo='bar')

The translation program xgettext, which is used by Django, does not care about the content to be translated. It just searches .py file for keywords such as ugettext_lazy or _ to collect the translatable strings(refs the manual of xgettext and Django code)

Furthermore, the .format() method above is a wrapper provided by the proxy object, like:

>>> ugettext_lazy(u'{foo}').format
<bound method __proxy__.__wrapper__ of <django.utils.functional.__proxy__ object at 0x102f19050>>

The invoking of the above .format() would get u'{foo}' to be translated to some unicode value, then call value.format with actual arguments. You could see that the translation and value.format happen in different stages.