Django ugettext_lazy not working for interpolated strings in forms

Question

I have form:

class UserForm(forms.Form):
fname = forms.CharField(max_length=30, label=_('First Name'), validators=[Alpha()])
lname = forms.CharField(max_length=30, label=_('Last Name {name}').format(name='XXXX'), validators=[Beta()])

i am rendering this form using form.as_p , when i change my language First Name gets translated but for some reason Last Name {name} which using format method for string interpolation is not getting translated, translation for this string is also present in the language catalog. I am on Django version 1.8 and python 2.7.

Update: It seems that for some reason django is not doing lazy evaluation of interpolated strings in my forms, It calls ugettext_lazy for the interpolated strings on very first request after server starts and never evaluates that string when it is actually gets rendered.

Answer 1

Very very old question but it bit me today, so here's my answer: the % operator is executed immediately on form initialization (ie on server start), so the label stops being a proxy and becomes a string, which obviously breaks the translation mechanism. By the way, as far as I can tell this would apply even if you used .format() instead of the interpolation operator, because the return of that function is still going to be a plain string instead of a proxy.

As for the solution: if you only need to use interpolation in a few form fields, the easy way is setting the label in __init__() like this:

class MyForm(forms.Form):
    myfield = forms.BooleanField()

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.fields['myfield'].label = _("Your interpolated %(value)s") % {'value': 'field'}

A better solution if you need a lot of fields would be to create a custom field (or multiple, with a mixin) that accepts a new label_mapping kwarg, something like:

class MyCustomField(Field):
    def __init__(label_mapping=None, label=None, *args, **kwargs):
        if label is not None and label_mapping is not None:
            label = label % label_mapping
        super().__init__(label=label, *args, **kwargs)

In my eyes, the main advantage of this, beside removing boilerplate code, would be to keep the declarative syntax of form fields.

Please note I only had a couple fields that required attention so I didn't actually use the custom field class approach, but I don't see why it shouldn't work (after proper testing).