How to use gettext with python >3.6 f-strings

Question

Previously you would use gettext as following:

_('Hey {},').format(username)

but what about new Python's f-string?

f'Hey {username}'

Answer 1

'Hey {},' is contained in your translation dictionary as is.

If you use f'Hey {username},', that creates another string, which won't be translated.

In that case, the format method remains the only one useable, but you could approach the f-string features by using named parameters

_('Hey {username},').format(username=username)

or if you have a dictionary containing your data, this cool trick where format picks the required information in the input dictionary:

d = {"username":"John", "city":"New York", "unused":"doesn't matter"}

_('Hey {username} from {city},').format(**d)

Answer 2

My solution is to make a function f() which performs the f-string interpolation after gettext has been called.

from copy import copy
from inspect import currentframe

def f(s):
    frame = currentframe().f_back
    kwargs = copy(frame.f_globals)
    kwargs.update(frame.f_locals)
    return eval(s.format(**kwargs))

Now you just wrap _(...) in f() and don’t preface the string with an f:

f(_('Hey, {username}'))

Note of caution

I’m usually against the use of eval as it could make the function potentially unsafe, but I personally think it should be justified here, so long as you’re aware of what’s being formatted. That said use at your own risk.

Remember

This isn’t a perfect solution, this is just my solution. As per PEP 498 states each formatting method “have their advantages, but in addition have disadvantages” including this.

For example if you need to change the expression inside the string then it will no longer match, therefore not be translated unless you also update your .po file as well. Also if you’re not the one translating them and you use an expression that’s hard to decipher what the outcome will be then that can cause miscommunication or other issues in translation.