Python: Capitalize a word using string.format()

Question

Is it possible to capitalize a word using string formatting? For example,

"{user} did such and such.".format(user="foobar")

should return "Foobar did such and such."

Note that I'm well aware of .capitalize(); however, here's a (very simplified version of) code I'm using:

printme = random.choice(["On {date}, {user} did la-dee-dah. ",
                         "{user} did la-dee-dah on {date}. "
                         ])

output = printme.format(user=x,date=y)

As you can see, just defining user as x.capitalize() in the .format() doesn't work, since then it would also be applied (incorrectly) to the first scenario. And since I can't predict fate, there's no way of knowing which random.choice would be selected in advance. What can I do?

Addt'l note: Just doing output = random.choice(['xyz'.format(),'lmn'.format()]) (in other words, formatting each string individually, and then using .capitalize() for the ones that need it) isn't a viable option, since printme is actually choosing from ~40+ strings.

Answer 1

As said @IgnacioVazquez-Abrams, create a subclass of string.Formatter allow you to extend/change the format string processing.

In your case, you have to overload the method convert_field

from string import Formatter
class ExtendedFormatter(Formatter):
    """An extended format string formatter

    Formatter with extended conversion symbol
    """
    def convert_field(self, value, conversion):
        """ Extend conversion symbol
        Following additional symbol has been added
        * l: convert to string and low case
        * u: convert to string and up case

        default are:
        * s: convert with str()
        * r: convert with repr()
        * a: convert with ascii()
        """

        if conversion == "u":
            return str(value).upper()
        elif conversion == "l":
            return str(value).lower()
        # Do the default conversion or raise error if no matching conversion found
        return super(ExtendedFormatter, self).convert_field(value, conversion)

# Test this code

myformatter = ExtendedFormatter()

template_str = "normal:{test}, upcase:{test!u}, lowcase:{test!l}"


output = myformatter.format(template_str, test="DiDaDoDu")
print(output)

Answer 2

You can pass extra values and just not use them, like this lightweight option

printme = random.choice(["On {date}, {user} did la-dee-dah. ",
                         "{User} did la-dee-dah on {date}. "
                         ])

output = printme.format(user=x, date=y, User=x.capitalize())

The best choice probably depends whether you are doing this enough to need your own fullblown Formatter.

Answer 3

You can create your own subclass of string.Formatter which will allow you to recognize a custom conversion that you can use to recase your strings.

myformatter.format('{user!u} did la-dee-dah on {date}, and {pronoun!l} liked it. ',
                      user=x, date=y, pronoun=z)

Answer 4

In python 3.6+ you can use fstrings now. https://realpython.com/python-f-strings/

>>> txt = 'aBcD'
>>> f'{txt.upper()}'
'ABCD'