Python f Strings interpret from list of messages

Question

I am writing a sample greeting program. I have a random list of greeting message with template string as name.

My code is as below

from random import  randint

random_greeting = [
    "Hello {name}, Hope you are doing well",
    "Hello {name}, Hope you are having a great day",
    "Hello {name}, Nice to meet you"
    ]

class Greetings(object):

    global random_greeting

    def read_input(self):
        return input("What is your name? ")

    def print_greeting_rand(self, name):
        greet_length = len(random_greeting)
        random_int = randint(0, greet_length-1)
        message = random_greeting[random_int]
        print(f"{message}") # Here {name} not coming into scope

if __name__ == "__main__":
    greet = Greetings()
    name = greet.read_input()
    greet.print_greeting_rand(name)

So random_greeting have list of messages with name as template string {name}. In the function print_greeting_rand I am fetching greeting message based on random index and printing using Python F-Strings, {name} is not interpreting as expected.

Any suggestions please ?

I don't think you can do that with f strings (except possibly with some nasty `eval` hack). You can just do `message.format(name=name)` — alani, Jul 18 '20 at 18:58
@alaniwi, yes, can you add more information about `eval` hack. Please. Thank you. — Raja G, Jul 18 '20 at 19:18
@rɑːdʒɑ re `eval`, someone has posted an answer to that effect on the page that wjandrea linked. But it is not to be recommended. — alani, Jul 18 '20 at 19:20
@Jan My point is that getting variables by name at run-time is inevitably going to be hacky. Hence no doubt the reason why you used a `kwargs` dictionary in your answer instead. — alani, Jul 18 '20 at 19:32

Jan · Accepted Answer · 2020-07-18T19:25:22.160

You may leverage a simple regular expression and **kwargs:

import random, re

class Greetings(object):
    greetings = [
        "Hello {name}, Hope you are doing well {when}",
        "Hello {name}, Hope you are having a great day",
        "Hello {name}, Nice to meet you"
    ]
    rx = re.compile(r'{([^{}]+)}')
    vars = {}

    def read_input(self):
        return input("What is your name? ")

    def print_greeting_rand(self, **kwargs):
        self.vars = kwargs
        tmpl = self.rx.sub(self.__replace__, random.choice(self.greetings))
        print(tmpl)

    def __replace__(self, m):
        var = m.group(1)
        if var in self.vars:
            return self.vars[var]
        return m.group(0)


if __name__ == "__main__":
    greet = Greetings()
    greet.print_greeting_rand(name=greet.read_input(), when="today")

Explanation:

This scans the string for any variable of the form { + variable_name + } and substitutes it when it finds it within the **kwargs parameter. If it is not found, it simply returns {variable_name}, that is leaving it untouched.

See a demo for the expression on regex101.com.

Could you please explain what above code will do ? Particularly about areas where `re` used. Thank you. — Raja G, Jul 18 '20 at 19:22

Harben · Answer 2 · 2020-07-18T19:05:20.397

1

Unfortunately with f-strings you can't do this as they are evaluated immediately and so what you can do instead is use:

from random import choice

random_greeting = [
    "Hello {name}, Hope you are doing well",
    "Hello {name}, Hope you are having a great day",
    "Hello {name}, Nice to meet you"
    ]

def print_greeting_rand(self, name):
    message = choice(random_greeting)
    print(message.format(name=name))

edited Jul 18 '20 at 19:05

answered Jul 18 '20 at 19:01

Harben

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The first of these is no longer recommended in new code. – alani Jul 18 '20 at 19:02
2

I would recommend completely removing the old-style formatting recommendation, and just keep `.format`. The [Python official docs](https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting) recommend against old-style formatting. – SethMMorton Jul 18 '20 at 19:03
@alaniwi I am just giving options – Harben Jul 18 '20 at 19:03
1

@Harben Options are fine when the options are good. In this case, the first option is not good. – SethMMorton Jul 18 '20 at 19:04

score 1 · Answer 3 · answered Jul 18 '20 at 19:03

1

You want

print(message.format(name=name))

The same holds true with an f-string as well.

print(f'{message}'.format(name=name))

answered Jul 18 '20 at 19:03

Balaji Ambresh

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1

Don't use `f'{message}'`, that's just adding an extra unnecessary step. – wjandrea Jul 18 '20 at 19:05
1

@wjandrea Agreed. I was just showing a way to format from the context of f-strings – Balaji Ambresh Jul 18 '20 at 19:06

Python f Strings interpret from list of messages

3 Answers3