In Python 3.6, the f-string, formatted string literal, was introduced(PEP 498). In short, it is a way to format your string that is more readable and fast.

Example:

agent_name = 'James Bond'
kill_count = 9


# old ways
print("%s has killed %d enemies" % (agent_name,kill_count))

print('{} has killed {} enemies'.format(agent_name,kill_count))
print('{name} has killed {kill} enemies'.format(name=agent_name,kill=kill_count))
    

# f-strings way
print(f'{agent_name} has killed {kill_count} enemies')

The f or F in front of strings tell Python to look at the values , expressions or instance inside {} and substitute them with the variables values or results if exists. The best thing about f-formatting is that you can do cool stuff in {}, e.g. {kill_count * 100}.

You can use it to debug using print e.g.

print(f'the {agent_name=}.')
# the agent_name='James Bond'

Formatting, such as zero-padding, float and percentage rounding is made easier:

print(f'{agent_name} shoot with {9/11 : .2f} or {9/11: .1%} accuracy')
# James Bond shoot with  0.82 or  81.8% accuracy 

Even cooler is the ability to nest and format. Example date

from datetime import datetime

lookup = {
    '1': 'st',
    '21': 'st',
    '31': 'st',
    '2': 'nd',
    '22': 'nd',
    '3': 'rd',
    '23': 'rd'
}

dato = datetime.now()

print(f"{dato: %B %-d{lookup.get(f'{dato:%-d}', 'th')} %Y}")

# April 23rd 2022

Pretty formatting is also easier

tax = 1234

print(f'{tax:,}') # separate 1k \w comma
# 1,234

print(f'{tax:,.2f}') # all two decimals 
# 1,234.00

print(f'{tax:~>8}') # pad left with ~ to fill eight characters or < other direction
# ~~~~1234

print(f'{tax:~^20}') # centre and pad
# ~~~~~~~~1234~~~~~~~~

The __format__ allows you to funk with this feature. Example

class Money:
    
    def __init__(self, value, currency='€'):
        self.currency = currency
        self.value = value
        
    def __repr__(self):
        return f'Money(value={self.value}, currency={self.currency})'
        
    def __format__(self, *_):
        
        return f"{self.currency}{float(self.value):.2f}"
        
        
tax = 12.3446
money = Money(tax, currency='$')

print(f'{money}')
# $12.34

print(money)
# Money(value=12.3446, currency=$)

There is much more. Readings:

• PEP 498 Literal String Interpolation
• Python String Formatting

Since any valid Python expression is allowed inside the braces in an f-string, you can simply use str.join() to produce the result you want:

>>> a = [1, 'a', 3, 'b']
>>> f'unpack a list: {" ".join(str(x) for x in a)}'
'unpack a list: 1 a 3 b'

You could of course also write a helper function, if your real-world use case makes the above more verbose than you'd like:

def unpack(s):
    return " ".join(map(str, s))  # map(), just for kicks

>>> f'unpack a list: {unpack(a)}'
'unpack a list: 1 a 3 b'