What are formatted string literals in Python 3.6?

19
votes

One of the features of Python 3.6 are formatted strings.

This SO question(String with 'f' prefix in python-3.6) is asking about the internals of formatted string literals, but I don't understand the exact use case of formatted string literals. In which situations should I use this feature? Isn't explicit better than implicit?

3
pythonpython-3.6f-string
For example generating queries for query languages (should use prepared queries where available) or you want a single string that has variable parameters like a list or formatted float etc. - Fma
I think this is somewhat broad and/or opinion-based, but I see three benefits: 1) it's shorter; 2) impossible to mess up the order of parameters in format (you can use named parameters, but then it's much shorter); 3) immediately clear which variable goes where without jumping back and forth between format string and parameter list. - tobias_k
Beware of f-strings! They are extremely addictive. After a few days of using them it's hard to go back to other kinds of formatting. :) - PM 2Ring
Possible duplicate of String with 'f' prefix in python-3.6 - Chris_Rands
Docs: docs.python.org/3/reference/… - handle

3 Answers

35
votes

Simple is better than complex.

So here we have formatted string. It gives the simplicity to the string formatting, while keeping the code explicit (comprared to other string formatting mechanisms).

title = 'Mr.'
name = 'Tom'
count = 3

# This is explicit but complex
print('Hello {title} {name}! You have {count} messages.'.format(title=title, name=name, count=count))

# This is simple but implicit
print('Hello %s %s! You have %d messages.' % (title, name, count))

# This is both explicit and simple. PERFECT!
print(f'Hello {title} {name}! You have {count} messages.')

It is designed to replace str.format for simple string formatting.

3
votes

Pro: F-literal has better performance.(See below)

Con: F-literal is a new 3.6 feature.

In [1]: title = 'Mr.'
   ...: name = 'Tom'
   ...: count = 3
   ...: 
   ...: 

In [2]: %timeit 'Hello {title} {name}! You have {count} messages.'.format(title=title, name=name, count=count)
330 ns ± 1.08 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [3]: %timeit 'Hello %s %s! You have %d messages.'%(title, name, count)
417 ns ± 1.76 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [4]: %timeit f'Hello {title} {name}! You have {count} messages.'
13 ns ± 0.0163 ns per loop (mean ± std. dev. of 7 runs, 100000000 loops each)
1
votes

Comparing Performance for all 4 ways of string formatting

title = 'Mr.' name = 'Tom' count = 3

%timeit 'Hello {title} {name}! You have {count} messages.'.format(title=title, name=name, count=count)

198 ns ± 7.88 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

%timeit 'Hello {} {}! You have {} messages.'.format(title, name, count)

329 ns ± 7.04 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

%timeit 'Hello %s %s! You have %d messages.'%(title, name, count)

264 ns ± 6.95 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

%timeit f'Hello {title} {name}! You have {count} messages.'

12.1 ns ± 0.0936 ns per loop (mean ± std. dev. of 7 runs, 100000000 loops each)

So the latest way of string formatting is from python3.6 is fastest also.