Can I import Python's 3.6's formatted string literals (f-strings) into older 3.x, 2.x Python?

The f-strings are created by the interpreter upon tokening the f prefix – that feature alone will kill any compatibility chances.

Your closest shot is to use the keyword formatting, like

'Foo is {age} {units} old'.format(age=age, units=units)

which can be more easily refactored upon the termination of requirement for compatibility.

future-fstrings brings f-strings to Python 2.7 scripts. (And I assume 3.3-3.5 based on the documentation.)

Once you pip install it via pip install future-fstrings, you have to place a special line at the top of your code. That line is:

# -*- coding: future_fstrings -*-

Then you can use formatted string literals (f-strings) within your code:

# -*- coding: future_fstrings -*-
var = 'f-string'
print(f'hello world, this is an {var}')

here’s what I use:

text = "Foo is {age} {units} old".format(**locals())

it unpacks (**) the dict returned by locals() which has all your local variables as a dict {variable_name: value}

Note this will not work for variables declared in an outer scope, unless you import it to the local scope with nonlocal (Python 3.0+).

you can also use

text.format(**locals(),**globals())

to include global variables in your string.

I just wrote a back-port compiler for f-string, called f2format. Just as you requests, you may write f-string literals in Python 3.6 flavour, and compile to a compatible version for end-users to run, just like Babel for JavaScript.

f2format provides an intelligent, yet imperfect, solution of a back-port compiler. It shall replace f-string literals with str.format methods, whilst maintaining the original layout of source code. You can simply use

f2format /path/to/the/file_or_directory

which will rewrite all Python files in place. For instance,

var = f'foo{(1+2)*3:>5}bar{"a", "b"!r}boo'

will be converted to

var = ('foo{:>5}bar{!r}boo').format(((1+2)*3), ("a", "b"))

String concatenation, conversion, format specification, multi-lines and unicodes are all treated right. Also, f2format will archive original files in case there’re any syntax breaches.

A dirty solution using simpleeval

import re
import simpleeval
test='_someString'
lst = ['_456']

s = '123123{lst[0]}{test}'

def template__format(template, context=None):
    if context is None:
        frame = inspect.currentframe()
        context = frame.f_back.f_locals        
        del frame
    ptn =  '([^{]?){([^}]+)}'
    class counter():
        i = -1

    def count(m):
        counter.i += 1
        return m.expand('\1{%d}'%counter.i)

    template = re.sub(ptn,string=s, repl= count)
    exprs = [x[1] for x in re.findall(ptn,s)]
    vals = map(simpleeval.SimpleEval(names=context).eval,exprs)
    res = template.format(*vals)
    return res

print (template__format(s))

I’ve been using 'str'.format(**locals()) for a while but made this after a while because the additional code was a bit cumbersome for each statement

def f(string):
    """
    Poor man's f-string for older python versions
    """
    import inspect

    frame = inspect.currentframe().f_back

    v = dict(**frame.f_globals)
    v.update(**frame.f_locals)

    return string.format(string, **v)

# Example
GLOBAL = 123


def main():
    foo = 'foo'
    bar = 'bar'

    print(f('{foo} != {bar} - global is {GLOBAL}'))


if __name__ == '__main__':
    main()

Using dict() to hold name-value pairs

• In addition to the approaches mentioned elsewhere in this thread (such as format(**locals()) ) the developer can create one or more python dictionaries to hold name-value pairs.

• This is an obvious approach to any experienced python developer, but few discussions enumerate this option expressly, perhaps because it is such an obvious approach.

• This approach is arguably advantageous relative to indiscriminate use of locals() specifically because it is less indiscriminate. It expressly uses one or more dictionaries a namespace to use with your formatted string.

• Python 3 also permits unpacking multiple dictionaries (e.g., .format(**dict1,**dict2,**dict3) … which does not work in python 2.7)

  ## init dict
  ddvars = dict()

  ## assign fixed values
  ddvars['firname']   =   'Huomer'
  ddvars['lasname']   =   'Huimpson'
  ddvars['age']       =   33
  pass

  ## assign computed values
  ddvars['comname']   =   '{firname} {lasname}'.format(**ddvars)
  ddvars['reprself']  =   repr(ddvars)
  ddvars['nextage']   =   ddvars['age'] + 1
  pass

  ## create and show a sample message
  mymessage = '''
  Hello {firname} {lasname}!
  Today you are {age} years old.
  On your next birthday you will be {nextage} years old!
  '''.format(**ddvars)

  print(mymessage)