Example from PEP-0498

>>> import datetime
>>> name = 'Fred'
>>> age = 50
>>> anniversary = datetime.date(1991, 10, 12)
>>> f'My name is {name}, my age next year is {age+1}, my anniversary is {anniversary:%A, %B %d, %Y}.'
'My name is Fred, my age next year is 51, my anniversary is Saturday, October 12, 1991.'
>>> f'He said his name is {name!r}.'
"He said his name is 'Fred'."

Sympathetic output from fstring427

>>> from fstring427.fstring import Fmt as f
>>> import datetime
>>> name = 'Fred'
>>> age = 50
>>> anniversary = datetime.date(1991,10,12)
>>> str(f('My name is {name}, my age next year is {age+1}, my anniversary is {anniversary:%A, %B %d, %Y}.'))
'My name is Fred, my age next year is 51, my anniversary is Saturday, October 12, 1991.'
>>>  f('He said his name is {name!r}')()
"He said his name is 'Fred'"

Note the major differences:

• f is a class, not a string type

• f() evaluates the string

• str() of a instance of f also evaluates the string

The underlying implementation is a subclass of the Python 2.7 Format class, and depends on internals. Obviously fragile and probably non-portable, but still serves my purpose.

## printf(), a convenience function

>>> printf('He said his name is {name!r}') He said his name is 'Fred'

which has the additional convenience of a temporary scope for kwargs

 >>> printf('He said his name is {name!r}', name="Sam")
He said his name is 'Sam'

Major incompatibilities

Python 3.6 f-strings were carefully designed, and cover edge cases that .format() does not, see https://mail.python.org/pipermail/python-ideas/2015-July/034726.html

fstring427 was implemented on top of .format() and shares the underlying implementation of lookups. If .format() can’t handle a {field}, fstring427 will evaluate field as a Python expression in the proper scope. In practice this means that:

a = 10
d = {'a': 'string', 10: 'int'}
printf("{d[a]")

prints string (Python 2.7 .format() behavior) instead of int (Python 3.6 f-string behavior). I’ve found this a small price to pay in my 2.7 code to get cleaner printing and string formatting.

Roadmap

• Add printf() style logging module

• Contemplate 2-3 port for printf() utility function (dealing with the kwargs scope)

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