Quote string value in F-string in Python

Question

I'm trying to quote one of the values I send to an f-string in Python:

f'This is the value I want quoted: \'{value}\''

This works, but I wonder if there's a formatting option that does this for me, similar to how %q works in Go. Basically, I'm looking for something like this:

f'This is the value I want quoted: {value:q}'
>>> This is the value I want quoted: 'value'

I would also be okay with double-quotes. Is this possible?

Answer 1

Use the explicit conversion flag !r:

>>> value = 'foo'
>>> f'This is the value I want quoted: {value!r}'
"This is the value I want quoted: 'foo'"

The r stands for repr; the result of f'{value!r}' should be equivalent to using f'{repr(value)}'. Conversion flags are a feature carried over from str.format, which predates f-strings.

For some reason undocumented in PEP 3101, but mentioned in the docs, there's also an !a flag which converts with ascii:

>>> f'quote {"🔥"!a}'
"quote '\\U0001f525'"

And there's an !s for str, which seems useless... unless you know that objects can override their formatter to do something different than object.__format__ does. It provides a way to opt-out of those shenanigans and use __str__ anyway.

>>> class What:
...     def __format__(self, spec):
...         if spec == "fancy":
...             return "𝓅𝑜𝓉𝒶𝓉𝑜"
...         return "potato"
...     def __str__(self):
...         return "spam"
...     def __repr__(self):
...         return "<wacky object at 0xcafef00d>"
... 
>>> obj = What()
>>> f'{obj}'
'potato'
>>> f'{obj:fancy}'
'𝓅𝑜𝓉𝒶𝓉𝑜'
>>> f'{obj!s}'
'spam'
>>> f'{obj!r}'
'<wacky object at 0xcafef00d>'

One example of where that !s might be useful in practice is with alignment/padding of types that don't otherwise support it:

>>> f"{[1,2]:.>10}"
TypeError: unsupported format string passed to list.__format__
>>> f"{[1,2]!s:.>10}"
'....[1, 2]'

Or types which "support" it, but not how you may have intended:

>>> from datetime import date
>>> f"{date.today():_^14}"
'_^14'
>>> f"{date.today()!s:_^14}"
'__2024-03-25__'