Conditional string formatting inside of string

Question

My objective is to have conditional formatting in a string where the condition is inside the string.

This can be done with f-strings like so:

>>> my_name = 'Steve'
>>> f"hello, {my_name if my_name else 'world'}"
'hello, Steve'

>>> my_name = ''
>>> f"hello, {my_name if my_name else 'world'}"
'hello, world'

But, as f-strings are just syntax and not objects, they are not useful for my use-case.

I could instead use the format method like so (as suggested in How to implement conditional string formatting?):

>>> my_name = 'Steve'
>>> s = 'hello, {name}'
>>> s.format(name=my_name if my_name else 'world')
'hello, Steve'

>>> my_name = ''
>>> s.format(name=my_name if my_name else 'world')
'hello, world'

But then the condition wouldn't be part of the string.

So I'm trying to achieve something that behaves like so:

>>> s = "hello, {name if name else 'world'}"
>>> s.format(name='Steve')
'hello, Steve'

>>> s.format(name='')
'hello, world'

But this fails with the following message: KeyError: "name if name else 'world'"

Any way to do this other then implementing it myself?

Why do you think you need to do it this way? What can this hypothetical approach solve that f-strings and regular string formatting can't? You mentioned that f-strings don't work for your use-case. What is your use-case, exactly? Is this your attempted solution to a larger problem, or are you just curious to see if it can be done? — Paul M., Apr 21 '22 at 19:54
just to mention that `{my_name if my_name else 'world'}` from the `f-string` is better as `{my_name or 'world'}` — buran, Apr 21 '22 at 19:59
s = lambda my_name = None: f"hello, {my_name if my_name else 'world'}" print(s('Carlos')) — lmielke, Apr 21 '22 at 20:06
@PaulM. This is for a college project. I don't necessarily need to do it this way, but it seemed like a simple approach. Currently I'm receiving a string of which I know nothing, I just know that I have some variables that might be arguments of keys in that string. So I would format the string providing those arguments and the string would know what to do with them. This works just fine for replacement, but not for conditional replacement. — CarlosHSF, Apr 21 '22 at 21:43

score 0 · Answer 1 · answered Apr 21 '22 at 20:08

If you want the condition to be a part of string, use a templating engine like Jinja2:

# pip install jinja2
from jinja2 import Template

s = Template("hello, {{ name|default('world') }}"

Output:

>>> s.render(name='Steeve')
'hello, Steeve'

>>> s.render()
'hello, world'

Conditional string formatting inside of string

1 Answers1