Trying to understand f-strings...

[pythoncoder]

This works with string.format, with the caller being able to define the format of a value computed at run time:

Code: Select all

def bar(s="{}"):
    v = 4.2
    print(s.format(v))

Is there a way to do this with f-strings? This fails on the first line because v is undefined.

Code: Select all

def foo(s=f"{v}"):
    v = 4.2
    print(s)

It seems that the f-string is compiled, with its value, when it is declared. So this delivers 99:

Code: Select all

v = 99
def foo(s=f"{v}"):
    v = 4.2
    print(s)

So is there a way to declare an f-string which formats a value which changes at run time?

[jim]

Would this work for your needs?

Code: Select all

def foo(s=None):
    v = 4.2
    if s is None:
        s = f"{v}"
    print(s)

[bulletmark]

Formatted string literals (i.e. f-strings) are not intended to replace str.format() in every case. When you have a string with embedded replacement fields which you need to replace later in time, then just call .format() on it at that time. No point trying to use an f-string here.

[joshfreeman]

I am having issues to understanding this.

[jim]

I think it is a similar issue as trying to use an object variable as a method parameter (default argument) within the same class, which is why I proposed the above.

[pythoncoder]

I think @bulletmark is right in this instance and string format must be used. Looking at your function:

Code: Select all

def foo(s=None):
    v = 4.2
    if s is None:
        s = f"{v}"
    print(s)

my typical call pattern would be:

Code: Select all

foo(f"{v:4.1f}")

This fails with NameError: name 'v' isn't defined.