How do I convert a string into an f-string?

Question

I was reading this blog on python's new f-strings and they seem really neat. However, I want to be able to load an f-string from a string or file.

I can't seem to find any string method or other function that does this.

From the example in my link above:

name = 'Fred'
age = 42
f"My name is {name} and I am {age} years old"

'My name is Fred and I am 42 years old'

But what if I had a string s? I want to be able to eff-ify s, something like this:

name = 'Fred'
age = 42
s = "My name is {name} and I am {age} years old"
effify(s)

---

Turns out I can already perform something similar to str.format and garner the performance pick up. Namely:

format = lambda name, age: f"My name is {name} and I am {age} years old"
format('Ted', 12)

'My name is Ted and I am 12 years old'

Answer 1

f-strings are code. Not just in the safe, "of course a string literal is code" way, but in the dangerous, arbitrary-code-execution way. This is a valid f-string:

f"{__import__('os').system('install ransomware or something')}"

and it will execute arbitrary shell commands when evaluated.

You're asking how to take a string loaded from a text file and evaluate it as code, and the answer boils down to eval. This is of course a security risk and probably a bad idea, so I recommend not trying to load f-strings from files.

If you want to load the f-string f"My name is {name} and I am {age} years old" from a file, then actually put

f"My name is {name} and I am {age} years old"

in the file, f and quotation marks and all.

Read it from the file, compile it and save it (so eval doesn't have to recompile it every time):

compiled_fstring = compile(fstring_from_file, '<fstring_from_file>', 'eval')

and evaluate it with eval:

formatted_output = eval(compiled_fstring)

If you do this, be very careful about the sources you load your f-strings from.