I was reading this blog post 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 the blog post:
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)
It 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'
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.