How to check if there is a valid substitution value for string?

Question

Given a string and a string template, how do I check if there is a valid variable that can be used as substitute?

For example,

def find_substitute(template: str, s: str) -> str:
    ...

template = "a_%(var)s_b_%(var)s_c"

find_substitute(template, "a_foo_b_foo_c")  # Should return foo
find_substitute(template, "a_foo_b_bar_c")  # Should raise, no valid substitution value
find_substitute(template, "a_foo_a_foo_a")  # Should raise, no valid substitution value

I could do template.split("%(var)s") and then try to match each section of the string, but I'm guessing there's a better way to do this, perhaps using regex.

Answer 1

You can use re.match for this.

import re

def find_substitute(template ,string):
    m = re.match(template, string)
    print(m.group(1)) if m else print("no match")

if __name__=="__main__":
    var = "foo"
    t = fr'a_({var})_b_(\1)_c'
    lines = ['a_foo_b_foo_c', 'a_bar_b_bar_c', 'a_foot_b_balls_c']
    for line in lines:
        find_substitute(t, line)

#output:
#foo
#no match
#no match

re.match returns a match object. You can use match object to get the full match or even the captured groups.