Question

I am using a re.sub callback to replace substrings with random values, but I would like the random values to be the same across different strings. Since the re.sub callback does not allow arguments, I am not sure how to do this.

Here is a simplified version of what I’m doing:

def evaluate(match):
    mappings = {'A': 1, 'B': 2}
    return str(eval(match.group(0)[2:-1], mappings))

# variables = {'A':[1,2,3,4,5], 'B':[1,2,3,4,5]}    
# mappings2 = {k:v[random.randint(0,len(v)-1)] for k, v in variables.items()}
string_one: "#{A} + #{B}"
string_two: "#{A+B}"
newstring_one = sub(r'\#\{([^#]+)\}', evaluate, string_one)
newstring_two = sub(r'\#\{([^#]+)\}', evaluate, string_two)

Now, as it stands, the strings will be properly evaluated: newstring_one is “1 + 2” and newstring_two is “3”. But I want to be able to pick the values randomly, and still have them replaced in both strings. This would involve deleting the ‘mappings’ line in ‘evaluate’, and using something like the two commented lines. How, though, can I get my randomly chosen mappings2 to be used when eval-ing both strings, if I cannot pass it as an argument in the re.sub callback function?

Many thanks.

Answer 1

The easiest way I guess is to make use of functools.partial, which allows you create a “partially evaluated” function:

from functools import partial

def evaluate(match, mappings):
    return str(eval(match.group(0)[2:-1], mappings))

mappings = {'A': 1, 'B': 2}  # Or whatever ...

newstring = sub(r'\#\{([^#]+)\}', partial(evaluate, mappings=mappings), string)