Parse Math Expression as "3 + 2 * temp + humidity", where replace alpha values from payload

Question

Math Expression can be anything. The variable or alpha character here in expression are also not fixed "3 + 2 * temp + humidity". The variable must look for json and get replaced by it's value from json.

environment = {"temp": 23, "humidity": 12, "airpressure":21.12}
expression = '3 + 2 * temp + humidity'

the response I am looking is as '3 + 2 * 23 + 12'.

as I just started exploring pyparsing I couldn't find the solution.

I won't upvote nor downvote, but members of the community will certainly ask you to show what have you have tried so far. — Haroldo_OK, Dec 14 '20 at 10:05
Do you just want to replace the strings `'temp'` and `'humidity'` by the numbers `23` and `12`, or do you also want to evaluate the resulting string `'3 + 2 * 23 + 12'` as an arithmetic expression to get the number 61? — mkrieger1, Dec 14 '20 at 10:13
BTW, you may want to take a look at this similar (but more complex) question: https://stackoverflow.com/questions/57744290/parse-a-list-of-expressions-using-pyparsing — Haroldo_OK, Dec 14 '20 at 17:23

score 2 · Answer 1 · answered Dec 14 '20 at 10:16

You could just use a regex to look for the variables, and use re.sub to replace each matched variable by its value:

import re

environment = {"temp": 23, "humidity": 12, "airpressure":21.12}
expression = '3 + 2 * temp + humidity'

def replace_var(m):
    return str(environment[m.group(0)])

def replace_vars(expression):
    return re.sub(r'[a-z]+', replace_var, expression)  # assuming your variables are all lowercase

print(replace_vars(expression))
# 3 + 2 * 23 + 12

This will raise a KeyError in case a variable doesn't exist in environment.

score 2 · Accepted Answer · answered Dec 14 '20 at 10:17

You can transform the keys of your environment dictionary to a regular expression, and then replace them with the corresponding value:

def resolve_vars(expression, environment):
    regex = r"\b({})\b".format("|".join([re.escape(k) for k in environment]))
    repl = lambda s: str(environment.get(s.group(), s.group()))
    return re.sub(regex, repl, expression)

Parse Math Expression as "3 + 2 * temp + humidity", where replace alpha values from payload

2 Answers2