Python's if key in dictionary not working correctly

Question

I'm working on a programming problem where user input can define variables, like so:

length = 1

When the user does this, I store the pair in a dictionary:

{'length': '1'}

The problem is, when I later try to check if a key exists in the dictionary, I always get False, even if it is definitely in there. Here is my code:

import math



def main():

    curr_formula = input()

    substitutions = dict()



    while(curr_formula != "0"):

        if("=" in curr_formula):

            split_formula = curr_formula.split("=")

            substitutions[split_formula[0]] = split_formula[1]



        curr_split = curr_formula.split(" ")

        for i in range(len(curr_split)):



            # this if statement never runs for some reason

            if(curr_split[i] in substitutions):

                curr_split[i] = substitutions[curr_split[i]]



        print(''.join(curr_split))



        curr_formula = input()

main()

The input "length = 1" and then "length + 2" should print "1 + 2" but instead cannot detect that the key already exists. Any insight on this issue would be much appreciated!

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Answer 1

When you split on '=', there is a space after the first term and before the second.

>>> 'length = 1'.split('=')
['length ', ' 1']

Whereas when you split on ' ' it's removed.

>>> 'length = 1'.split(' ')
['length', '=', '1']

As a result, the keys in your dict are probably different.

Python's if key in dictionary not working correctly

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