How would you parse a list in python to combine integer while leaving the operator of an equation alone

Question

How would you parse a list in python to combine integer while leaving the operator of an equation alone?

list =['(' ,'1', '+', '2', ')', '+', '(', '2', '0'. '0', '/', '2', '5', ')']

I would like to ultimately be able to use a list like this to create a calculable equation taken from user input. Is it possible?

Answer 1

If the object is to do the calculation, skip converting to integers entirely and do an eval . Suppose you have this string from input '(1+2)+(200/25)' , you can do:

calc_result = eval(calc_input) # equals 11

However be careful, the use of eval can run any python code that is given to it, so if it's a program for public use someone will figure out a way to use this part to insert code you never intended to allow.

eval and exec should generally be avoided because they can be a security risk. For details, please see Eval really is dangerous by SO veteran Ned Batchelder (thanks to @PM2Ring for the comment)

Answer 2

if you do not mind using sympy :

from sympy import sympify

lst = ['(', '1', '+', '2', ')', '+', '(', '2', '0', '0', '/', '2', '5', ')']

res = sympify(''.join(lst))
print(res)  # 11

(not sure what you mean by leaving the operator of an equation alone . your expression is not an equation...).

Answer 3

To do this sort of thing properly, you will need a parser. You can create your own: parsing algorithms are well-documented. However, you don't need to build a parser from scratch, you can use a package like pyparsing to create a parser.

If you can guarantee that your user input is totally safe, then you can get Python to evaluate it for you via the built-in eval function. However, eval is slow, and it is a security hole because it can be used to execute arbitrary code, as mentioned in the article by Ned Batchelder that's linked in Ofer Sadan's answer.

But to answer your immediate question, we can easily grab the digits from that list of strings, join them together, and convert the resulting number strings into integers. Grouping the digits and separating them from the non-digits is easily done using itertools.groupby . You just need to give it a function it can use to identify the groups; we can use str.isdigit for that. Here's a short demo:

from itertools import groupby

lst = ['(' ,'1', '+', '2', ')', '+', '(', '2', '0', '0', '/', '2', '5', ')']
a = [int(''.join(g)) if k else next(g) for k, g in groupby(lst, str.isdigit)]
print(a)

output

['(', 1, '+', 2, ')', 200, '/', 25, ')']

If you also want to handle + and - signs and decimal points, it's not too hard to write a function that does that. Proper handling of all floating-point numbers is a little trickier, and it's possibly better to use a parser that already knows how to do that. ;)