Parsing a string input into a lambda function (Python)

Question

How can I parse a string input into the proper form for it to be executed as mapping regulation in a lambda function?

 fx=type(input("Enter a polynomial: "))

This is my input, I want to enter arbirtray polynomials.

f= lambda x: fx

Now i want my lambda function to be able to execute the strings from the input function just as if they were normal mapping regulations like x**2 for instance.

Your question is not clear. Do you mean you want to input a string that represents a polynomial and store it in variable fx, then parse it into a new string format that can be evaluated more easily? Or do you want a function that evaluates that string and can be used in another, lambda function? Or something else? I hope you realize that your lambda expression as written is meaningless. — Rory Daulton
– Rory Daulton, Commented Nov 27, 2016 at 12:29
Yes, i would like to do the second option. The expression is part of a bigger program, this is only the part that causes me my problem — Christian Singer
– Christian Singer, Commented Nov 27, 2016 at 12:34

Community · Accepted Answer · 2017-05-23 12:24:31Z

9

First things first, input() behaves differently in Python 2 and Python 3, as specified in this answer.

eval() is one of the simplest options:

Python 3

>>> fx = input("Enter a polynomial: ")
Enter a polynomial: x**2 + 2*x + 1
>>> f = lambda x: eval(fx)
>>> f(1)
4

Python 2

>>> fx = raw_input("Enter a polynomial: ")
Enter a polynomial: x**2 + 2*x + 1
>>> f = lambda x: eval(fx)
>>> f(1)
4

Be careful though, as eval() can execute arbitrary code.

edited May 23, 2017 at 12:24

CommunityBot

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answered Nov 27, 2016 at 12:41

farsil

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