Clever Geek Handbook

Evaluating strings as values

Is it possible in Python to calculate a member in a string? For example:

string_a = "4 ** (3 - 2)"

unknown_function(string_a) = 4

Is it possible? Is there a function that mimics "unknown_function" in my example?

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3 answers


Exist eval 

eval(string_a)
# 4


But don't use this under any circumstances if it string_a

comes from someone other than you, because they can easily hack into your systems and destroy your files!

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Just as sympy

was a helpful module for your last question , it can be applied here:

>>> import sympy
>>> sympy.sympify("4**(3-2)")
4

and even

>>> sympy.sympify("2*x+y")
2*x + y
>>> sympy.sympify("2*x+y").subs(dict(x=2, y=3))
7


Note that this will return objects sympy

, and if you want to get an integer or float from it, you have to do the conversion explicitly:

>>> type(sympy.sympify("4**(3-2)"))
<class 'sympy.core.numbers.Integer'>
>>> int(sympy.sympify("4**(3-2)"))
4

I hacked a recipe to turn string expressions into functions here , which is pretty cute.

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Yes, you can use the function eval

.

>>> string_a = "4 ** (3 - 2)"
>>> eval(string_a)
4
>>>

More details can be found in the documentation

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