CODEWITHSUNDEEP
pythonstringpython-3.xeval
Vaibhav
Vaibhav

Reputation: 1234

Python eval(string == string)

I'm trying to use eval function to read a string and evaluate the content that's inside the string.

a = 'red'
b = '=='
c = 'red'
print(eval(a+b+c))

it throw me an error 

NameError: name 'red' is not defined

Note: i know it evaluate to red==red and read it as an variable instead of string but dont know how to make it string during eval process i.e 'red'=='red'

Than i tried with ast 

import ast
ast.literal_eval(a+b+c)

but gives an error :

ValueError: malformed node or string: <_ast.Compare object at 0x035706F0>

Than i tried ast.parse but i dont want to write seperate function/method based on operators if it can be done with eval only function/method

Any advice or links to useful resources would be appreciated.

Upvotes: 0

Views: 1016

Answers (2)

Mad Physicist
Mad Physicist

Reputation: 114478

While what you are doing is a terrible idea and can be done in other ways, you need to add quotes within your string:

a = '\'red\''
b = ' == '
c = '\'red\''

OR

a = '"red"'
b = ' == '
c = '"red"'

OR

a = "'red'"
b = ' == '
c = "'red'"

Any of these should work. The choice is yours based on the type of quotes you want and the amount of escaping you are willing to do.

On a side note, you don't need to use eval for your particular example:

a == c

will give the result of the eval. If you are worried about not knowing the operator, use a dict and the operator module:

import operator

ops = {
    '==': operator.eq,
    '<': operator.lt,
    '<=': operator.le,
    ...
}
ops[b](a, c)

Upvotes: 2

internet_user
internet_user

Reputation: 3279

a = "'red'"
c = "'red'"

A more general way to do this is using repr:

repr('red') # -> "'red'"
repr('"blue"') # -> '\'"blue"\'' (this does actually evaluate to '"blue"')

Upvotes: 1

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