How to get the value of a variable given its name in a string?

Question

For simplicity this is a stripped down version of what I want to do:

def foo(a):
    # I want to print the value of the variable
    # the name of which is contained in a

I know how to do this in PHP:

function foo($a) {
    echo $$a;
}

global $string = "blah"; // might not need to be global but that's irrelevant
foo("string"); // prints "blah"

Any way to do this?

Solution 1

If it's a global variable, then you can do:

>>> a = 5
>>> globals()['a']
5

A note about the various "eval" solutions: you should be careful with eval, especially if the string you're evaluating comes from a potentially untrusted source -- otherwise, you might end up deleting the entire contents of your disk or something like that if you're given a malicious string.

(If it's not global, then you'll need access to whatever namespace it's defined in. If you don't have that, there's no way you'll be able to access it.)

Solution 2

Edward Loper's answer only works if the variable is in the current module. To get a value in another module, you can use getattr:

import other
print getattr(other, "name_of_variable")

https://docs.python.org/3/library/functions.html#getattr

Solution 3

Assuming that you know the string is safe to evaluate, then eval will give the value of the variable in the current context.

>>> string = "blah"
>>> string
'blah'
>>> x = "string"
>>> eval(x)
'blah'

Solution 4

>>> x=5
>>> print eval('x')
5

tada!

View more solutions

Comments

[mrivard]

locals() for locals.

[Dirk]

And if you want to use this method to get a value in the same module that you are calling getattr(), use import sys; sys.modules[__name__] to get a reference to the current module.

[Guillermo Mosse]

do you have an alternative for python3?

[eresonance]

@GuillermoMosse no, it should be the same: docs.python.org/3.8/library/functions.html#getattr

[Sapphire_Brick]

Security holes: "tada!".

[m0bbin]

@Sapphire_Brick to be fair as long as the str that is being eval is a constant it shouldn't be a big problem.

[Ethan]

@m0bbin it is still a major security issue, if attackers know what the string is they can search for it in memory and edit it easily