In Python, how do I get a variable via a string representation of the name of the variable?

Question

I'm using Python 2.7.

Take, for example, the following code:

class Apple:
    def __init__(self, n):
        self.n = n
    def printVariable(self, s): # print variable named s
        if hasattr(self, s):
            print ...

What would I replace ... with to print self.'s'. For example, if I called printVariable('n'), what would I put in place of ... to print n?

Of course, self.s would not work because first of all there is no attribute self.s, but more importantly, that's printing a different variable, not the variable self.'s' I want to print the variable whose name is represented by the string s that is passed to the method.

I'm sorry about the inherently confusing nature of self.s and self.'s' and s in this question.

It's worth a note that this can often be a sign of a bigger design issue - often this means you want a data structure and not an attribute. What are you actually trying to achieve wiht this? — Gareth Latty, Aug 01 '14 at 20:19
You should always try to keep data out of your variable names: http://nedbatchelder.com/blog/201112/keep_data_out_of_your_variable_names.html — TheSoundDefense, Aug 01 '14 at 20:21

Robᵩ · Accepted Answer · 2014-08-01T20:34:13.867

2

If hasattr(self,s), is sufficient to your needs, then you want getattr() :

if hasattr(self, s):
    print getattr(self, s)

In fact, you may be able to skip the hasattr altogether, depending upon your precise requirement. getattr() takes a default value to return if the attribute is missing:

print gettattr(self, s, 'No such attribute: '+s)

If you want to find variables outside of the current object (say, in local scope or global scope, or in another object), try one of these:

locals()[s]
globals()[s]
getattr(other_object, s)

Note: using locals(), globals(), and to a lesser extent, hasattr(self,s), outside of a few limited cases, is a code smell. It almost very likely means that your design is flawed.

edited Aug 01 '14 at 20:34

answered Aug 01 '14 at 20:18

Robᵩ

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As a note, the `locals()` and `globals()` calls are extremely rarely a good idea. – Gareth Latty Aug 01 '14 at 20:24
I wholeheartedly agree. – Robᵩ Aug 01 '14 at 20:24

score 1 · Answer 2 · answered Aug 01 '14 at 20:18

1

I think this is what you're after, but not totally sure...

print getattr(self, s)

You can also use it to specify what to return if it doesn't exist. See the getattr docs

print getattr(self, s, 'default value')

answered Aug 01 '14 at 20:18

Matt Dodge

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In Python, how do I get a variable via a string representation of the name of the variable?

2 Answers2