Python: access class property from string

Question

I have a class like the following:

class User:
    def __init__(self):
        self.data = []
        self.other_data = []

    def doSomething(self, source):
        // if source = 'other_data' how to access self.other_data

I want to pass a string for the source variable in doSomething and access the class member of the same name.

I have tried getattr which only works on functions (from what I can tell) as well as having User extend dict and using self.__getitem__, but that doesn't work either. What is the best way to do this?

score 383 · Accepted Answer · edited Oct 01 '17 at 15:04

383

x = getattr(self, source) will work just perfectly if source names ANY attribute of self, including the other_data in your example.

edited Oct 01 '17 at 15:04

serv-inc

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answered Jul 22 '09 at 18:55

Alex Martelli

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(realizing this is an old thread). I was confused/always forget this because of dicts. If I want to get a value from a dict, I can say: myDict.get('keyy'), so I would expect attributes to work the same way: myObject.getattr('attr_name'). But instead they take the object as a first argument...ok to do, but the apparent inconsistency is why I had trouble. – Robert Lugg Nov 20 '14 at 22:53

score 264 · Answer 2 · answered Jul 22 '09 at 20:45

264

A picture's worth a thousand words:

>>> class c:
        pass
o = c()
>>> setattr(o, "foo", "bar")
>>> o.foo
'bar'
>>> getattr(o, "foo")
'bar'

answered Jul 22 '09 at 20:45

Robert Rossney

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I like this answer in particular because it illustrates, simply, that the getattr() function works outside class methods as well. – Eric Dec 08 '11 at 04:49

score 109 · Answer 3 · answered Oct 10 '16 at 04:01

109

getattr(x, 'y') is equivalent to x.y
setattr(x, 'y', v) is equivalent to x.y = v
delattr(x, 'y') is equivalent to del x.y

answered Oct 10 '16 at 04:01

Yas

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1

How I can access `x.y` according to the value string of `v`? – user2284570 Sep 04 '17 at 01:48
1

@user2284570: If `v` contains the string `'y'` then `getattr(x, v)` gives the value of `x.y`. – md2perpe Nov 02 '17 at 11:31
@md2perpe except it is not the case. – user2284570 Nov 02 '17 at 11:45
3

@user2284570: How was your case, then? – md2perpe Nov 02 '17 at 13:49

score 5 · Answer 4 · answered Jul 22 '09 at 19:50

Extending Alex's answer slightly:

class User:
    def __init__(self):
        self.data = [1,2,3]
        self.other_data = [4,5,6]
    def doSomething(self, source):
        dataSource = getattr(self,source)
        return dataSource

A = User()
print A.doSomething("data")
print A.doSomething("other_data")

will yield:

[1, 2, 3]
[4, 5, 6]

However, personally I don't think that's great style - getattr will let you access any attribute of the instance, including things like the doSomething method itself, or even the __dict__ of the instance. I would suggest that instead you implement a dictionary of data sources, like so:

class User:
    def __init__(self):

        self.data_sources = {
            "data": [1,2,3],
            "other_data":[4,5,6],
        }

    def doSomething(self, source):
        dataSource = self.data_sources[source]
        return dataSource

A = User()

print A.doSomething("data")
print A.doSomething("other_data")

again yielding:

[1, 2, 3]
[4, 5, 6]

I didn't explain properly... my apologies. I am able to get the data via the string. However, I am having problems setting the data using the above methods. Specifically I have tried getattr(self, "other_data") = [1, 2, 3] as well as self.__setitem__("other_data", [1, 2, 3]) — sberry, Jul 22 '09 at 20:39
@sberry2A, `getattr()` is only for getting, `setattr()` is for setting. You can't assign to the return value from `getattr()`. — smci, Aug 16 '11 at 23:44

Python: access class property from string

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