use one function to getattr for either functions or regular attributes

Question

I have the following code:

In [38]: %paste
def set_session_attribute(obj, attribute):
    if attribute.endswith('()'):
        attribute = attribute.replace('()', '')
        return getattr(obj, attribute)()
    else:
        return getattr(obj, attribute)


class Thing(object):
    def __init__(self):
        self.legs = 4
    def length(self):
        return 6

## -- End pasted text --

In [39]: x = Thing()

In [40]: y = set_session_attribute(x, 'legs')

In [41]: y
Out[41]: 4

In [42]: z = set_session_attribute(x, 'length()')

In [43]: z
Out[43]: 6

This is because calling with "length()" didn't work (AttributeError, no attribute length())

Is there a shorter, more maintainable way to make functions like this? Thank you.

Mike Müller · Accepted Answer · 2016-04-04T20:28:50.263

5

Solution 1

You can make length a property:

class Thing(object):
    def __init__(self):
        self.legs = 4
    @property
    def length(self):
        return 6

>>> thing = Thing()
>>> thing.legs
4
>>> thing.length
6

If you really want to use your function:

def set_session_attribute(obj, attribute):
    return getattr(obj, attribute)

>>> set_session_attribute(thing, 'legs')
4
>>> set_session_attribute(thing, 'length')
6

If you cannot change the source of thing directly, you can do after importing the class:

class Thing(object):
    def __init__(self):
        self.legs = 4
    def length(self):
        return 6

Here:

Thing.length2 = property(Thing.length)

>>> thing = Thing()
>>> thing.length2
6

Solution 2

Alternatively, you can check if the attribute is callable:

class Thing(object):
    def __init__(self):
        self.legs = 4
    def length(self):
        return 6

def set_session_attribute(obj, attribute):
    attr = getattr(obj, attribute)
    if hasattr(attr, '__call__'):  # may use `callable()`
        return attr()
    return attr

>> thing = Thing()
>>> set_session_attribute(thing, 'legs')
4
>>> set_session_attribute(thing, 'length')
6

edited Apr 04 '16 at 20:28

answered Apr 04 '16 at 19:55

Mike Müller

82,630
20
166
161

I can't change the function I'm trying to call – codyc4321 Apr 04 '16 at 20:05
1

Do you mean the function `set_session_attribute` or a method in the class `Thing` or another function? – Mike Müller Apr 04 '16 at 20:07
I can't change anything in the classes I'm accessing, like Thing(), a huge system depends on those – codyc4321 Apr 04 '16 at 20:08
1

Added a second solution that does not need to change `Thing`. – Mike Müller Apr 04 '16 at 20:55
that if/return is better than what I ended with yesterday. ty – codyc4321 Apr 05 '16 at 14:23

score 1 · Answer 2 · answered Apr 04 '16 at 20:14

Decorate those methods with @property, which you feel should have semantics of data rather than a method.

class Thing:
    @property
    def length(self):
        return somedata

    @property.setter
    def length(self, value):
        somename = value

t = Thing()
l0 = t.length
t.length = l1

So this is a getter. There is also a setter @property.setter, where you can do t.length = somedata instead of something like t.set_length(somedata)

use one function to getattr for either functions or regular attributes

2 Answers2

Solution 1

Solution 2