Why does `getattr` not support consecutive attribute retrievals?

Question

class A(): pass

a = A()
b = A()

a.b = b
b.c = 1

a.b     # this is b
getattr(a, "b") # so is this

a.b.c   # this is 1   
getattr(a, "b.c") # this raises an AttributeError

It seemed very natural to me to assume the latter. I'm sure there is a good reason for this. What is it?

Answer 1

You can't put a period in the getattr function because getattr is like accessing the dictionary lookup of the object (but is a little bit more complex than that, due to subclassing and other Python implementation details).

If you use the 'dir' function on a, you'll see the dictionary keys that correspond to your object's attributes. In this case, the string "b.c" isn't in the set of dictionary keys.

The only way to do this with getattr is to nest calls:

getattr(getattr(a, "b"), "c")

Luckily, the standard library has a better solution!

import operator
operator.attrgetter("b.c")(a)

Answer 2

Python's built-in reduce function enables the functionality you're looking for. Here's a simple little helper function that will get the job done:

class NoDefaultProvided(object):
    pass

def getattrd(obj, name, default=NoDefaultProvided):
    """
    Same as getattr(), but allows dot notation lookup
    Discussed in:
    http://stackoverflow.com/questions/11975781
    """

    try:
        return reduce(getattr, name.split("."), obj)
    except AttributeError, e:
        if default != NoDefaultProvided:
            return default
        raise

Test proof;

>>> getattrd(int, 'a')
AttributeError: type object 'int' has no attribute 'a'

>>> getattr(int, 'a')
AttributeError: type object 'int' has no attribute 'a'

>>> getattrd(int, 'a', None)
None

>>> getattr(int, 'a', None)
None

>>> getattrd(int, 'a', None)
None

>>> getattrd(int, '__class__.__name__')
type

>>> getattrd(int, '__class__')
<type 'type'>

Answer 3

I think your confusion arises from the fact that straight dot notation (ex a.b.c) accesses the same parameters as getattr(), but the parsing logic is different. While they both essentially key in to an object's __dict__ attribute, getattr() is not bound to the more stringent requirements on dot-accessible attributes. For instance

setattr(foo, 'Big fat ugly string.  But you can hash it.', 2)

Is valid, since that string just becomes a hash key in foo.__dict__, but

foo.Big fat ugly string.  But you can hash it. = 2

and

foo.'Big fat ugly string.  But you can hash it.' = 2

are syntax errors because now you are asking the interpreter to parse these things as raw code, and that doesn't work.

The flip side of this is that while foo.b.c is equivalent to foo.__dict__['b'].__dict__['c'], getattr(foo, 'b.c') is equivalent to foo.__dict__['b.c']. That's why getattr doesn't work as you are expecting.

Answer 4

Because getattr doesn't work that way. getattr gets attribute of a given object (first argument) with a given name (second argument). So your code:

getattr(a, "b.c") # this raises an AttributeError

means: Access "b.c" attribute of object referenced by "a". Obviously your object doesn't have attribute called "b.c".

To get "c" attribute you must use two getattr calls:

getattr(getattr(a, "b"), "c")

Let's unwrap it for better understanding:

b = getattr(a, "b")
c = getattr(b, "c")

Answer 5

I think the most straight forward way to achieve what you want is to use operator.attrgetter.

>>> import operator
>>> class B():
...   c = 'foo'
... 
>>> class A():
...   b = B()
... 
>>> a = A()
>>> operator.attrgetter('b.c')(a)
'foo'

If the attribute doesn't exist then you'll get an AttributeError

>>> operator.attrgetter('b.d')(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: B instance has no attribute 'd'

Answer 6

You can call the multiple getattr without calling a function within function by splitting the dot operators and performing a getattr() for each dot operator

def multi_getattr(self,obj, attr, default = None):
          attributes = attr.split(".")
          for i in attributes:
              try:
                  obj = getattr(obj, i)
              except AttributeError:
                  if default:
                      return default
                  else:
                      raise
          return obj

If suppose you wish to call a.b.c.d you can do it via a.multi_getattr('b.c.d'). This will generalise the operation without worrying about the count of dot operation one has in the string.

Answer 7

What should return getattr('a.b', {'a': None}, 'default-value'}? Should it raise AttributeError or just return 'default-value'? That's why complex keys if introduced in getattr would make it obscure to use.

So, it's more natural to view getattr(..) function as get method of dictionary of object attributes.