What is getattr() exactly and how do I use it?

Question

I was reading about the getattr() function. The problem is that I still can't grasp the idea of its usage. The only thing I understand about getattr() is that getattr(li, "pop") is the same as calling li.pop.

I didn't understand when the book mentioned how you use it to get a reference to a function without knowing its name until run-time. Maybe this is me being a noob in programming, in general. Could anyone shed some light to the subject? When and how do I use this exactly?

Answer 1

Quite frequently when I am creating an XML file from data stored in a class I would frequently receive errors if the attribute didn't exist or was of type None. In this case, my issue wasn't not knowing what the attribute name was, as stated in your question, but rather was data ever stored in that attribute.

class Pet:
    def __init__(self):
        self.hair = None
        self.color = None

If I used hasattr to do this, it would return True even if the attribute value was of type None and this would cause my ElementTree set command to fail.

hasattr(temp, 'hair')
>>True

If the attribute value was of type None, getattr would also return it which would cause my ElementTree set command to fail.

c = getattr(temp, 'hair')
type(c)
>> NoneType

I use the following method to take care of these cases now:

def getRealAttr(class_obj, class_attr, default = ''):
    temp = getattr(class_obj, class_attr, default)
    if temp is None:
        temp = default
    elif type(temp) != str:
        temp = str(temp)
    return temp

This is when and how I use getattr.

Answer 2

A pretty common use case for getattr is mapping data to functions.

For instance, in a web framework like Django or Pylons, getattr makes it straightforward to map a web request's URL to the function that's going to handle it. If you look under the hood of Pylons's routing, for instance, you'll see that (by default, at least) it chops up a request's URL, like:

http://www.example.com/customers/list

into "customers" and "list". Then it searches for a controller class named CustomerController. Assuming it finds the class, it creates an instance of the class and then uses getattr to get its list method. It then calls that method, passing it the request as an argument.

Once you grasp this idea, it becomes really easy to extend the functionality of a web application: just add new methods to the controller classes, and then create links in your pages that use the appropriate URLs for those methods. All of this is made possible by getattr.

Answer 3

# getattr

class hithere():

    def french(self):
        print 'bonjour'

    def english(self):
        print 'hello'

    def german(self):
        print 'hallo'

    def czech(self):
        print 'ahoj'

    def noidea(self):
        print 'unknown language'


def dispatch(language):
    try:
        getattr(hithere(),language)()
    except:
        getattr(hithere(),'noidea')()
        # note, do better error handling than this

dispatch('french')
dispatch('english')
dispatch('german')
dispatch('czech')
dispatch('spanish')

Answer 4

Another use of getattr() in implementing a switch statement in Python. It uses both reflection to get the case type.

import sys

class SwitchStatement(object):
    """ a class to implement switch statement and a way to show how to use gettattr in Pythion"""

    def case_1(self):
        return "value for case_1"

    def case_2(self):
        return "value for case_2"

    def case_3(self):
        return "value for case_3"

    def case_4(self):
        return "value for case_4"

    def case_value(self, case_type=1):
        """This is the main dispatchmethod, that uses gettattr"""
        case_method = 'case_' + str(case_type)
        # fetch the relevant method name
        # Get the method from 'self'. Default to a lambda.
        method = getattr(self, case_method, lambda: "Invalid case type")
        # Call the method as we return it
        return method()

def main(_):
    switch = SwitchStatement()
    print swtich.case_value(_)

if __name__ == '__main__':
    main(int(sys.argv[1]))

Answer 5

I sometimes use getattr(..) to lazily initialise attributes of secondary importance just before they are used in the code.

Compare the following:

class Graph(object):
    def __init__(self):
        self.n_calls_to_plot = 0

    #...
    #A lot of code here
    #...

    def plot(self):
        self.n_calls_to_plot += 1

To this:

class Graph(object):
    def plot(self):
        self.n_calls_to_plot = 1 + getattr(self, "n_calls_to_plot", 0)

The advantage of the second way is that n_calls_to_plot only appears around the place in the code where it is used. This is good for readability, because (1) you can immediately see what value it starts with when reading how it's used, (2) it doesn't introduce a distraction into the __init__(..) method, which ideally should be about the conceptual state of the class, rather than some utility counter that is only used by one of the function's methods for technical reasons, such as optimisation, and has nothing to do with the meaning of the object.

Answer 6

Objects in Python can have attributes -- data attributes and functions to work with those (methods). Actually, every object has built-in attributes.

For example you have an object person, that has several attributes: name, gender, etc.

You access these attributes (be it methods or data objects) usually writing: person.name, person.gender, person.the_method(), etc.

But what if you don't know the attribute's name at the time you write the program? For example you have attribute's name stored in a variable called attr_name.

if

attr_name = 'gender'

then, instead of writing

gender = person.gender

you can write

gender = getattr(person, attr_name)

Some practice:

Python 3.4.0 (default, Apr 11 2014, 13:05:11)

>>> class Person():
...     name = 'Victor'
...     def say(self, what):
...         print(self.name, what)
... 
>>> getattr(Person, 'name')
'Victor'
>>> attr_name = 'name'
>>> person = Person()
>>> getattr(person, attr_name)
'Victor'
>>> getattr(person, 'say')('Hello')
Victor Hello

getattr will raise AttributeError if attribute with the given name does not exist in the object:

>>> getattr(person, 'age')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Person' object has no attribute 'age'

But you can pass a default value as the third argument, which will be returned if such attribute does not exist:

>>> getattr(person, 'age', 0)
0

You can use getattr along with dir to iterate over all attribute names and get their values:

>>> dir(1000)
['__abs__', '__add__', ..., '__trunc__', '__xor__', 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']

>>> obj = 1000
>>> for attr_name in dir(obj):
...     attr_value = getattr(obj, attr_name)
...     print(attr_name, attr_value, callable(attr_value))
... 
__abs__ <method-wrapper '__abs__' of int object at 0x7f4e927c2f90> True
...
bit_length <built-in method bit_length of int object at 0x7f4e927c2f90> True
...

>>> getattr(1000, 'bit_length')()
10

A practical use for this would be to find all methods whose names start with test and call them.

Similar to getattr there is setattr which allows you to set an attribute of an object having its name:

>>> setattr(person, 'name', 'Andrew')
>>> person.name  # accessing instance attribute
'Andrew'
>>> Person.name  # accessing class attribute
'Victor'
>>>