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Python Object-Oriented Python (retired) Inheritance Instance Methods

Tobias Edwards
Tobias Edwards
14,458 Points
Posted by Tobias Edwards
Tobias Edwards
14,458 Points

What does setattr() actually do?

In this series, Kenneth keeps using setattr(), but why?

What actually does this function do and how do you use it?

Thank you

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

As stated in the docs, setattr() is the counterpart of getattr(). The arguments are an object, a string and an arbitrary value. The string may name an existing attribute or a new attribute. The function assigns the value to the attribute, provided the object allows it. For example, setattr(x, 'foobar', 123) is equivalent to x.foobar = 123.

setattr() can be used when the name of the attribute you wish to set is contained in a variable.. Say you have a set of keyword variables you wish to add to an object as attributes. You can loop through the kwargs and add them using setattr():

$ ipython3
Python 3.4.0 (default, Jun 19 2015, 14:20:21) 
Type "copyright", "credits" or "license" for more information.

IPython 1.2.1 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: class myobj():
   ...:         pass
   ...: 

In [2]: kwargs = {'username': "snakeherder", 'email': "sherder@example.com"}

In [3]: for key, value in kwargs.items():
   ...:     setattr(myobj, key, value)
   ...: 

In [4]: myobj.username
Out[4]: 'snakeherder'

In [5]: myobj.email
Out[5]: 'sherder@example.com'
Tobias Edwards
Tobias Edwards
14,458 Points
Tobias Edwards
Tobias Edwards
14,458 Points

Awesome, thanks! Now, what does getattr() mean?

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

From the docs: getattr(object, name[, default]) Returns the value of the named attribute of object. name must be a string. If the string is the name of one of the object’s attributes, the result is the value of that attribute. For example, getattr(x, 'foobar') is equivalent to x.foobar. If the named attribute does not exist, default is returned if provided, otherwise AttributeError is raised.

Using getattr(x, 'foobar') instead of x.foobar has the advantage of being able to specify a default. This prevents an error when looking up an attribute that may not exist. For example,

username = getattr(user, 'username', None)
if username:
    # do something

This is easier to read then having to wrap code in a try statement:

try:
    username = user.username
except AttributeError:
    username = None
else:
    # do something
fahad lashari
fahad lashari
7,693 Points
fahad lashari
fahad lashari
7,693 Points

Chris Freeman Thank you Chris for this brilliant explanation and the perfect code example! I had been struggling to understand this for a while now. Much appreciated!

Kind regards,

Fahad