So what does setattr() and getattr() actually do?

Question

In reference to the code in this thread:

https://teamtreehouse.com/community/my-address-book-class-with-search-function-regular-expressions-in-python-extra-credit

I'm wondering exactly is the purpose of this part of the code:

class Contact(object):
    def __init__(self, match):
        for key, value in match.groupdict().items():
            setattr(self, key, value)

    def __str__(self):
        return '\n'.join(
            sorted(
                ["\t{}: {}".format(
                                   key, val
                                   ) for key, val in self.__dict__.items()]))

In python documentation: it says that setattr(x, 'foobar', 123) is the same as x.foobar = 123.

So why when I substitute setattr(self, key, value) for self.key = value, the code behaves differently?

[MOD: fixed formatting -cf]

Answer 1 · 2015-10-29T02:04:11Z

October 29, 2015 2:04am

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. If the name of the attribute you are trying to set is in held in a variable, then x.variable_name = "some value" won't work, but instead it will set the attribute name to literally "variable_name".

You can loop through kwargs and add them using setattr() without having to know the precise names of the keywords:

$ 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'

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.

The same case as above, when the attribute name is in a variable, trying to access x.variable_name won't work.

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

Digging specifically into your example code:

class Contact(object):
    def __init__(self, match):
        '''Initialize an instance using contents of 'match'

        match is an 're.match()' object
        match.groupdict() is a dict of successful matches
        '''
        # unpack each dict itemin to key:value pair 
        for key, value in match.groupdict().items():
            # set the attribute 'key' to 'value
            setattr(self, key, value)

    def __str__(self):
        '''Instance string method. 

        Returns a string of attributes in the form "key: value".
        The string is constructed by generating a list of strings
        where each string is in the form "key: value". The 'key', 
        'value'attributes are generated by unpacking the output
        the '__dict__.items()' method which lists all attributes
        in a (key, value) tuple. This list of strings is then sorted
        then joined together using a newline character to form
        a multi-line string.
        '''
        return '\n'.join(
            sorted(
                ["\t{}: {}".format(
                                   key, val
                                   ) for key, val in self.__dict__.items()]))

Answer 2 · 2018-02-16T20:48:24Z

February 16, 2018 8:48pm

I'd like to suggest adding a key bit of explanation to this course: how to display/see the values that are collected using setattr. It's not obvious at all in the Python documentation, and is easy to misunderstand. This could be as simple as "To see the values that were set via **kwargs and setattr(), use ...." and provide an example. For a beginner, setattr does some opaque magic that it's not obvious how to see inside of.

Answer 3 · 2016-09-15T12:13:06Z

September 15, 2016 12:13pm

Chris Freeman ,Don't you feel our python teacher ,which Mr Love, doesn't really explain it in more detail. He just explain some "key part" within few second ,such as setattr. It is just like he writes his code and doesn't really explain everything.

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So what does setattr() and getattr() actually do?

3 Answers

Chris Freeman

Chris Freeman

David Yao

David Yao

Chris Freeman

Chris Freeman

James Gill

James Gill

Chris Freeman

Chris Freeman

Name:GoogleSearch orJonathan Sum

Name:GoogleSearch orJonathan Sum

Chris Freeman

Chris Freeman