Empty class object in Python

Question

I'm teaching a Python class on object-oriented programming and as I'm brushing up on how to explain classes, I saw an empty class definition:

class Employee:
    pass

The example then goes on to define a name and other attributes for an object of this class:

john = Employee()
john.full_name = "john doe"

Interesting!

I'm wondering if there's a way to dynamically define a function for an instance of a class like this? something like:

john.greet() = print 'Hello, World!'

This doesn't work in my Python interpreter, but is there another way of doing it?

Possible? Yes. A good idea? Rarely (outside of some metaprogramming, of course). — , Jun 01 '11 at 16:43
The shortest empty class definition I've seen is from http://www.gossamer-threads.com/lists/python/python/832915#832915: `Employee=type('Employee',(),{})` and then `john=Employee()` etc. — eudoxos, Jun 03 '15 at 09:18

Katriel · Accepted Answer · 2011-06-01T16:37:31.390

A class is more or less a fancy wrapper for a dict of attributes to objects. When you instantiate a class you can assign to its attributes, and those will be stored in foo.__dict__; likewise, you can look in foo.__dict__ for any attributes you have already written.

This means you can do some neat dynamic things like:

class Employee: pass
def foo(self): pass
Employee.foo = foo

as well as assigning to a particular instance. (EDIT: added self parameter)

score 19 · Answer 2 · edited Oct 10 '15 at 14:07

19

Try with lambda:

john.greet = lambda : print( 'hello world!' )

The you'll be able to do:

john.greet()

EDIT: Thanks Thomas K for the note - this works on Python 3.2 and not for Python2, where print appeared to be statement. But this will work for lambdas, without statements (right? Sorry, I know only python3.2 (: )

edited Oct 10 '15 at 14:07

Sjon

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answered Jun 01 '11 at 15:52

Kiril Kirov

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2

This doesn't actually work: lambdas can't contain statements. – Katriel Jun 01 '11 at 15:58
@katrielalex - Works perfect on my python 3.2 (with added parentheses for `print`, of course). What statemets? – Kiril Kirov Jun 01 '11 at 16:00
1

@Kirii: In Python 2, `print` is a statement. You're adding brackets because in Python 3, it became a function. – Thomas K Jun 01 '11 at 16:05
@Thomas K - ah, thanks! I didn't know, that `print` is statement in Python2. – Kiril Kirov Jun 01 '11 at 16:06
2

Since the OP wanted to dynamically define a function, perhaps the `print` example wasn't the best. +1 – Eric Wilson Jun 01 '11 at 16:07

score 1 · Answer 3 · answered Sep 08 '16 at 12:20

You could also use "named tuples" from the collection standard module. Named tuples work like "ordinary" tuples but the elements have names and you can access the elements using the "dot syntax". From the collection docs:

>>> # Basic example
>>> Point = namedtuple('Point', ['x', 'y'])
>>> p = Point(11, y=22)     # instantiate with positional or keyword arguments
>>> p[0] + p[1]             # indexable like the plain tuple (11, 22)
33
>>> x, y = p                # unpack like a regular tuple
>>> x, y
(11, 22)
>>> p.x + p.y               # fields also accessible by name
33
>>> p                       # readable __repr__ with a name=value style
Point(x=11, y=22)

score 0 · Answer 4 · answered Feb 24 '15 at 17:53

0

You could use AttrDict

>>> from attrdict import AttrDict
>>> my_object = AttrDict()
>>> my_object.my_attribute = 'blah'
>>> print my_object.my_attribute
blah
>>>

Install attrdict from PyPI:

pip install attrdict

It's useful in other situations too - like when you need attribute access to dict keys.

answered Feb 24 '15 at 17:53

Sudheer

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sure, more dependencies for the God of Dependencies – iperov Jan 03 '22 at 12:11

Empty class object in Python

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