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). — user395760
– user395760, Commented Jun 1, 2011 at 16:43
The shortest empty class definition I've seen is from gossamer-threads.com/lists/python/python/832915#832915: Employee=type('Employee',(),{}) and then john=Employee() etc. — eudoxos
– eudoxos, Commented Jun 3, 2015 at 9:18

Katriel · Accepted Answer · 2011-06-01 16:37:31Z

55

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)

edited Jun 1, 2011 at 16:37

answered Jun 1, 2011 at 15:57

Katriel

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Sjon · Accepted Answer · 2015-10-10 14:07:56Z

21

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, 2015 at 14:07

Sjon

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answered Jun 1, 2011 at 15:52

Kiril Kirov

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6 Comments

Katriel Over a year ago

This doesn't actually work: lambdas can't contain statements.

Kiril Kirov Over a year ago

@katrielalex - Works perfect on my python 3.2 (with added parentheses for print, of course). What statemets?

Thomas K Over a year ago

@Kirii: In Python 2, print is a statement. You're adding brackets because in Python 3, it became a function.

Kiril Kirov Over a year ago

@Thomas K - ah, thanks! I didn't know, that print is statement in Python2.

Eric Wilson Over a year ago

Since the OP wanted to dynamically define a function, perhaps the print example wasn't the best. +1

|

András Aszódi · Accepted Answer · 2016-09-08 12:20:53Z

2

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)

answered Sep 8, 2016 at 12:20

András Aszódi

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Sudheer · Accepted Answer · 2015-02-24 17:53:59Z

-1

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, 2015 at 17:53

Sudheer

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1 Comment

iperov Over a year ago

sure, more dependencies for the God of Dependencies

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