CODEWITHSUNDEEP
pythonclass
Baf
Baf

Reputation: 2251

Classes How I understand them. Correct me if Im wrong please

I really hope this is not a question posed by millions of newbies, but my search didn t really give me a satisfying answer.

So my question is fairly simple. Are classes basically a container for functions with its own namespace? What other functions do they have beside providing a separate namespace and holding functions while making them callable as class atributes? Im asking in a python context.

Oh and thanks for the great help most of you have been!

Upvotes: 3

Views: 234

Answers (4)

iancoleman
iancoleman

Reputation: 2786

A class is the definition of an object. In this sense, the class provides a namespace of sorts, but that is not the true purpose of a class. The true purpose is to define what the object will 'look like' - what the object is capable of doing (methods) and what it will know (properties).

Note that my answer is intended to provide a sense of understanding on a relatively non-technical level, which is what my initial trouble was with understanding classes. I'm sure there will be many other great answers to this question; I hope this one adds to your overall understanding.

Upvotes: 3

Fred Foo
Fred Foo

Reputation: 363797

  1. More importantly than functions, class instances hold data attributes, allowing you to define new data types beyond what is built into the language; and
  2. they support inheritance and duck typing.

For example, here's a moderately useful class. Since Python files (created with open) don't remember their own name, let's make a file class that does.

class NamedFile(object):
    def __init__(self, name):
        self._f = f
        self.name = name

    def readline(self):
        return self._f.readline()

Had Python not had classes, you'd probably be working with dicts instead:

def open_file(name):
    return {"name": name, "f": open(name)}

Needless to say, calling myfile["f"].readline() all the time will cause your fingers to hurt at some point. You could of course introduce a function readline in a NamedFile module (namespace), but then you'd always have to use that exact function. By contrast, NamedFile instances can be used anywhere you need an object with a readline method, so it would be a plug-in replacement for file in many situation. That's called polymorphism, one of the biggest benefits of OO/class-based programming.

(Also, dict is a class, so using it violates the assumption that there are no classes :)

Upvotes: 9

Vivek S
Vivek S

Reputation: 5550

In most languages, classes are just pieces of code that describe how to produce an object. That's kinda true in Python too:

>>> class ObjectCreator(object):
...       pass
... 

>>> my_object = ObjectCreator()
>>> print my_object
<__main__.ObjectCreator object at 0x8974f2c>

But classes are more than that in Python. Classes are objects too.

Yes, objects.

As soon as you use the keyword class, Python executes it and creates an OBJECT. The instruction:

>>> class ObjectCreator(object):
...       pass
...

creates in memory an object with the name ObjectCreator.

This object (the class) is itself capable of creating objects (the instances), and this is why it's a class.

But still, it's an object, and therefore:

  • you can assign it to a variable
  • you can copy it
  • you can add attributes to it
  • you can pass it as a function parameter

e.g.:

>>> print ObjectCreator # you can print a class because it's an object
<class '__main__.ObjectCreator'>
>>> def echo(o):
...       print o
... 
>>> echo(ObjectCreator) # you can pass a class as a parameter
<class '__main__.ObjectCreator'>
>>> print hasattr(ObjectCreator, 'new_attribute')
False
>>> ObjectCreator.new_attribute = 'foo' # you can add attributes to a class
>>> print hasattr(ObjectCreator, 'new_attribute')
True
>>> print ObjectCreator.new_attribute
foo
>>> ObjectCreatorMirror = ObjectCreator # you can assign a class to a variable
>>> print ObjectCreatorMirror.new_attribute
foo
>>> print ObjectCreatorMirror()
<__main__.ObjectCreator object at 0x8997b4c>

Upvotes: 8

tkone
tkone

Reputation: 22758

Classes (or objects) are used to provide encapsulation of data and operations that can be performed on that data.

They don't provide namespacing in Python per se; module imports provide the same type of stuff and a module can be entirely functional rather than object oriented.

You might gain some benefit from looking at OOP With Python, Dive into Python, Chapter 5. Objects and Object Oriented Programming or even just the Wikipedia article on object oriented programming

Upvotes: 4

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