foo and _foo - about variables inside a class

Question

class ClassName(object):
    """
    """    
    def __init__(self, foo, bar):
        """
        """
        self.foo = foo # read-write property
        self.bar = bar # simple attribute

    def _set_foo(self, value):
        self._foo = value

    def _get_foo(self):
        return self._foo

    foo = property(_get_foo, _set_foo)

a = ClassName(1,2)
#a._set_foo(3)
print a._get_foo()

When I print a._get_foo() the function _get_foo prints the variable self._foo . But where does it come from? self._foo and self.foo are different, aren't they?

EDIT: The problem is that I still not understand what property does. Give me some time.

Answer 1

According to your code, it "comes from" _set_foo. In your init, when you do self.foo = foo, that calls _set_foo(1), which performs self._foo = 1.

You can see this more clearly if you add a print statement inside _set_foo().

Answer 2

Docs for property explain how it's supposed to be used. There is no difference between these lines:

self.foo = foo # read-write property

and

a.foo = 3

You're also not supposed to call setters and getters manually.

eta: if you don't understand what property does after looking at the examples provided in the docs and reading our answers, you perhaps should just abstain from using it. It really is not the most essential of Python's features. You might want to start with a simpler example, which is easy to understand:

>>> class ClassName(object):
    def __init__(self, foo):
        self.foo = foo


>>> a = ClassName(1, 2)
>>> a.foo
1
>>> a.foo = 42
>>> a.foo
42

Answer 3

You define foo to be a property with getters and setters.

Those getters and setters use the variable self._foo, since you coded it that way:

def _set_foo(self, value):
  self._foo = value

In your case you would get a problem if you had a slightly different implementation:

class ClassName(object):
    """
    """    
    def __init__(self,):
        """
        """
        pass

    def _set_foo(self, value):
        self._foo = value

    def _get_foo(self):
        return self._foo

    foo = property(_get_foo, _set_foo)


 a = ClassName()
 print a.foo
 -> AttributeError

 b = ClassName()
 b.foo = 1 # implicitely sets self._foo !
 print b.foo
 >> OK, this is 1

As it happens you indeed should set self.foo in the __init__ since the setter creates self._foo. (Or initialize self._foo in the __init__ directly of course).

So, where do self.foo and self._foo come from? It's explicit in the code. self.foo is a property, and self._foo is the variable in which you decided to keep the value of that property.

Answer 4

I really recommend this site:

http://adam.gomaa.us/blog/2008/aug/11/the-python-property-builtin/

A good explanation of the property-function.