How does the @property decorator work in Python?

Question

I would like to understand how the built-in function property works. What confuses me is that property can also be used as a decorator, but it only

Answer 1

Here is another example:

##
## Python Properties Example
##
class GetterSetterExample( object ):
    ## Set the default value for x ( we reference it using self.x, set a value using self.x = value )
    __x = None


##
## On Class Initialization - do something... if we want..
##
def __init__( self ):
    ## Set a value to __x through the getter / setter... Since __x is defined above, this doesn't need to be set...
    self.x = 1234

    return None


##
## Define x as a property, ie a getter - All getters should have a default value arg, so I added it - it will not be passed in when setting a value, so you need to set the default here so it will be used..
##
@property
def x( self, _default = None ):
    ## I added an optional default value argument as all getters should have this - set it to the default value you want to return...
    _value = ( self.__x, _default )[ self.__x == None ]

    ## Debugging - so you can see the order the calls are made...
    print( '[ Test Class ] Get x = ' + str( _value ) )

    ## Return the value - we are a getter afterall...
    return _value


##
## Define the setter function for x...
##
@x.setter
def x( self, _value = None ):
    ## Debugging - so you can see the order the calls are made...
    print( '[ Test Class ] Set x = ' + str( _value ) )

    ## This is to show the setter function works.... If the value is above 0, set it to a negative value... otherwise keep it as is ( 0 is the only non-negative number, it can't be negative or positive anyway )
    if ( _value > 0 ):
        self.__x = -_value
    else:
        self.__x = _value


##
## Define the deleter function for x...
##
@x.deleter
def x( self ):
    ## Unload the assignment / data for x
    if ( self.__x != None ):
        del self.__x


##
## To String / Output Function for the class - this will show the property value for each property we add...
##
def __str__( self ):
    ## Output the x property data...
    print( '[ x ] ' + str( self.x ) )


    ## Return a new line - technically we should return a string so it can be printed where we want it, instead of printed early if _data = str( C( ) ) is used....
    return '\n'

##
##
##
_test = GetterSetterExample( )
print( _test )

## For some reason the deleter isn't being called...
del _test.x

Basically, the same as the C( object ) example except I'm using x instead... I also don't initialize in __init - ... well.. I do, but it can be removed because __x is defined as part of the class....

The output is:

[ Test Class ] Set x = 1234
[ Test Class ] Get x = -1234
[ x ] -1234

and if I comment out the self.x = 1234 in init then the output is:

[ Test Class ] Get x = None
[ x ] None

and if I set the _default = None to _default = 0 in the getter function ( as all getters should have a default value but it isn't passed in by the property values from what I've seen so you can define it here, and it actually isn't bad because you can define the default once and use it everywhere ) ie: def x( self, _default = 0 ):

[ Test Class ] Get x = 0
[ x ] 0

Note: The getter logic is there just to have the value be manipulated by it to ensure it is manipulated by it - the same for the print statements...

Note: I'm used to Lua and being able to dynamically create 10+ helpers when I call a single function and I made something similar for Python without using properties and it works to a degree, but, even though the functions are being created before being used, there are still issues at times with them being called prior to being created which is strange as it isn't coded that way... I prefer the flexibility of Lua meta-tables and the fact I can use actual setters / getters instead of essentially directly accessing a variable... I do like how quickly some things can be built with Python though - for instance gui programs. although one I am designing may not be possible without a lot of additional libraries - if I code it in AutoHotkey I can directly access the dll calls I need, and the same can be done in Java, C#, C++, and more - maybe I haven't found the right thing yet but for that project I may switch from Python..

Note: The code output in this forum is broken - I had to add spaces to the first part of the code for it to work - when copy / pasting ensure you convert all spaces to tabs.... I use tabs for Python because in a file which is 10,000 lines the filesize can be 512KB to 1MB with spaces and 100 to 200KB with tabs which equates to a massive difference for file size, and reduction in processing time...

Tabs can also be adjusted per user - so if you prefer 2 spaces width, 4, 8 or whatever you can do it meaning it is thoughtful for developers with eye-sight deficits.

Note: All of the functions defined in the class aren't indented properly because of a bug in the forum software - ensure you indent it if you copy / paste