On 2/25/2012 4:34 AM, Dave Angel wrote:
On 02/25/2012 03:31 AM, Chris Kavanagh wrote:
Hey Everyone,
I came across this code in 'A Byte Of Python' & realized there was a
line I didn't understand. The line is "howMany =
staticmethod(howMany)" (Complete code pasted below.)
I don't think, in my very short Python career, I've heard of a
staticmethod or classmethod. There's very little explanation of them
in the book. I've googled them, but am still confused on exactly what
they are & why they should be used. The only sense I can make of them
is, a staticmethod can be called WITHOUT actually creating an object
first.
Anyways, if someone could give me a simple explanation with a very
simple example, I will be elated! As always, thank you in advance for
any help!! Code Below:
PS: Please keep in mind, Python is my 1st language, & I'm very new to
it (4 to 5 months).
class Robot:
'''Represents a robot, with a name.'''
# A class variable, counting the number of robots
population = 0
def __init__(self, name):
'''Initializes the data.'''
self.name = name
print('(Initializing {0})'.format(self.name))
# When this person is created, the robot
# adds to the population
Robot.population += 1
def __del__(self):
'''I am dying.'''
print('{0} is being destroyed!'.format(self.name))
Robot.population -= 1
if Robot.population == 0:
print('{0} was the last one.'.format(self.name))
else:
print('There are still {0:d} robots working.'.format(Robot.population))
def sayHi(self):
'''Greeting by the robot.
Yeah, they can do that.'''
print('Greetings, my masters call me {0}.'.format(self.name))
def howMany():
'''Prints the current population.'''
print('We have {0:d} robots.'.format(Robot.population))
howMany = staticmethod(howMany)
droid1 = Robot('R2-D2')
droid1.sayHi()
Robot.howMany()
droid2 = Robot('C-3PO')
droid2.sayHi()
Robot.howMany()
print("\nRobots can do some work here.\n")
print("Robots have finished their work. So let's destroy them.")
del droid1
del droid2
Robot.howMany()
Defining a function inside a class makes it a method. Two distinctions
exist between an ordinary function and a method. One is which namespace
the name is known in, and the other is this mysterious thing called self.
The namespace thing means that "sayHi" for example is not an attribute
of the module, but of the instance of the class. So other classes might
have a method of the same name without conflict.
The self thing means that when you say droid2.sayHi(), there is an extra
parameter added to the list (only one in this case), the 'self' parameter.
What staticmethod() does is to strip out the second feature. For methods
that don't need a 'self' parameter, (ie. that don't care which instance
they're called on, and are willing to hardcode any refs to other class
things) this works out great.
classmethod() converts the method so it supplies the class of the object
as its first parameter (cls). This can be useful if you have more than
one class derived from each other, and the method still needs to know
which class it actually was called on.
Does this help some?
Yes & No, lol. BTW, thanks for the reply Dave, it's much appreciated.
Maybe if I tell you what I know, it will make it easier for you to
understand what I don't know. Such as, I know a method is just a
function inside a class. . .
Ok, 1st, I learned Classes & Objects using Python 'Old Style' Classes &
Objects. I hadn't thought (until now) there was much difference other
than terminology, ie. using class(object): in the parent class. Instead
of class(self): ect.
2nd, I understood 'self' is just a place holder for the object that will
be created.
I get that a staticmethod does not need the 'self', but why would this
be necessary?? You said,
"For methods
> that don't need a 'self' parameter, (ie. that don't care which instance
> they're called on, and are willing to hardcode any refs to other class
> things)"
I know this will sound stupid, but why would the method care which
instance it's called on?? I think this is where my confusion is coming
in. . .I hope I've made sense here. . .
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