error in tutorial for 3.0, section 9.3.3
Section 9.3.3 says that given, class MyClass: A simple example class i = 12345 def f(self): return 'hello world' and x = MyClass() then this x.counter = 1 while x.counter 10: x.counter = x.counter * 2 print(x.counter) del x.counter will print 16 link, http://docs.python.org/3.0/tutorial/classes.html#a-first-look-at-classes I am reading this section so to learn about classes but if this is right I think I need to start over. Thanks Vincent Davis 720-301-3003 -- http://mail.python.org/mailman/listinfo/python-list
Re: error in tutorial for 3.0, section 9.3.3
let me add that I see that this could be right if x.counter = 1 and counter need not have anything to do with MyClass but this could be more clear. Thanks Vincent Davis 720-301-3003 On Sat, May 23, 2009 at 7:08 AM, Vincent Davis vinc...@vincentdavis.netwrote: Section 9.3.3 says that given, class MyClass: A simple example class i = 12345 def f(self): return 'hello world' and x = MyClass() then this x.counter = 1 while x.counter 10: x.counter = x.counter * 2 print(x.counter) del x.counter will print 16 link, http://docs.python.org/3.0/tutorial/classes.html#a-first-look-at-classes I am reading this section so to learn about classes but if this is right I think I need to start over. Thanks Vincent Davis 720-301-3003 -- http://mail.python.org/mailman/listinfo/python-list
Re: error in tutorial for 3.0, section 9.3.3
Thank you that makes sense to me. Much more clear then the tutorial, I think so anyway. If you are learning about classes that you kinda expect MyClass to have counter in it. I might be nice to show that x.counter = 1 creates an instance that would look like (is this correct?) class MyClass: A simple example class i = 12345 counter = 1 def f(self): return 'hello world' Thanks again Vincent Davis On Sat, May 23, 2009 at 8:24 AM, Benjamin Kaplan benjamin.kap...@case.eduwrote: On Sat, May 23, 2009 at 9:13 AM, Vincent Davis vinc...@vincentdavis.netwrote: let me add that I see that this could be right if x.counter = 1 and counter need not have anything to do with MyClass but this could be more clear. Thanks Vincent Davis 720-301-3003 On Sat, May 23, 2009 at 7:08 AM, Vincent Davis vinc...@vincentdavis.netwrote: Section 9.3.3 says that given, class MyClass: A simple example class i = 12345 def f(self): return 'hello world' and x = MyClass() then this x.counter = 1 while x.counter 10: x.counter = x.counter * 2 print(x.counter) del x.counter will print 16 link, http://docs.python.org/3.0/tutorial/classes.html#a-first-look-at-classes I am reading this section so to learn about classes but if this is right I think I need to start over. The code given is correct, though the description in the tutorial could be clearer. Basically, a class in Python is represented by a dict with strings mapping to other stuff. Internally, x.counter = 1 is just a shortcut for x.__dict__['counter'] = 1. This appears in the code as dynamically adding the variable counter to the instance of MyClass. Unlike in static languages, an instance variable in python doesn't need to be declared inside the class for you to use it. It also doesn't need to appear in every instance of the class. The last line in the code (del x.counter) removes the counter key from x so that the instance variable disappears. That's how the code works without leaving a trace. Thanks Vincent Davis 720-301-3003 -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: error in tutorial for 3.0, section 9.3.3
Vincent writes: you kinda expect MyClass to have counter in it. Yeah, that makes sense. These instance variables are often initialized in the __init__ method: class Counter(object): def __init__(self,initialvalue): self.value=initialvalue def inc(self): self.value+=1 def dec(self): self.value-=1 beans=Counter(123) beans.inc() beans.inc() beans.dec() print beans.value # the output of the program is: 124 Greetings, -- The ability of the OSS process to collect and harness the collective IQ of thousands of individuals across the Internet is simply amazing. - Vinod Valloppillil http://www.catb.org/~esr/halloween/halloween4.html -- http://mail.python.org/mailman/listinfo/python-list
Re: error in tutorial for 3.0, section 9.3.3
On Sat, May 23, 2009 at 10:46 AM, Vincent Davis vinc...@vincentdavis.netwrote: Thank you that makes sense to me. Much more clear then the tutorial, I think so anyway. If you are learning about classes that you kinda expect MyClass to have counter in it. I might be nice to show that x.counter = 1 creates an instance that would look like (is this correct?) class MyClass: A simple example class i = 12345 counter = 1 def f(self): return 'hello world' That's how it would work in almost any language other than Python. In Python, anything declared in that scope is a member of the class. For something immutable like an int, it doesn't matter. If you can change it however, you get problems. This is one of the biggest sources of problems for python beginners (you get the same behavior with default method arguments btw) class Foo : ...lst = [] ... a = Foo() b = Foo() b.lst [] a.lst.append(1) b.lst [1] In Python, if you want something to be a part of the instance, you have to add it to the instance. That's what the self parameter in the method argument list is. x.f() is just syntax sugar for MyClass.f(x). Dynamically adding variables can get very confusing, so people usually declare everything (or almost everything) they're going to use in the __init__ method discussed in section 9.3.2. Thanks again Vincent Davis On Sat, May 23, 2009 at 8:24 AM, Benjamin Kaplan benjamin.kap...@case.edu wrote: On Sat, May 23, 2009 at 9:13 AM, Vincent Davis vinc...@vincentdavis.netwrote: let me add that I see that this could be right if x.counter = 1 and counter need not have anything to do with MyClass but this could be more clear. Thanks Vincent Davis 720-301-3003 On Sat, May 23, 2009 at 7:08 AM, Vincent Davis vinc...@vincentdavis.net wrote: Section 9.3.3 says that given, class MyClass: A simple example class i = 12345 def f(self): return 'hello world' and x = MyClass() then this x.counter = 1 while x.counter 10: x.counter = x.counter * 2 print(x.counter) del x.counter will print 16 link, http://docs.python.org/3.0/tutorial/classes.html#a-first-look-at-classes I am reading this section so to learn about classes but if this is right I think I need to start over. The code given is correct, though the description in the tutorial could be clearer. Basically, a class in Python is represented by a dict with strings mapping to other stuff. Internally, x.counter = 1 is just a shortcut for x.__dict__['counter'] = 1. This appears in the code as dynamically adding the variable counter to the instance of MyClass. Unlike in static languages, an instance variable in python doesn't need to be declared inside the class for you to use it. It also doesn't need to appear in every instance of the class. The last line in the code (del x.counter) removes the counter key from x so that the instance variable disappears. That's how the code works without leaving a trace. Thanks Vincent Davis 720-301-3003 -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: error in tutorial for 3.0, section 9.3.3
Vincent Davis wrote: Section 9.3.3 says that given, class MyClass: A simple example class i = 12345 def f(self): return 'hello world' and x = MyClass() then this x.counter = 1 while x.counter 10: x.counter = x.counter * 2 print(x.counter) del x.counter will print 16 link, http://docs.python.org/3.0/tutorial/classes.html#a-first-look-at-classes I am reading this section so to learn about classes but if this is right I think I need to start over. Thanks Vincent Davis 720-301-3003 Yes, this code will display 16, and it's not specific to 3.0, but works in earlier Pythons as well. I'm not sure why you're puzzled; it could be the question of why 16, but i suspect it's because you can't see who creates this x attribute. Unlike languages like Java and C++, object attributes are not fixed at the time the class is compiled. Some attributes are created by the placement of the code, for example the method name f. But others are created in the code of the class (typically the __init__() method), and others can be created by anyone, at any time. I don't know why the tutorial starts with this, but it illustrates that an object is really just a container for attributes, some of which are callable (methods), and some of which are data (what Java would call fields). Anybody with an object reference can create, modify or delete those attributes. If it's being done inside the class methods, you use the syntax self.counter. But it also works outside, as you see. -- http://mail.python.org/mailman/listinfo/python-list
Re: Re: error in tutorial for 3.0, section 9.3.3
Vincent Davis wrote: Thank you that makes sense to me. Much more clear then the tutorial, I think so anyway. If you are learning about classes that you kinda expect MyClass to have counter in it. I might be nice to show that x.counter = 1 creates an instance that would look like (is this correct?) class MyClass: A simple example class i = 12345 counter = 1 def f(self): return 'hello world' Thanks again Vincent Davis On Sat, May 23, 2009 at 8:24 AM, Benjamin Kaplan benjamin.kap...@case.eduwrote: On Sat, May 23, 2009 at 9:13 AM, Vincent Davis vinc...@vincentdavis.netwrote: let me add that I see that this could be right if x.counter = 1 and counter need not have anything to do with MyClass but this could be more clear. Thanks Vincent Davis 720-301-3003 On Sat, May 23, 2009 at 7:08 AM, Vincent Davis vinc...@vincentdavis.netwrote: Section 9.3.3 says that given, class MyClass: A simple example class i = 12345 def f(self): return 'hello world' and x = MyClass() then this x.counter = 1 while x.counter 10: x.counter = x.counter * 2 print(x.counter) del x.counter will print 16 link, http://docs.python.org/3.0/tutorial/classes.html#a-first-look-at-classes I am reading this section so to learn about classes but if this is right I think I need to start over. The code given is correct, though the description in the tutorial could be clearer. Basically, a class in Python is represented by a dict with strings mapping to other stuff. Internally, x.counter = 1 is just a shortcut for x.__dict__['counter'] = 1. This appears in the code as dynamically adding the variable counter to the instance of MyClass. Unlike in static languages, an instance variable in python doesn't need to be declared inside the class for you to use it. It also doesn't need to appear in every instance of the class. The last line in the code (del x.counter) removes the counter key from x so that the instance variable disappears. That's how the code works without leaving a trace. Thanks Vincent Davis 720-301-3003 -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list (You accidentally top-posted. That makes the context much harder for others to follow.) No, the attribute 'counter' you demonstrated is an attribute of the class. To duplicate what x.counter=1 does, you have to add it to the instance. Normally, this would be done in the __init__() method, but that would put it in each instance of the class, as it's being created. But this one is only in the 'x' instance. -- http://mail.python.org/mailman/listinfo/python-list
