On Thu, 23 Oct 2014 02:31:57 +0100, MRAB <pyt...@mrabarnett.plus.com> wrote:
>On 2014-10-23 01:10, Seymore4Head wrote: >> On Thu, 23 Oct 2014 11:05:08 +1100, Steven D'Aprano >> <steve+comp.lang.pyt...@pearwood.info> wrote: >> >>>Seymore4Head wrote: >>> >>>> Those string errors were desperate attempts to fix the "append" error >>>> I didn't understand. >>> >>>Ah, the good ol' "make random changes to the code until the error goes away" >>>technique. You know that it never works, right? >>> >>>Start by *reading the error message*, assuming you're getting an error >>>message. I'm the first person to admit that Python's error messages are not >>>always as clear as they should be, especially syntax errors, but still >>>there is a lot of information that can be gleamed from most error messages. >>>Take this attempt to use append: >>> >>>py> mylist.append(23) >>>Traceback (most recent call last): >>> File "<stdin>", line 1, in <module> >>>NameError: name 'mylist' is not defined >>> >>>That tells me that I have forgotten to define a variable mylist. So I fix >>>that: >>> >>>py> mylist = 23 >>>py> mylist.append(23) >>>Traceback (most recent call last): >>> File "<stdin>", line 1, in <module> >>>AttributeError: 'int' object has no attribute 'append' >>> >>> >>>That tells me that I can't append to a int. After googling for "Python >>>append" I learn that I can append to a list, so I try again: >>> >>>py> mylist = [] >>>py> mylist.append(23) >>>py> print(mylist) >>>[23] >>> >>> >>>Success! >>> >>>If you are familiar with other programming languages, it might help to think >>>of append() as being like a procedure in Pascal, for example. You call >>>append() with an argument, but don't expect a return result. >>> >>>Technically, *all* functions and methods in Python return something, even if >>>just the special value None, which can lead to "Gotchas!" like this one: >>> >>>py> mylist = mylist.append(42) # Don't do this! >>>py> print(mylist) # I expect [23, 42] but get None instead. >>>None >>> >>>Oops. One of the small annoyances of Python is that there is no way to tell >>>ahead of time, except by reading the documentation, whether something is a >>>proper function that returns a useful value, or a procedure-like function >>>that returns None. That's just something you have to learn. >>> >>>The interactive interpreter is your friend. Learn to experiment at the >>>interactive interpreter -- you do know how to do that, don't you? If not, >>>ask. At the interactive interpreter, if a function or method returns a >>>value, it will be printed, *except for None*. So a function that doesn't >>>print anything might be procedure-like, and one which does print something >>>might not be: >>> >>>py> mylist = [1, 5, 2, 6, 4, 3] >>>py> sorted(mylist) # proper function returns a value >>>[1, 2, 3, 4, 5, 6] >>>py> mylist.sort() # procedure-like function returns None >>>py> print(mylist) # and modifies the list in place >>>[1, 2, 3, 4, 5, 6] >> >> I am going to get around to learning the interpreter soon. >> >Why wait? > >You're trying to learn the language _now_, and checking things >interactively will help you. Because most of the practice I am getting is not using Python. I use Codeskulptor. OK.........Now is as good a time as ever. Thanks -- https://mail.python.org/mailman/listinfo/python-list
