I am out of trial and error again Lists

Seymore4Head Seymore4Head at Hotmail.invalid
Thu Oct 23 03:35:19 CEST 2014


On Thu, 23 Oct 2014 02:31:57 +0100, MRAB <python at 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.python at 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



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