I am out of trial and error again Lists

Seymore4Head Seymore4Head at Hotmail.invalid
Thu Oct 23 03:57:57 CEST 2014


On Wed, 22 Oct 2014 21:35:19 -0400, Seymore4Head
<Seymore4Head at Hotmail.invalid> wrote:

>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

Now I remember why...........nothing happens
http://i.imgur.com/MIRpqzY.jpg

If I click on the shell window, I can get the grayed options to show
up for one turn.
I hit step and everything goes gray again.

http://i.imgur.com/NtMdmU1.jpg

Not a very fruitful exercise.  :(



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