I am out of trial and error again Lists

Mark Lawrence breamoreboy at yahoo.co.uk
Thu Oct 23 09:20:32 CEST 2014


On 23/10/2014 02:57, Seymore4Head wrote:
> 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.  :(
>

If you were to read and digest what is written it would help.  You're 
trying to run IDLE.  We're talking the interactive interpreter.  If (at 
least on Windows) you run a command prompt and then type python<cr> you 
should see something like this.

Python 3.4.2 (v3.4.2:ab2c023a9432, Oct  6 2014, 22:16:31) [MSC v.1600 64 
bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence




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