I am out of trial and error again Lists

Seymore4Head Seymore4Head at Hotmail.invalid
Thu Oct 23 02:10:16 CEST 2014


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.

Thanks



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