[Tutor] Functions returning multiple values

[Christian Witts]

Giorgio wrote:
> I have an update:
>
> I can easily undertand why this example doesn't work:
>
> def nochange(x):
>     x = 0
>
> y = 1
> nochange(y)
> print y # Prints out 1
>
> X is a local variable, and only gets modified in the function, that 
> doesn't return any value.
>
> But it's very difficult for me to understand WHY this works:
>
> def change(some_list):
>     some_list[1] = 4
>
> x = [1,2,3]
> change(x)
> print x # Prints out [1,4,3]
>
> some_list is a "local" list, isn't it? Maybe i can't have lists that 
> are only existing in a function?
>
> Thankyou all
>
> 2010/2/22 Kent Johnson <kent37 at tds.net <mailto:kent37 at tds.net>>
>
>     On Mon, Feb 22, 2010 at 9:13 AM, Giorgio
>     <anothernetfellow at gmail.com <mailto:anothernetfellow at gmail.com>>
>     wrote:
>
>     > And, i have some difficulties understanding the other "strange"
>     example in
>     > that howto. Just scroll down to: "However, the point is that the
>     value
>     > of x is picked up from the environment at the time when the
>     function is
>     > defined. How is this useful? Let’s take an example — a function
>     which
>     > composes two other functions."
>     > He is working on a function that compose other 2 functions. This
>     is the
>     > final solution
>     > def compose(fun1, fun2):
>     >     def inner(x, fun1=fun1, fun2=fun2):
>     >         return fun1(fun2(x))
>     >     return inner
>     > But also tries to explain why this example:
>     > # Wrong version
>     > def compose(fun1, fun2):
>     >     def inner(x):
>     >         return fun1(fun2(x))
>     >     return inner
>     > def fun1(x):
>     >     return x + " world!"
>     > def fun2(x):
>     >     return "Hello,"
>     > sincos = compose(sin,cos)  # Using the wrong version
>     > x = sincos(3)
>     > Won't work. Now, the problem is that the "inner" function gets
>     fun1 and fun2
>     > from other 2 functions.
>     > My question is: why? inner is a sub-function of compose, where
>     fun1 and fun2
>     > are defined.
>
>     It does work:
>     In [6]: def compose(fun1, fun2):
>       ...:     def inner(x):
>       ...:         return fun1(fun2(x))
>       ...:     return inner
>       ...:
>
>     In [7]: def fun1(x):
>       ...:         return x + " world!"
>       ...:
>
>     In [8]: def fun2(x):
>       ...:         return "Hello,"
>       ...:
>
>     In [9]: from math import sin, cos
>
>     In [10]: sincos = compose(sin,cos)  # Using the wrong version
>
>     In [11]:
>
>     In [12]: x = sincos(3)
>
>     In [13]:
>
>     In [14]: x
>     Out[14]: -0.8360218615377305
>
>     That is a very old example, from python 2.1 or before where nested
>     scopes were not supported. See the note "A Note About Python 2.1 and
>     Nested Scopes" - that is now the default behaviour.
>
>     Kent
>
>
>
>
> -- 
> --
> AnotherNetFellow
> Email: anothernetfellow at gmail.com <mailto:anothernetfellow at gmail.com>
> ------------------------------------------------------------------------
>
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> To unsubscribe or change subscription options:
> http://mail.python.org/mailman/listinfo/tutor
>   
Take a look at the Python gothcha's:
http://www.ferg.org/projects/python_gotchas.html#contents_item_6

-- 
Kind Regards,
Christian Witts