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Help understanding list operatoins inside functions in python 3

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  Help understanding list operatoins inside functions in python 3 stephen.boulet@gmail.com - 2015-01-13 04:51 -0800
    Re: Help understanding list operatoins inside functions in python 3 Joel Goldstick <joel.goldstick@gmail.com> - 2015-01-13 08:39 -0500
    Re: Help understanding list operatoins inside functions in python 3 Laurent Pointal <laurent.pointal@laposte.net> - 2015-01-13 14:43 +0100
    Re: Help understanding list operatoins inside functions in python 3 mortoxa <mortoxa@gmx.com> - 2015-01-14 00:49 +1100
    Re: Help understanding list operatoins inside functions in python 3 Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-01-13 09:25 -0500

#83687 — Help understanding list operatoins inside functions in python 3

I'm a bit confused why in the second case x is not [1,2,3]:

x = []

def y():
    x.append(1)

def z():
    x = [1,2,3]

y()
print(x)
z()
print(x)

Output:
[1]
[1]

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#83689

[Multipart message — attachments visible in raw view] — view raw

On Tue, Jan 13, 2015 at 7:51 AM, <stephen.boulet@gmail.com> wrote:

> I'm a bit confused why in the second case x is not [1,2,3]:
>
> x = []
>
> def y():
>     x.append(1)
>
> def z():
>     x = [1,2,3]
>
> y()
> print(x)
> z()
> print(x)
>
> Output:
> [1]
> [1]
>

x in the outer scope is not x in the z() scope

> --
> https://mail.python.org/mailman/listinfo/python-list
>



-- 
Joel Goldstick
http://joelgoldstick.com

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#83690

Hello,

stephen.boulet@gmail.com wrote:

> I'm a bit confused why in the second case x is not [1,2,3]:
> 
> x = []
> 
> def y():
>     x.append(1)
> 
> def z():
>     x = [1,2,3]

Here x is a local, so global x is not modified.
If you want to modify gobal x, write:
def z():
    global x
    x = [1,2,3]

You can read the Python FAQ about global/local rules:

https://docs.python.org/3/faq/programming.html#what-are-the-rules-for-local-and-global-variables-in-python

Or Dive Into Python

http://www.diveintopython.net/html_processing/locals_and_globals.html

(and for your problem, its the same with Python2 and Python3)

Or…

A+
Laurent.

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#83693

[Multipart message — attachments visible in raw view] — view raw

On 01/13/15 23:51, stephen.boulet@gmail.com wrote:
> I'm a bit confused why in the second case x is not [1,2,3]:
>
> x = []
>
> def y():
>      x.append(1)
>
> def z():
>      x = [1,2,3]
>
> y()
> print(x)
> z()
> print(x)
>
> Output:
> [1]
> [1]

In your y() function, as you are appending data, the list must already 
exist. So the global list x is used

In your z() function, you are creating a local list x which only exists 
as long as you are in the function.
Anything you do to that list has no effect on the global list x. That is 
why the list does not change.

If you specifically wanted to change the global list x, you could do this:

def z():
     global x
     x = [1, 2, 3]

Output:
[1]
[1, 2, 3]

Or better

def z():
     x = [1, 2, 3]
     return x

y()
print(x)
x = z()
print(x)

Output:
[1]
[1, 2, 3]

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#83697

On Tue, 13 Jan 2015 04:51:19 -0800 (PST), stephen.boulet@gmail.com
declaimed the following:

>I'm a bit confused why in the second case x is not [1,2,3]:
>
>x = []
>
>def y():
>    x.append(1)

	MODIFY contents of object identified by the name "x"; "x" is not a
local name, search module (global) level to find it.

>
>def z():
>    x = [1,2,3]

	Create an anonymous list containing three elements; attach the LOCAL
name "x" to that list; never looks at module "x:
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
    wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/

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