help?? on functions

[lokeshkoppaka at gmail.com]

On Monday, May 27, 2013 11:18:34 AM UTC+5:30, Steven D'Aprano wrote:
> On Sun, 26 May 2013 21:48:34 -0700, lokeshkoppaka wrote:
> 
> 
> 
> > def shuffle(input, i, j):
> 
> >     pass
> 
> >     input = input[i:j+1] +input[0:i] + input[j+1:]
> 
> 
> 
> "pass" does nothing. Take it out.
> 
> 
> 
> 
> 
> 
> 
> > def test_shuffle():
> 
> >     input = [1, 2, 3, 4, 5, 6]
> 
> >     shuffle(input, 1, 2)
> 
> >     assert [2, 3, 1, 4, 5, 6] == input
> 
> > 
> 
> > 
> 
> > i had done the above code but the problem is i had manipulated the
> 
> > "input" in function shuffle(input, i, j) but once i get back to the
> 
> > test_shuffle() function again the variable "input" does not reflect the
> 
> > changes made in shuffle(input, i, j) why ,please can any one describe
> 
> > why . and help how to reflect that change to the variable "input".
> 
> 
> 
> The line of code:
> 
> 
> 
>     input = input[i:j+1] +input[0:i] + input[j+1:]
> 
> 
> 
> 
> 
> takes the input list, makes three slices from that list, creates a new 
> 
> list, and then reassigns the LOCAL variable "input". This does not touch 
> 
> the variable on the outside of the function.
> 
> 
> 
> This will be more clear if you use different names:
> 
> 
> 
> # Outside the function.
> 
> mylist = [1, 2, 3, 4, 5, 6]
> 
> shuffle(mylist, 1, 2)
> 
> 
> 
> 
> 
> Inside the function "shuffle", "input" is a local variable, and when you 
> 
> reassign to it, the variable "mylist" on the outside is not changed. Try 
> 
> this small function to see what I mean:
> 
> 
> 
> def demo(input):
> 
>     print('local variable, before:', input)
> 
>     input = 100
> 
>     print('local variable, after:', input)
> 
>     print('non-local variable', mylist)
> 
> 
> 
> 
> 
> mylist = [1, 2, 3, 4, 5, 6]
> 
> demo(mylist)
> 
> 
> 
> 
> 
> 
> 
> So, what can you do to fix this? You have two choices:
> 
> 
> 
> 
> 
> 1) You can return the shuffled list. Add this line to the end of your 
> 
> shuffle function:
> 
> 
> 
>     return input
> 
> 
> 
> 
> 
> and then inside the test function, do this:
> 
> 
> 
> def test_shuffle():
> 
>     input = [1, 2, 3, 4, 5, 6]
> 
>     input = shuffle(input, 1, 2)
> 
>     assert [2, 3, 1, 4, 5, 6] == input
> 
> 
> 
> 
> 
> 2) You can modify the input list in place.
> 
> 
> 
> In this case, instead of reassigning the local variable "input" with the 
> 
> new list, you simply tell Python to stuff the new list inside the 
> 
> original list. You do that with a slice:
> 
> 
> 
> 
> 
>     input[:] = input[i:j+1] + input[0:i] + input[j+1:]
> 
> 
> 
> 
> 
> That's a small difference from what you wrote, just three characters [:], 
> 
> but it makes a big difference in the effect. Instead of reassigning the 
> 
> local variable to the new list, it takes the existing list, and replaces 
> 
> each value inside it with the values taken from the new list. For example:
> 
> 
> 
> 
> 
> py> mylist = [100, 200, 300, 400, 500, 600]
> 
> py> mylist[3:5] = ['A', 'B', 'C']
> 
> py> mylist
> 
> [100, 200, 300, 'A', 'B', 'C', 600]
> 
> 
> 
> py> mylist[1:] = [99, 98, 97]
> 
> py> mylist
> 
> [100, 99, 98, 97]
> 
> 
> 
> 
> 
> Any questions?
> 
> 
> 
> 
> 
> -- 
> 
> Steven

Steven 
wow, wonderful explanation ,i got it thanks a lot