[Tutor] WAIDW - copying list within a recursive function

[Lloyd Kvam]

This function will return a None if it ends though your failsafe else OR
if it ends through a non-zero i.  It is not ending through the failsafe
branch since we do not see the printing of failsafe.

When the i==0 path returns the deck, it is returning it to the i==1
invocation of shuffle. That invocation does NOT have an explicit return,
so it returns None to the i == 2 invocation and so on.

The fix is:
....
     if i > 0:
         print "if i > 0"
         shuffle(i-1, deck)
         return deck
....

Ole Jensen wrote:

> WAIDW == (W)hat (A)m (I) (D)oing (W)rong
>  
> (Question put short:
> Why does this function return; None?
> ###
> def shuffle(i=24, deck=[]):
>     a = [ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, jack, queen, king]
>     deck.append(a[:])
>     print deck
>     if i > 0:
>         print "if i > 0"
>         shuffle(i-1, deck)
>     elif i == 0:
>         print "elif i == 0"
>         print deck
>         return deck
>     else: print "failsafe"
> ###
>  
> Read below for more detailed describtion of my wrong doings ;)
> )
>  
> I'm trying to create a list representing a stack of cards containing six 
> card decks, I do not really care much of the about colour so my stack 
> would come to look like this:
>  
> ###
> stack = [[ace, 2, 3,....., king], [ace, 2, 3,....., king],........,[ace, 
> 2, 3,...., king]
> ###
>  
> The above should symbolise a list (stack) with 24 lists of different 
> colours (allthough the colours e.g. clubs or hearts, are not shown as such).
>  
> Now call me lazy but I would prefer not to type all those those 
> identical lists manually (both easthatically and labour wise), so I saw 
> this as a good time to try to play around  with recursive functions 
> (easily done in a forloop with two lines, but now I'm stubborn).
> The best bit of code I have come up with untill now is this:
>  
> ###
> def shuffle(i=24, deck=[]):
>     a = [ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, jack, queen, king]
>     deck.append(a[:])
>     print deck
>     if i > 0:
>         print "if i > 0"
>         shuffle(i-1, deck)
>     elif i == 0:
>         print "elif i == 0"
>         print deck
>         return deck
>     else: print "failsafe"
> ###
> (The print statements are for testing)
>  
> Which of course doesn't work (Otherwise I wouldn't be writing this mail)!
> The the output ends up like this:
> [[11, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10, 10]]
> if i > 0
> [[11, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10, 10], [11, 2, 3, 4, 5, 6, 7, 8, 
> 9, 10, 10, 10, 10]]
> if i > 0
> etc. etc.
> These few lines above shows me that everything shaping about nicely, the 
> list is looking the way it's supposed to.
>  
> In the end of the output i finally equals 0, and the list is printed a 
> final time looking complete (len(deck) == 24). When the next command in 
> the sourcecode reads: return deck.
>  
> ###
> elif i == 0:
>     print "elif i == 0"
>     print deck
>     return deck
> ###
>  
> My question is this:
> If the list "deck" is as it is supposed to (and it looks to be), then 
> why does the function return; None?
>  
> In advance thanks for any enlightment.
>  
> 
> 
> ------------------------------------------------------------------------
> 
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-- 
Lloyd Kvam
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