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I am not a newbie to python. But recently I encountered an error, due to some misconception. Someone please help me to clarify it. Entire program is here : http://www.codeskulptor.org/#user39_cFs3Z8mAtf_0.py

I am having a function

def mc_trial(board, player):
"""
Plays a game starting with the given player by making random 
moves and alternating between players.
"""
   while board.check_win() == None:
    # Get a random empty square
      empty_squares = random.choice(board.get_empty_squares())

    # Move the player in a random position
      board.move(empty_squares[0], empty_squares[1], player)

    # Switch the player
      player = provided.switch_player(player)

# Game has ended
return

scores = [[0 for dummy in range(board.get_dim())] \
    for dummy in range(board.get_dim())]
board_clone = board.clone()
for dummy in range(trials):
    print board_clone ## Empty board
    mc_trial(board_clone, player)
    print board_clone #### Here value is changing after function call. How ??

My doubt is "board_clone" is passing to a function mc_trial(). the return statement there is not providing anything relating to return a value except None. But after the call, when I am printing "board_clone" the value is changing. I tried to clarify it with ipython through a sample program. But, there value remains unchanged as in the local scope . For clarification, i run a sample program, there it behaves as i expected.

def func1(var):
    x = 0
    while x < 1:
        var[1:5]
        x = x+1
    return

var1 = [1,2,3,4,5,6,7,8,9]
print "B F", var1
func1(var1)
print "A F", var1
thefourtheye
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user3116355
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2 Answers2

2

This is normal Python behaviour. Lists are passed by reference, so if you change the items in a list in a function, the changes will persist:

def f(mylist):
    mylist[0] = 1

>>> l = [0, 0, 0]
>>> mylist(l)
>>> l
[1, 0, 0]

The reason your second example isn't changing the list is because the var[1:5] statement doesn't do anything. It just creates a temporary variable which is a slice of the var list, and then throws it away. It's just like saying a + 1; this would create a temporary variable but would not modify a even inside the function, let alone outside of it.

Andrew Magee
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  • Actually the second example was a sample check. I know what you said. But, how come the changes to "board_clone" persist even after the function. Even if we are passing a list to a function, the changes made to that will be availble to the calling function ???? I dont know about that. – user3116355 Mar 06 '15 at 04:13
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    Yes, that's exactly what happens. – Andrew Magee Mar 06 '15 at 04:13
1

I'd guess your problem is that your "clone" has references to datastructs in the board object. The clone code doesn't appear in the link you posted?.. and as far as I can recall it's not an in-built python thing (I may be wrong there).?

Read this How to clone or copy a list?

and/or this

https://docs.python.org/2/library/copy.html

Community
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demented hedgehog
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  • yeah. "board_clone" is provided by codeskulptor as a part of problem. But how come the changes to the value persist in another function. they are passing board_clone , and in that they modified it with board. ( Normal argument reference ). But how come that changes persist to the old variable board_clone. The scope has be unchanged unless it is a global variable right . – user3116355 Mar 06 '15 at 04:15
  • it's hard to be sure if we don't know what board.clone() does and also what the board class looks like. Andrew Magee and I are both saying essentially the same thing here. If you're not careful about how you "clone" the object you'll just get another pointer to the existing data struct. At that point the board_clone is basically just an alias for the board so any changes to board_clone effect the board because they are two different names for the same object. – demented hedgehog Mar 06 '15 at 04:22
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    (they're actually names that get used to look up objects in namespace dicts, e.g. see this http://stackoverflow.com/questions/61517/python-dictionary-from-an-objects-fields ) or the builtin locals and globals methods. – demented hedgehog Mar 06 '15 at 04:22