My code is for a Tic Tac Toe game and checking for a draw state but I think this question could be more useful in a general sense.
I have a list that represents the board, it looks like this:
board = [1,2,3,4,5,6,7,8,9]
When a player makes a move, the integer they moved on is replaced with their marker ('x' or 'o'). I already have checks in place to look for a winning state. What I can't do is check for a draw state, where none of the list values are integers but a winning state has not been set.
The code I have so far:
if any(board) != playerOne or any(board) != playerTwo:
print 'continue'
elif all(board) == playerOne or playerTwo:
print 'Draw'
The if statement works, the elif does not. I think the problem is my 'or' operator. What I want to check for is: if the every item on the board is either playerOne
marker or playerTwo
marker. If I were to make the code:
elif all(board) == playerOne or all(board) == playerTwo:
I would be checking to see if every place on the board was playerOne
or every place on the board is playerTwo
, which it won't be.
So how do I check if the board is taken up by a combination of playerOne
markers and playerTwo
markers?
Generally speaking:
all
and any
are functions that take some iterable and return True
, if
all
, no values in the iterable are falsy;any
, at least one value is truthy.A value x
is falsy iff bool(x) == False
.
A value x
is truthy iff bool(x) == True
.
Any non-boolean elements in the iterable are perfectly acceptable — bool(x)
maps, or coerces, any x
according to these rules:
0
, 0.0
, None
, []
, ()
, []
, set()
, and other empty collections are mapped to False
True
.The docstring for bool
uses the terms 'true'/'false' for 'truthy'/'falsy', and True
/False
for the concrete boolean values.
For example:
if all(x > 0 for x in xs) or any(x > 100 for x in xs):
# if nothing is zero or something is over a hundred …
In your specific code samples:
You’ve slightly misunderstood how these functions work. The following does something completely different from what you thought:
if any(foobars) == big_foobar:
...because any(foobars)
would first be evaluated to either True
or False
, and then that boolean value would be compared to big_foobar
, which generally always gives you False
(unless big_foobar
coincidentally happened to be the same boolean value).
Note: the iterable can be a list, but it can also be a generator or a generator expression (≈ lazily evaluated/generated list), or any other iterator.
What you want instead is:
if any(x == big_foobar for x in foobars):
which basically first constructs an iterable that yields a sequence of booleans—for each item in foobars
, it compares the item to the value held by big_foobar
, and (lazily) emits the resulting boolean into the resulting sequence of booleans:
tmp = (x == big_foobar for x in foobars)
then any
walks over all items in tmp
and returns True
as soon as it finds the first truthy element. It's as if you did the following:
In [1]: foobars = ['big', 'small', 'medium', 'nice', 'ugly']
In [2]: big_foobar = 'big'
In [3]: any(['big' == big_foobar, 'small' == big_foobar, 'medium' == big_foobar, 'nice' == big_foobar, 'ugly' == big_foobar])
Out[3]: True
Note: As DSM pointed out, any(x == y for x in xs)
is equivalent to y in xs
but the latter is more readable, quicker to write and runs faster.
Some examples:
In [1]: any(x > 5 for x in range(4))
Out[1]: False
In [2]: all(isinstance(x, int) for x in range(10))
Out[2]: True
In [3]: any(x == 'Erik' for x in ['Erik', 'John', 'Jane', 'Jim'])
Out[3]: True
In [4]: all([True, True, True, False, True])
Out[4]: False
See also: http://docs.python.org/2/library/functions.html#all