list comprehension syntax..?

[Duncan Booth]

Gregory Guthrie wrote:

> Sorry for a simple question- but I don't understand how to parse this
> use of a list comprehension.
> 
> The "or" clauses are odd to me.
> 
> It also seems like it is being overly clever (?) in using a lc
> expression as a for loop to drive the recursion.

You are spot on there. It is a list comprehension, but the resulting list 
is just thrown away, so using a list comprehension is a complete waste of 
time serving only to confuse the issue. Presumably it saved the author a 
character or two.

[ exp for var in seq ]

when the result isn't used can be rewritten as:

for var in seq:
   exp

and:

   exp1 or exp2

when the result is thrown away is just:

   if not exp1:
       exp2


So:

 [ m in [(i-j)%9*(i/9^j/9)*(i/27^j/27|i%9/3^j%9/3)
          or board[j] for j in range(81) ]
     or solve(board[:i]+m+board[i+1:]) for m in'%d'%5**18 ] 

is equivalent to:

inner = [(i-j)%9*(i/9^j/9)*(i/27^j/27|i%9/3^j%9/3) or board[j] for j in 
range(81) ]
for m in '3814697265625':
    if m not in inner:
         solve(board[:i]+m+board[i+1:])

(That inner list comprehension doesn't depend on m, so it doesn't need to 
be reevaluated each time round the loop except, again, to save a few 
characters.)

The '%d'%5**18 looks to be a silly way to iterate through all possible 
digits for m even though it does some of them twice while saving one 
character over writing range(1,10).

The strange expression I called 'inner' is a list containing the string 
value board[j] if j is in the same row, column or block as i, or an integer 
for any other cells. So 'm not in inner' is true only if the value for m is 
not already used in that row column or block and is therefore a possible 
candidate for that location in the board.