'int' object is not iterable when I'm not trying to iterate

Question

The following piece of code attempts to create a map that shows the minimum number of moves it would take to get from each square on that map to the specified location. The function as a whole is largely irrelevant to the problem, but I thought I should provide my problem in context. I've also imported deque from collections. The strange part comes in at line 7. I get TypeError: 'int' object not iterable. But the statement "distance_from_loc, f_loc = squares_to_check.popleft()" shouldn't be attempting to iterating anything to the best of knowledge. Any help would be greatly appreciated.

    def complex_distance(self, loc):
        row, col = loc
        squares_to_check = deque((0, loc))
        self.complex_distance_map = zeros((self.height, self.width), int) + 999
        self.complex_distance_map[row][col] = 0
        while squares_to_check:
            distance_from_loc, f_loc = squares_to_check.popleft()
            distance_from_loc += 1
            for d in AIM:
                n_loc = self.destination(f_loc, d)
                n_row, n_col = n_loc
                if distance_from_loc < self.complex_distance_map[n_row][n_col] and not self.map[n_row][n_col] == -4:
                    squares_to_check.append((distance_from_loc, n_loc))
                    self.complex_distance_map[n_row][n_col] = distance_from_loc

Answer 1

The line does try to iterate:

>>> a, b = 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable

The line

squares_to_check = deque((0, loc))

initialises the deque with the two elements 0 and loc, not with the single element (0, loc). Use

squares_to_check = deque([(0, loc)])

to get the desired result.