BFS that blows up the queue

1from collections import deque
2 
3def shortest_path(grid, start, goal):
4    rows, cols = len(grid), len(grid[0])
5    q = deque([(start, 0)])
6    visited = set()
7 
8    while q:
9        (r, c), d = q.popleft()
10        if (r, c) in visited:
11            continue
12        visited.add((r, c))
13        if (r, c) == goal:
14            return d
15        for dr, dc in ((1, 0), (-1, 0), (0, 1), (0, -1)):
16            nr, nc = r + dr, c + dc
17            if 0 <= nr < rows and 0 <= nc < cols and grid[nr][nc] == 0:
18                q.append(((nr, nc), d + 1))
19    return -1