human
Solve in Python:
In a 1 million by 1 million grid, the coordinates of each grid square are (x, y) with 0 <= x, y < 10^6.
We start at the source square and want to reach the target square.  Each move, we can walk to a 4-directionally adjacent square in the grid that isn't in the given list of blocked squares.
Return true if and only if it is possible to reach the target square through a sequence of moves.
 
Example 1:
Input: blocked = [[0,1],[1,0]], source = [0,0], target = [0,2]
Output: false
Explanation: 
The target square is inaccessible starting from the source square, because we can't walk outside the grid.

Example 2:
Input: blocked = [], source = [0,0], target = [999999,999999]
Output: true
Explanation: 
Because there are no blocked cells, it's possible to reach the target square.

 
Note:

0 <= blocked.length <= 200
blocked[i].length == 2
0 <= blocked[i][j] < 10^6
source.length == target.length == 2
0 <= source[i][j], target[i][j] < 10^6
source != target

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Can you provide a the Python program you requested?
assistant
#Here is the Python program you requested.

class Solution:
    def isEscapePossible(self, blocked: List[List[int]], source: List[int], target: List[int]) -> bool:

        #Column and row of source and target (used for escape condition)
        sc, sr = source
        tc, tr = target
        R, C = 10**6, 10**6

        def dist(r1,c1,r2,c2):
            '''Calculates Manhattan distance from (r1,c1) to (r2,c2)'''
            return abs(r2-r1)+abs(c2-c1)

        #Two priority queues: one starting from target and one from source
        #Two visited sets: one for nodes visited by path from target and the other from source
        q = [[(0,*source[::-1])], [(0,*target[::-1])]]
        v = [set(),set()]
        b = set((tuple(b[::-1]) for b in blocked))

        if (tuple(source) in b) or (tuple(target) in b):
            return False

        #if source and target can reach 200 distance from their origin
        #it is safe to say 200 blocked spaces cannot contain them
        source_escape = False
        target_escape = False

        while q[0] and q[1]:

            index = 0 if len(q[0]) <= len(q[1]) else 1

            d, r, c = heapq.heappop(q[index])

            for i, j in ((r+1, c), (r-1, c), (r, c+1), (r, c-1)):
                if (0 <= i < R) and (0 <= j < C) and ((i,j) not in b) and ((i,j) not in v[index]):

                    if (i,j) in v[1-index]:
                        return True

                    v[index].add((i,j))
                    r_target, c_target = q[1-index][0][1:]
                    heapq.heappush(q[index], (dist(i,j,r_target,c_target), i, j))

            if not source_escape and not index:
                source_escape = dist(r, c, sr, sc) > 200
            if not target_escape and index:
                target_escape = dist(r, c, tr, tc) > 200

            if source_escape and target_escape:
                return True

        return False