Peter Norvig's Game of Life

Question

i was looking at this code Peter Norvig's Game of Life implementation when i noticed this:

def next_generation(world):
"The set of live cells in the next generation."
possible_cells = counts = neighbor_counts(world)
return {cell for cell in possible_cells
        if (counts[cell] == 3) 
        or (counts[cell] == 2 and cell in world)}

What is the reason to use counts instead of possible_cells?

Answer 1

Norvig explains in the comments (immediately above the code in the IPython cell In [1]):

Note that in next_generation the neighbor_counts is used two ways so I decided to use two different names for clarity: possible_cells is used to iterate over all cells that might be live, and counts is used to check if a cell has the right number of neighbors.