here’s a code that generates power sets over a list.

1
2
3

out = [[]]
for num in num_list:
  out.extend(x+[num] for x in out)

it’s lovely. it’s concise. it’s pythonic. it’s mathematically elegant. it also contains an infinite loop!

the code inside the extend function evaluates to a generator expression, which is lazily loaded. modifying what you iterate over is never a good idea!

but this outcome is non-trivial. one would imagine that extend extends a chunk to an existing list. but with generator expressions, this chunk is loaded lazily. as extend pumps elements to the tail of the list, the generator sees a broader horizon to iterate over, and the loop goes on forever.