Python function 返回语句后继续运行？

Question

The following function ends with the TypeError: Unsupported operand types float and NoneType. How is it possible for a base case to fire a return None and the program continue with factorial() = None . At least this is how I perceive it.

def factorial(i):
  if i < 0:
    return None # Why does the program not stop here?...
  if i == 0:
    return 1
  return i * factorial(i-1) #factorial(i-1) returns None after i < 0?

print(factorial(3.01))

Answer 1

You're getting that error because return i * factorial(i-1) is getting called in your function, and factorial(i-1) is evaluating to None .

The reason it's evaluating to None is the input value you gave - 3.01 - this will result in the following:

factorial(3.01) # returns 3.01 * factorial(3.01-1)
factorial(2.01) # returns 2.01 * factorial(2.01-1)
factorial(1.01) # returns 1.01 * factorial(1.01-1)
factorial(0.01) # returns 0.01 * factorial(0.01-1)
factorial(-0.99) # returns None (because i < 0)

Therefore, the line 0.01 * None is run, which results in your error.

Answer 2

The last recursive call in your function is

return 0.01 * factorial(-0.99)

factorial(-0.99) invokes the if i < 0: case, so it returns None . Then the above code tries to multiply 0.01 * None , which causes the error you got.

Your definition of factorial is only valid for non-negative integers.