奇怪的TypeError：“ NoneType”对象不可迭代

Question

I encounter a weird error. As these codes, both foo() and bar() return None , but it raises TypeError only when iterating bar()

def foo():
  if True:
    return

  yield 1, 2

def bar():
  return

for a, b in foo():
  print a + b

for a, b in bar():
  print a + b

Answer 1

Because foo includes a yield statement, it is a generator, so the result from the return is always a generator object even if the actual yield statement can't be reached. A generator is true in a boolean sense, hence your result.

Answer 2

If you

print foo()
print bar()

you get

<generator object foo at 0x7f8a79fd5eb0>
None

Answer 3

In your functions foo and bar , they both reach an empty return statement. They implicitly return None . foo will nonetheless generate an iterator as it includes a yield statement.

Therefore as you are looping over the output of the functions, foo will use the iterator values, while bar will not, resulting in a TypeError .

Answer 4

The yield 1, 2 makes the difference. foo returns:

<generator object foo at 0x7f9e01a91d70>

bar returns:

None

If you'd comment the yield part the code would crash on foo() too.

You should return two values for each of the methods to solve it.

Answer 5

If your function does not include statement and you want it to return "empty iterator" you should write

def empty()
    raise StopIteration

for each in empty():
    print('this will never show up')