[Python] how to catch throw and other exceptions in coroutine with 1 yield?

I have var DICTIONARY, which is a dictionary where the keys are English letters and the values are words that start with the corresponding letter. The initial filling of DICTIONARY looks like this:

DICTIONARY = {
'a': 'apple',
'b': 'banana',
'c': 'cat',
'd': 'dog',
}


My code has 2 while loops, since higher try-except would end the entire generator loop:

def alphabet():
while True:
try:
letter = yield #waiting for first input from send
while True:
try:
letter = yield DICTIONARY[letter]
except KeyError:
letter = yield 'default' # return 'default', if key is not found
except Exception:
letter = yield 'default'
except KeyError:
letter = yield 'default'
except Exception:
letter = yield 'default'


However for the input:

coro = alphabet()
next(coro)
print(coro.send('apple'))
print(coro.send('banana'))
print(coro.throw(KeyError))
print(coro.send('dog'))
print(coro.send('d'))


expected output:

default
default
default
default
dog


But I don't catch last default - it's None:

default
default
default
None
dog


What is wrong?

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