python generator

[bree...@gmail.com]

code show as below:

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def generator():

while 1:

recv=yield a

print('recv :',recv)

if __name__=='__main__':

g=generator()

>>> a=10

>>> g.send(None)

10

>>> g.send(None)

recv : None

10

>>> a=11

>>> g.send('next')

recv : next

11

>>> 

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Why do you claim that its internal variable a, even if it is not declared, does not report an error, and can access the external variable of the same name --a. This makes me puzzled. Can anyone tell me why?