Generator functions must maintain state between invocations. A generator function is compiled into two classes. One is anenvironment class that maintains the local variables of the function, and the label/location where execution will continue(__mypyc_next_label__ attribute). The second class implements the actual generator. The environment class is similar to what mypyc uses to support nested functions.

For example, consider this simple generator:

deff(x:int)->Iterator[int]:print('a')yieldxprint('b')

This is compiled to something roughly like this (in heavily simplified pseudo-python):

deff(x:int)->Iterator[int]:_env=f_env()_env.x=x_env.__mypyc_next_label__=0_gen=f_gen()_gen.__mypyc_env__=_envreturn_genclassf_env:x:int__mypyc_next_label__:intclassf_gen:__mypyc_env__:f_envdef__next__(self)->int:_env=self.__mypyc_env___label=_env.__mypyc_next_label__if_label==0:print('a')_ret=_env.x_env.__mypyc_next_label__=1return_retelif_label==1:print('b')_env.__mypyc_next_label__=-1raiseStopIteration()else:raiseStopIteration()# Already completed

The actual generated code is more complicated because of error handling, andclose/throw/send methods.