class ssource:
    def __init__(self, n):
        self.list1=[]
        self.list2=[]
        self.i=0
        self.max=n
        self.At=stream(self, self.list1)
        self.Bt=stream(self, self.list2)
        
    def __next__(self):
        if self.i>=self.max: raise StopIteration
        print(f"NEXTSYNC:{self.i}")
        self.list1.append(f'A{self.i}')
        self.list2.append(f'B{self.i}')
        self.i += 1

src=ssource(10)
Ai=src.Ait
Bi=src.Bit

for i in range(5):
    print(f'A->{next(Ai)}')
for i in range(8):
    print(f'B->{next(Bi)}')
for a in Ai : print(f'A->{a}')

Error:
NEXTSYNC:0
A->A0
NEXTSYNC:1
A->A1
NEXTSYNC:2
A->A2
NEXTSYNC:3
A->A3
NEXTSYNC:4
A->A4
B->B0
B->B1
B->B2
B->B3
B->B4
NEXTSYNC:5
B->B5
NEXTSYNC:6
B->B6
NEXTSYNC:7
B->B7
A->A5
A->A6
A->A7
NEXTSYNC:8
A->A8
NEXTSYNC:9
A->A9

---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-36-b322da59c757> in stream(src, fifo)
      4     while True:
----> 5         if len(fifo)==0: next(src)
      6         yield(fifo.pop(0))

<ipython-input-36-b322da59c757> in __next__(self)
     17     def __next__(self):
---> 18         if self.i>=self.max: raise StopIteration
     19         print(f"NEXTSYNC:{self.i}")

StopIteration: 

The above exception was the direct cause of the following exception:

RuntimeError                              Traceback (most recent call last)
<ipython-input-39-9b42e44489d5> in <module>
      7 for i in range(8):
      8     print(f'B->{next(Bi)}')
----> 9 for a in Ai : print(f'A->{a}')

RuntimeError: generator raised StopIteration

def stream(src, flist):
    try:
        while True:
            while len(flist)==0: next(src) 
            yield(flist.pop(0))
    except StopIteration:
        pass
#        raise StopIteration