Python | local variable 'xxxx' referenced before assignment

>>> def func(num):
...     def func_in():
...             num += 1
...             print(num)
...     return func_in
... 
>>> fun = func(10)
>>> fun
<function func.<locals>.func_in at 0x1034410d0>
>>> fun()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in func_in
UnboundLocalError: local variable 'num' referenced before assignment

　Error local variables func_in caused by the undefined num:  local variable 'num' referenced before Assignment

　Even func_in function is defined in the function func, but it's still a variable is a local variable, not related to the parameters of function func num.