为什么不使用全局关键字就没有错误

Question

In this file I am using a dictionary named modelDict declared globally and I am using it in multiple functions ( addCharToModelDict, dumpModelDict ). I haven't used global keyword inside these functions to use the global modelDict .
addCharToModelDict is updating it and dumpModelDict is writing it back to file in the end.

Everything works fine!!

Why this is happening?? Isn't using global keyword is necessary??

Answer 1

The global keyword is only required when rebinding the name. Your operations mutate the object instead.

Answer 2

You are working with modelDict variable from globals (python trying to find modelDict in locals but can't then It tries to find it in globals and succeeded). This works If You use variable defined in outer code either read or update.

d = {}
def foo():
    a = d.get('x')
    d[4] = True
foo()

If You will try to reassign new data to variable with this name (rebind it) You will get Error.

>>> d = {}
>>> def foo():
        a = d.get('x')
        d = {4: True}

>>> foo()

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in foo
UnboundLocalError: local variable 'd' referenced before assignment