python3 print() in exec()

Question

In python3, when I run

>>> exec("","","")
TypeError: exec() arg 2 must be a dict, not str
>>> exec( "print('Hello')", print("World"), print("!") )
World
!
Hello
>>> type(print("World"))
World
<class 'NoneType'>

I mean in the Python3, the arg2 of exec() expects a dict, but we can still put a print() function which is not a dict. why?

Answer 1

Simple!

It's acceptable because its value is None (it can accept None or a dict), which is the default value for the argument.

In a example, a call such as:

exec("print('Hello')")

Is the same as:

exec("print('Hello')", None, None)

Answer 2

print返回None ，这是可选参数的有效参数。