为什么python在函数和表达式中对星号的处理方式不同？

Question

In the following codes there are two places which asterisk(*) is used :

Asterisk in function:

def function(*asterisk):
    print(1, type(asterisk))

Asterisk in expressions:

a, *b = (1, 2, 3)
print(2, type(b))

a, *b = [1, 2, 3]
print(3, type(b))

I know the meaning of asterisk in functions and how it works.

My question is why asterisk in print number 1 has a tuple type whereas in print number 2 and 3 has a list type ?

Why python interprets function *parameter as tuple regardless of argument's type and interprets *variable as list regardless of assigned numbers' type in expressions?

In the end, are those * different operators?!

Answer 1

The devs just figured a list would be easier to process in the assignment case. Quoting the PEP for a, *b = ... syntax:

After a short discussion on the python-3000 list [1] , the PEP was accepted by Guido in its current form. Possible changes discussed were:

• ...

• Make the starred target a tuple instead of a list. This would be consistent with a function's *args, but make further processing of the result harder.

And the mailing list discussion :

IMO, it's likely that you would like to further process the resulting sequence, including modifying it.

The mailing list discussion is pretty short ( starts here , only 28 messages), so don't be afraid to read the whole thing.

Answer 2

The unpacking in assignment (python 3 specific) is extended iterable unpacking (when the unpacking term refers to the right hand side , the variable that follows * in the left hand side isn't unpacked, but created ), whereas the other one (also available in python 2) is just positional argument unpacking.

The * "operator" is very different in both cases. The fact that extended iterable unpacking creates a list is probably designed so the caller can extend that list.

In parameter unpacking, it's better to have a fixed iterable like tuple . Once again those are completely different mechanisms.