How to limit function parameter as array of fixed-size?

What about checking the length inside of the function? Here I just raised an error, but you could do anything.

def func(array):
    if len(array) != 2:
        raise ValueError("array with length 2 was expected")
    # code here runs if len(array) == 2

1st way (you'll most probably want to use this)

You can just check for all the criteria inside your function by using if statements:

def func(a):
    if not isinstance(a, collections.abc.Sequence):
        raise TypeError("The variable has a wrong type")
    elif len(a) != 2:
        raise ValueError("Wrong length given for list")
    # rest of the code goes here

2nd way (only for debugging)

You can use assert as a workaround solution (meant for debugging):

def func(a):
    assert isinstance(a, collections.abc.Sequence) and len(a) == 2, "Wrong input given."
    # rest of the code goes here

So this will check if both criteria are met, otherwise an assertion error will be raised with the message Wrong Input Type.

If you don't mind unpacking your arg list then you could do this to limit the second arg to a fixed size collection of two elements.

> def f(a,(b1,b2), c):
    b=[b1,b2]
    print(a)
    print(b)
    print(c)

Examples :

# ok to call with 2 elem array
> b=[1,2]
> f("A",l,"C")
A
[1, 2]
C

# but falls if call with other size array
> b=[1,2,3]
> f("A",b,"C")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in f
ValueError: too many values to unpack


# btw also ok if call with 2 elem tuple    
> f("A",(1,2),"B")
A
[1, 2]
C