You can use the numpy.asarray function to convert any sequence-like input to an array:
>>> import numpy
>>> numpy.asarray([1,2,3])
array([1, 2, 3])
>>> numpy.asarray(numpy.array([2,3]))
array([2, 3])
>>> numpy.asarray(1)
array(1)
>>> numpy.asarray((2,3))
array([2, 3])
>>> numpy.asarray({1:3,2:4})
array({1: 3, 2: 4}, dtype=object)
It's important to note that as the documentation says No copy is performed if the input is already an ndarray. This is really nice since you can pass an existing array in and it just returns the same array.
Once you convert it to a numpy array, just check the length if that's a requirement. Something like:
>>> def f(x):
... x = numpy.asarray(x)
... if len(x) != 2:
... raise Exception("invalid argument")
...
>>> f([1,2])
>>> f([1,2,3])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in f
Exception: invalid argument
Update:
Since you asked, here's a "magic" function that will except *args as an array also:
>>> def f(*args):
... args = numpy.asarray(args[0]) if len(args) == 1 else numpy.asarray(args)
... return args
...
>>> f(7,3,5)
array([7, 3, 5])
>>> f([1,2,3])
array([1, 2, 3])
>>> f((2,3,4))
array([2, 3, 4])
>>> f(numpy.array([1,2,3]))
array([1, 2, 3])
