I want to use a use a name to refer to an index range for array/list indexing. Plus, I want to be able to do arithmetic on it. For instance, I want to do
myrange = 3:10
myrange2 = myrange + 5
arr2[myrange2] = arr1[myrange] # This being equivalent to arr2[8:15] = arr1[3:10]
myrange3 = myrange * 2 + 5
arr2[myrange3] = arr1[myrange] # This being equivalent to arr2[11:25] = arr1[3:10]
Is this possible with python lists and numpy arrays?
You can simply use a slice():
myrange = slice(3, 10)
myrange2 = slice(3 + 5, 10 + 5)
arr2[myrange2] = arr1[myrange]
This can be made into a function easily:
def add_to_slice(slice_, offset):
return slice(slice_.start + offset, slice_.stop + offset)
myrange = slice(3, 10)
myrange2 = add_to_slice(myrange, 5)
arr2[myrange2] = arr1[myrange]
If you want to use + you would need to patch the slice class (since slice is not an acceptable base class) to include a __add__ method, i.e.:
class Slice(slice):
pass
TypeError: type 'slice' is not an acceptable base type
A "perhaps close enough" approach would be wrap a slice around another class, say Slice and, optionally, make it easy to access the inner slice, e.g.:
class Slice(object):
def __init__(self, *args):
self.slice_ = slice(*args)
def __call__(self):
return self.slice_
def __add__(self, offset):
return Slice(self.slice_.start + offset, self.slice_.stop + offset, self.slice_.step)
myrange = Slice(2, 5)
myrange2 = myrange + 3
mylist = list(range(100))
print(mylist[myrange()])
print(mylist[myrange2()])
(You may want to refine the code for production to handle the possibility of the attributes of slice being None).
slice * 2 + 4). With this approach, it seems to me that is also straightforward, by defining __multiply__, etc. Would you think so?
operator module to see what to implement (e.g. * operator is __mul__, etc.). Note that you could also accept a str in the constructor with the colon syntax, which would allow you to write Slice('1:10') in place of Slice(1, 10), although that may be useful for handling user input than to simplify syntax in the code.You can use slice
a = slice(1,3)
'hello'[a] -> ell
You can access the attributes with a.start/a.stop/a.step. The attributes are read only but you can create a new slice from them for the arithmetic requirement
__int__or__index__doesn't seem to work.slilceobject, and passes that to the__getitem__method. By itself a slice object is quite simple, with just a couple of attributes.np.s_andnp.r_are specialnumpyclass instances that help generate and expand slices.myrange = 3:10. That's aninvalid syntaxerror.3:10, but being able to name a slice)...