Use of None in Array indexing in Python

Question

I am using the LSTM tutorial for Theano (http://deeplearning.net/tutorial/lstm.html). In the lstm.py (http://deeplearning.net/tutorial/code/lstm.py) file, I don\'t understan

Answer 1

I think the Theano vector's __getitem__ method expects a tuple as an argument! like this:

class Vect (object):
    def __init__(self,data):
        self.data=list(data)

    def __getitem__(self,key):
        return self.data[key[0]:key[1]+1]

a=Vect('hello')
print a[0,2]

Here print a[0,2] when a is an ordinary list will raise an exception:

>>> a=list('hello')
>>> a[0,2]
Traceback (most recent call last):
  File "", line 1, in 
TypeError: list indices must be integers, not tuple

But here the __getitem__ method is different and it accepts a tuple as an argument.

You can pass the : sign to __getitem__ like this as : means slice:

class Vect (object):
    def __init__(self,data):
        self.data=list(data)

    def __getitem__(self,key):
        return self.data[0:key[1]+1]+list(key[0].indices(key[1]))

a=Vect('hello')
print a[:,2]

Speaking about None, it can be used when indexing in plain Python as well:

>>> 'hello'[None:None]
'hello'