I think you have it dead on, for anybody who still doesn't understand how the ord(c) >> 5 part works, this is the binary representation of characters and digits:

>>> bin(ord('0'))
'0b0110000'
>>> bin(ord('9'))
'0b0111001'
>>> bin(ord('A'))
'0b1000001'
>>> bin(ord('Z'))
'0b1011010'
>>> bin(ord('a'))
'0b1100001'
>>> bin(ord('z'))
'0b1111010'

The first 2 binary digits will separate the characters into digits, lower case, upper case. So right shifting the value of ord(c) will just give those digits, e.g.

>>> bin(ord('a') >> 5)
'0b11'
which as conkosidowski says is 3 when represented as int.