Found this great table at http://wiki.python.org/moin/MovingToPythonFromOtherLanguages
Python indexes and slices for a six-element list.
Indexes enumerate the elements, slices enumerate the spaces between the elements.
Index from rear: -6 -5 -4 -3 -2 -1 a=[0,1,2,3,4,5] a[1:]==[1,2,3,4,5]
Index from front: 0 1 2 3 4 5 len(a)==6 a[:5]==[0,1,2,3,4]
+---+---+---+---+---+---+ a[0]==0 a[:-2]==[0,1,2,3]
| a | b | c | d | e | f | a[5]==5 a[1:2]==[1]
+---+---+---+---+---+---+ a[-1]==5 a[1:-1]==[1,2,3,4]
Slice from front: : 1 2 3 4 5 : a[-2]==4
Slice from rear: : -5 -4 -3 -2 -1 :
b=a[:]
b==[0,1,2,3,4,5] (shallow copy of a)
In python 2.7
Slicing in python
[a:b:c]
len = length of string, tuple or list
c -- default is +1. sign of c indicates forward or backward, absolute value of c indicates steps. Default is forward with step size 1. Positive means forward, negative means backward.
a -- when c is positive or blank, default is 0. when c is negative, default is -1.
b -- when c is positive or blank, default is len. when c is negative, default is -(len+1).
Understanding index assignment is very important.
In forward direction, starts at 0 and ends at len-1
In backward direction, starts at -1 and ends at -len
when you say [a:b:c] you are saying depending on sign of c (forward or backward), start at a and end at b ( excluding element at bth index). Use the indexing rule above and remember you will only find elements in this range
-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1
but this range continues in both directions infinitely
...,-len -2 ,-len-1,-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1, len, len +1, len+2 , ....
e.g.
0 1 2 3 4 5 6 7 8 9 10 11
a s t r i n g
-9 -8 -7 -6 -5 -4 -3 -2 -1if your choice of a , b and c allows overlap with the range above as you traverse using rules for a,b,c above you will either get a list with elements (touched during traversal) or you will get an empty list.
One last thing: if a and b are equal , then also you get an empty list
>>> l1 [2, 3, 4] >>> l1[:] [2, 3, 4] >>> l1[::-1] # a default is -1 , b default is -(len+1) [4, 3, 2] >>> l1[:-4:-1] # a default is -1 [4, 3, 2] >>> l1[:-3:-1] # a default is -1 [4, 3] >>> l1[::] # c default is +1, so a default is 0, b default is len [2, 3, 4] >>> l1[::-1] # c is -1 , so a default is -1 and b default is -(len+1) [4, 3, 2] >>> l1[-100:-200:-1] # interesting [] >>> l1[-1:-200:-1] # interesting [4, 3, 2] >>> l1[-1:5:1] [4] >>> l1[-1:-1:1] [] >>> l1[-1:5:1] # interesting [4] >>> l1[1:-7:1] [] >>> l1[1:-7:-1] # interesting [3, 2]