Python Language Basic Slicing


Example

For any iterable (for eg. a string, list, etc), Python allows you to slice and return a substring or sublist of its data.

Format for slicing:

iterable_name[start:stop:step]

where,

  • start is the first index of the slice. Defaults to 0 (the index of the first element)
  • stop one past the last index of the slice. Defaults to len(iterable)
  • step is the step size (better explained by the examples below)

Examples:

a = "abcdef"
a            # "abcdef" 
             # Same as a[:] or a[::] since it uses the defaults for all three indices
a[-1]        # "f"
a[:]         # "abcdef" 
a[::]        # "abcdef" 
a[3:]        # "def" (from index 3, to end(defaults to size of iterable)) 
a[:4]        # "abcd" (from beginning(default 0) to position 4 (excluded)) 
a[2:4]       # "cd" (from position 2, to position 4 (excluded)) 

In addition, any of the above can be used with the step size defined:

a[::2]       # "ace" (every 2nd element)
a[1:4:2]     # "bd" (from index 1, to index 4 (excluded), every 2nd element)

Indices can be negative, in which case they're computed from the end of the sequence

a[:-1]     # "abcde" (from index 0 (default), to the second last element (last element - 1))
a[:-2]     # "abcd" (from index 0 (default), to the third last element (last element -2))
a[-1:]     # "f" (from the last element to the end (default len()) 

Step sizes can also be negative, in which case slice will iterate through the list in reverse order:

a[3:1:-1]   # "dc" (from index 2 to None (default), in reverse order)

This construct is useful for reversing an iterable

a[::-1]     # "fedcba" (from last element (default len()-1), to first, in reverse order(-1))

Notice that for negative steps the default end_index is None (see http://stackoverflow.com/a/12521981 )

a[5:None:-1] # "fedcba" (this is equivalent to a[::-1])
a[5:0:-1]    # "fedcb" (from the last element (index 5) to second element (index 1)