python slice how to, I know the python slice but how can I use built-in slice object for it?

Question

I want to know what's the use of built-in function slice and how can I use it, the direct way of pythonic slicing I know l1[start:stop:step], I want to know if I have object of slice then howto?

Answer 1

There is good deal of discussion on slicing at SO

• http://stackoverflow.com/questions/509211/good-primer-for-python-slice-notation

Are you looking for something specific?

Answer 2

The slice function returns slice objects. Slice objects are one of Python's internal types, which are optimized for read performance - all of their attributes are read-only.

Altering slice could be useful if wish to change the default behaviour. For example, lxml uses slice notation to access DOM elements (however, I haven't confirmed how they did that myself).

Answer 3

You create a slice by calling slice with the same fields you would use if doing [start:end:step] notation:

sl = slice(0,4)

To use the slice, just pass it as if it were the index into a list or string:

>>> s = "ABCDEFGHIJKL"
>>> sl = slice(0,4)
>>> print s[sl]
'ABCD'

Let's say you have a file of fixed-length text fields. You could define a list of slices to easily extract the values from each "record" in this file.

data = """\
0010GEORGE JETSON    12345 SPACESHIP ST   HOUSTON       TX
0020WILE E COYOTE    312 ACME BLVD        TUCSON        AZ
0030FRED FLINTSTONE  246 GRANITE LANE     BEDROCK       CA
0040JONNY QUEST      31416 SCIENCE AVE    PALO ALTO     CA""".splitlines()


fieldslices = [slice(*fielddef) for fielddef in [
    (0,4), (4, 21), (21,42), (42,56), (56,58),
    ]]
fields = "id name address city state".split()

for rec in data:
    for field,sl in zip(fields, fieldslices):
        print field, ':', rec[sl]
    print

Prints:

id : 0010
name : GEORGE JETSON    
address : 12345 SPACESHIP ST   
city : HOUSTON       
state : TX

id : 0020
name : WILE E COYOTE    
address : 312 ACME BLVD        
city : TUCSON        
state : AZ

id : 0030
name : FRED FLINTSTONE  
address : 246 GRANITE LANE     
city : BEDROCK       
state : CA

id : 0040
name : JONNY QUEST      
address : 31416 SCIENCE AVE    
city : PALO ALTO     
state : CA

Answer 4

Square brackets following a sequence denote either indexing or slicing depending on what's inside the brackets:

>>> "Python rocks"[1]    # index
'y'
>>> "Python rocks"[1:10:2]    # slice
'yhnrc'

Both of these cases are handled by the __getitem__() method of the sequence (or __setitem__() if on the left of an equals sign.) The index or slice is passed to the methods as a single argument, and the way Python does this is by converting the slice notation, (1:10:2, in this case) to a slice object: slice(1,10,2).

So if you are defining your own sequence-like class or overriding the __getitem__ or __setitem__ or __delitem__ methods of another class, you need to test the index argument to determine if it is an int or a slice, and process accordingly:

def __getitem__(self, index):
    if isinstance(index, int):
        ...    # process index as an integer
    elif isinstance(index, slice):
        start, stop, step = index.indices(len(self))    # index is a slice
        ...    # process slice
    else:
        raise TypeError("index must be int or slice")

A slice object has three attributes: start, stop and step, and one method: indices, which takes a single argument, the length of the object, and returns a 3-tuple: (start, stop, step).

Answer 5

>>> class sl:
...  def __getitem__(self, key): print key
... 
>>> s = sl()
>>> s[1:3:5]
slice(1, 3, 5)
>>>