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

Question

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 a slice object, then how do I use it?

Answer 1

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("{} : {}".format(field, rec[sl]))     print('')  # or this same code using itemgetter, to make a function that # extracts all slices from a string into a tuple of values import operator rec_reader = operator.itemgetter(*fieldslices) for rec in data:     for field, field_value in zip(fields, rec_reader(rec)):         print("{} : {}".format(field, field_value))     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 2

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).