[Tutor] Alternating patterns

Kent Johnson kent37 at tds.net
Wed Mar 29 15:29:20 CEST 2006

kevin parks wrote:
>>From: Kent Johnson <kent37 at tds.net>
>>itertools.cycle() will repeat a sequence indefinitely:
>>In [2]: from itertools import cycle
>>In [3]: i=cycle([1,2])
>>In [5]: for j in range(6):
>>    ...:     print i.next()
>>    ...:
>>    ...:
>>For non-repeating sequences I would look at writing a generator 
>>for the sequences.
> okay.. i am painfully unaware of generators, iterators, sets, genexes 
> and a lot of the new stuff post 2.3 & 2.4 stuffs... my problem is that 
> i find the online docs kind of terse and few of the Python books yet 
> cover these newer constructs in detail....
> itertools looks very cool.... are there any toots on the above and on 
> Sets & itertools? It really seems like it would help to know these for 
> my work... That liddo bit right up there with itertools.cycle already 
> has me a drooling... (so helpful that would be!)

The best introduction to new features is usually in the What's New 
document accompanying the release where the feature was added. Of course 
it helps to know when the feature was added...here are some generator 

Generators are excellent for encapsulating the generation of a sequence 
when there is state that must be maintained between elements. For 
example here is a generator that takes a sequence argument, and yields 
this sequence of sequences:
   the original sequence
   the original sequence with the first element incremented by one
   the original sequence
   the original sequence with the second element incremented by one
   etc until each element has been incremented

In [2]: def change_each(seq):
    ...:     seq = list(seq) # Copy and ensure it's a list
    ...:     for i in range(len(seq)):
    ...:         yield seq
    ...:         seq[i] += 1
    ...:         yield seq
    ...:         seq[i] -= 1

In [3]: s = [1, 3]

In [5]: for n in change_each(s):
    ...:     print n
[1, 3]
[2, 3]
[1, 3]
[1, 4]

If you wanted to repeat this sequence indefinitely you could just wrap 
it with itertools.cycle().

The module docs for itertools contain quite a few examples:

itertools.cycle() is pretty simple, it just loops endlessly over the 
sequence you give it.

The itertools docs shows equivalent Python functions for each of the 
itertools functions. Most of them are implemented using generator 
functions so by looking at them you can learn about itertools and 
generators at the same time.


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