a few extensions for the itertools

[Steven D'Aprano]

On Sun, 19 Nov 2006 21:35:24 +0100, Mathias Panzenboeck wrote:

> I wrote a few functions which IMHO are missing in python(s itertools).
> 
> You can download them here:
> http://sourceforge.net/project/showfiles.php?group_id=165721&package_id=212104
> 
> A short description to all the functions:
> 
> icmp(iterable1, iterable2) -> integer
> 	Return negative if iterable1 < iterable2,
> 	zero if iterable1 == iterable1,
> 	positive if iterable1 > iterable1.


What does it mean for an iterable to be less than another iterable? That
it has fewer items? How do these two iterables compare?

iter([1, 2, None, "foo", 3+2j])

def ones():
    while 1:
        yield 1

Which is smaller?



> isum(iterable, start=0) -> value
> 	Returns the sum of the elements of a iterable
> 	plus the value of parameter 'start'.  When the
> 	iterable is empty, returns start.


You mean just like the built-in sum()?

>>> sum(xrange(12), 1000)
1066


> iproduct(iterable, start=0) -> value
> 	Returns the product of the elements of a iterable
> 	times the value of parameter 'start'.  When the
> 	iterable is empty, returns start.

If I recall, product() was requested about the same time that sum() was
introduced, and Guido rejected it as a built-in because it was really only
useful for calculating geometric means, and it is easy to do if you need
it:

def product(it, start=1):
    # default value of 1 is more sensible than 0
    # 1 is the multiplicative identity
    p = start
    for x in it:
        p *= x
    return p


> forall(predicate, iterable, default=True) -> bool
> 	Returns True, when for all elements x in iterable
> 	predicate(x) is True. When the iterable is empty,
> 	returns default.
> 
> 
> forany(predicate, iterable, default=False) -> bool
> 	Returns True, when for any element x in iterable
> 	predicate(x) is True. When the iterable is empty,
> 	returns default.


I vaguely recall plans for all() and any() builtins -- perhaps for Python
2.5?


> take(n,iterable) -> iterator
> 	returns a iterator over the first n
> 	elements of the iterator

Just like itertools.islice(iterable, n).

>>> list(itertools.islice(xrange(10), 5))
[0, 1, 2, 3, 4]


> drop(n,iterable) -> iterable
> 	drops the first n elemetns of iterable and
> 	return a iterator over the rest

Just like itertools.islice(iterable, n, None)

>>> list(itertools.islice(xrange(20), 15, None))
[15, 16, 17, 18, 19]

(Aside: I think islice would be so much cleaner if it took keyword
arguments.)



> heads(iterable) -> iterator over all heads
> tails(iterable) -> iterator over all tails

What would you use these for?


> fcain(funct,*functs) -> function(...,***)
> 	fcain(f1,f2,...,fn)(*args,*kwargs) equals f1(f2(...fn(*args,*kwargs)))


The usual term for this is function composition.


-- 
Steven.