Insert item before each element of a list

[Steven D'Aprano]

On Mon, 08 Oct 2012 19:34:26 -0700, rusi wrote:

> How about a 2-paren version?
> 
>>>> x = [1,2,3]
>>>> reduce(operator.add,  [['insert', a] for a in x])
> ['insert', 1, 'insert', 2, 'insert', 3]

That works, but all those list additions are going to be slow. It will be 
an O(N**2) algorithm.


If you're going to be frequently interleaving sequences, a helper 
function is a good solution. Here's my muxer:

def mux(*iterables):
    """Muxer which yields items interleaved from each iterator or
    sequence argument, stopping when the first one is exhausted.

    >>> list( mux([1,2,3], range(10, 15), "ABCD") )
    [1, 10, 'A', 2, 11, 'B', 3, 12, 'C']
    """
    for i in itertools.izip(*iterables):  # in Python 3 use builtin zip
        for item in i:
            yield item


Then call it like this:

py> list(mux(itertools.repeat("insert me"), range(5)))
['insert me', 0, 'insert me', 1, 'insert me', 2, 'insert me', 3, 'insert 
me', 4]



-- 
Steven