CODEWITHSUNDEEP
pythonlistpython-3.xlist-comprehension
Jukurrpa
Jukurrpa

Reputation: 4168

Inserting an element before each element of a list

I'm looking to insert a constant element before each of the existing element of a list, i.e. go from:

['foo', 'bar', 'baz']

to:

['a', 'foo', 'a', 'bar', 'a', 'baz']

I've tried using list comprehensions but the best thing I can achieve is an array of arrays using this statement:

[['a', elt] for elt in stuff]

Which results in this:

[['a', 'foo'], ['a', 'bar'], ['a', 'baz']]

So not exactly what I want. Can it be achieved using list comprehension? Just in case it matters, I'm using Python 3.5.

Upvotes: 6

Views: 3032

Answers (3)

garg10may
garg10may

Reputation: 6179

You already have 

   l =  [['a', 'foo'], ['a', 'bar], ['a', 'baz']]

You can flatten it using 

[item for sublist in l for item in sublist]

this even works for arbitrary length nested list.

Upvotes: 2

mgilson
mgilson

Reputation: 309841

A simple generator function works nicely here too:

def add_between(iterable, const):
    # TODO: think of a better name ... :-)
    for item in iterable:
        yield const
        yield item

list(add_between(['foo', 'bar', 'baz'], 'a')

This lets you avoid a nested list-comprehension and is quite straight-forward to read and understand at the cost of being slightly more verbose.

Upvotes: 3

Martijn Pieters
Martijn Pieters

Reputation: 1121486

Add another loop:

[v for elt in stuff for v in ('a', elt)]

or use itertools.chain.from_iterable() together with zip() and itertools.repeat() if you need an iterable version rather than a full list:

from itertools import chain, repeat
try:
    # Python 3 version (itertools.izip)
    from future_builtins import zip
except ImportError:
    # No import needed in Python 3

it = chain.from_iterable(zip(repeat('a'), stuff))

Upvotes: 13

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