Joining two lists into tuples in python

Question

I want to use the join function in python (not any other function) to merge two lists into nested lists, assuming the lists are of equal length, for example:

list1 = [1, 2, 3]

list2 = ["a", "b", "c"]

I want it to produce a new list like this:

[[1,"a"], [2,"b"], [3,"c"]]

Answer 1

I don't think you understand what str.join is for.

str.join is a string method that takes an iterable (usually a list) of strings and returns a new string object that is a concatenation of those strings separated by the string that the method was invoked on.

Below is a demonstration:

>>> strs = ['a', 'b', 'c']
>>> ''.join(strs)
'abc'
>>> '--'.join(strs)
'a--b--c'
>>>

This means that you would not use str.join for what you are trying to do. Instead, you can use zip and a list comprehension :

>>> list1 = [1, 2, 3]
>>> list2 = ["a", "b", "c"]
>>> [list(x) for x in zip(list1, list2)]
[[1, 'a'], [2, 'b'], [3, 'c']]
>>>

Note however that, if you are on Python 2.x, you may want to use itertools.izip instead of zip :

>>> from itertools import izip
>>> list1 = [1, 2, 3]
>>> list2 = ["a", "b", "c"]
>>> [list(x) for x in izip(list1, list2)]
[[1, 'a'], [2, 'b'], [3, 'c']]
>>>

Like the Python 3.x zip , itertools.izip will return an iterator (instead of a list like the Python 2.x zip ). This makes it more efficient, especially when dealing with larger lists.

Answer 2

Eureka! The join function you are looking for in this context, my friend, is the following:

def join(l1, l2):
    return [list(x) for x in zip(l1, l2)]

print join(list1, list2) 

:-)

Answer 3

list(map(list, zip(list1, list2)))