If I have a list of chars:

a = ['a','b','c','d']

How do I convert it into a single string?

a="abcd"

Use the join method of the empty string to join all of the strings together with the empty string in between, like so:

>>> a = ['a', 'b', 'c', 'd']
>>> ''.join(a)
'abcd'

This works in many popular languages like JavaScript and Ruby, why not in Python?

>>> ['a', 'b', 'c'].join('')
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute 'join'

Strange enough, in Python the join method is on the str class:

# this is the Python way
"".join(['a','b','c','d'])

Why join is not a method in the list object like in JavaScript or other popular script languages? It is one example of how the Python community thinks. Since join is returning a string, it should be placed in the string class, not on the list class, so the str.join(list) method means: join the list into a new string using str as a separator (in this case str is an empty string).

Somehow I got to love this way of thinking after a while. I can complain about a lot of things in Python design, but not about its coherence.

If your Python interpreter is old (1.5.2, for example, which is common on some older Linux distributions), you may not have join() available as a method on any old string object, and you will instead need to use the string module. Example:

a = ['a', 'b', 'c', 'd']

try:
    b = ''.join(a)

except AttributeError:
    import string
    b = string.join(a, '')

The string b will be 'abcd'.

This may be the fastest way:

>> from array import array
>> a = ['a','b','c','d']
>> array('B', map(ord,a)).tostring()
'abcd'

The reduce function also works

import operator
h=['a','b','c','d']
reduce(operator.add, h)
'abcd'

If the list contains numbers, you can use map() with join().

Eg:

>>> arr = [3, 30, 34, 5, 9]
>>> ''.join(map(str, arr))
3303459

h = ['a','b','c','d','e','f']
g = ''
for f in h:
    g = g + f

>>> g
'abcdef'

besides str.join which is the most natural way, a possibility is to use io.StringIO and abusing writelines to write all elements in one go:

import io

a = ['a','b','c','d']

out = io.StringIO()
out.writelines(a)
print(out.getvalue())

prints:

abcd

When using this approach with a generator function or an iterable which isn’t a tuple or a list, it saves the temporary list creation that join does to allocate the right size in one go (and a list of 1-character strings is very expensive memory-wise).

If you’re low in memory and you have a lazily-evaluated object as input, this approach is the best solution.

You could also use operator.concat() like this:

>>> from operator import concat
>>> a = ['a', 'b', 'c', 'd']
>>> reduce(concat, a)
'abcd'

If you’re using Python 3 you need to prepend:

>>> from functools import reduce

since the builtin reduce() has been removed from Python 3 and now lives in functools.reduce().