How would you make a comma-separated string from a list of strings?

Question

What would be your preferred way to concatenate strings from a sequence such that between every two consecutive pairs a comma is added. That is, how do you map, for instance

Answer 1

Why the map/lambda magic? Doesn't this work?

>>> foo = ['a', 'b', 'c']
>>> print(','.join(foo))
a,b,c
>>> print(','.join([]))

>>> print(','.join(['a']))
a

In case if there are numbers in the list, you could use list comprehension:

>>> ','.join([str(x) for x in foo])

or a generator expression:

>>> ','.join(str(x) for x in foo)

Answer 2

my_list = ['a', 'b', 'c', 'd']
my_string = ','.join(my_list)

'a,b,c,d'

This won't work if the list contains integers

---

And if the list contains non-string types (such as integers, floats, bools, None) then do:

my_string = ','.join(map(str, my_list)) 

Answer 3

Don't you just want:

",".join(l)

Obviously it gets more complicated if you need to quote/escape commas etc in the values. In that case I would suggest looking at the csv module in the standard library:

https://docs.python.org/library/csv.html

Answer 4

My two cents. I like simpler an one-line code in python:

>>> from itertools import imap, ifilter
>>> l = ['a', '', 'b', 1, None]
>>> ','.join(imap(str, ifilter(lambda x: x, l)))
a,b,1
>>> m = ['a', '', None]
>>> ','.join(imap(str, ifilter(lambda x: x, m)))
'a'

It's pythonic, works for strings, numbers, None and empty string. It's short and satisfies the requirements. If the list is not going to contain numbers, we can use this simpler variation:

>>> ','.join(ifilter(lambda x: x, l))

Also this solution doesn't create a new list, but uses an iterator, like @Peter Hoffmann pointed (thanks).

Answer 5

@jmanning2k using a list comprehension has the downside of creating a new temporary list. The better solution would be using itertools.imap which returns an iterator

from itertools import imap
l = [1, "foo", 4 ,"bar"]
",".join(imap(str, l))

Answer 6

Unless I'm missing something, ','.join(foo) should do what you're asking for.

>>> ','.join([''])
''
>>> ','.join(['s'])
's'
>>> ','.join(['a','b','c'])
'a,b,c'

(edit: and as jmanning2k points out,

','.join([str(x) for x in foo])

is safer and quite Pythonic, though the resulting string will be difficult to parse if the elements can contain commas -- at that point, you need the full power of the csv module, as Douglas points out in his answer.)