How to concatenate (join) items in a list to a single string

Question

How do I concatenate a list of strings into a single string?

For example, given ['this', 'is', 'a', 'sentence'], how do I get "this-is-a-sentence"?

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_{For handling a few strings in separate variables, see How do I append one string to another in Python?.}

_{For the opposite process - creating a list from a string - see How do I split a string into a list of characters? or How do I split a string into a list of words? as appropriate.}

Ricard

Answer 1

Use str.join:

>>> words = ['this', 'is', 'a', 'sentence']
>>> '-'.join(words)
'this-is-a-sentence'
>>> ' '.join(words)
'this is a sentence'

Centum

Didn't understand, what's the '-' on the left supposed to be?? – Nammu

@LawrenceDeSouza The string you want to be joined; see the documentation, or this answer which goes into a bit more detail. – Centum

I kind of expected sentence.join(" ") to work as well, since the reverse operation is list.split(" "). Any idea if this is going to be added to Python's methods for lists? – Marthena

@Wouter, it will not. On the one hand, only lists of strings can be joined; so list.join would be inappropriate for an arbitrary list. On the other, the argument of str.join can be any "iterable" sequence of strings, not just a list. The only thing that would make sense is a built-in function join(list, sep); there's one in the (basically obsolete) string module if you really want it. – Mestas

@Mestas That indeed makes sense, that a Python list can contain elements of mixed types, and that it is not always join-able. Thanks! – Marthena

What if we want Output as this is a sentence .? – Jenette

To summarize the comments, for those of us who just want to join the strings, the right command is then ''.join(sentence) on an empty string (see @6pack kid's answer below). – Daughter

What if one of the item is in another type how can I handle it? – Josephinejosephson

What if its a list like ['TEST ID', BATCHID', 'TEST2'] and I ONLY want to join ADD ID to one string? Output should be ['TESTID', BATCHID', 'TEST2']. – Nares

what if the elements of the list were custom class objects and not strings ? – Hattiehatton

@Hattiehatton you would have to call the str() method on the object, and that would convert it to the its string representation, ''.join(map(str, [obj1,obj2,obj3])) – Centum

Answer 2

A more generic way (covering also lists of numbers) to convert a list to a string would be:

>>> my_lst = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> my_lst_str = ''.join(map(str, my_lst))
>>> print(my_lst_str)
12345678910

Soilasoilage

why is list comprehension necessary when the input is string? also map(str, my_lst) would be enough without enumerating the list =) – Ricard

Most of the answers to this question are considering the case when the list has strings in it. Mine is a generic way to convert lists to strings. In my example, the list is of type int but it could be any type that can be represented as a string. – Soilasoilage

To add to this, map would also be useful when used with a lambda function. For example ' '.join(map(lambda x: ' $'+ str(x), my_lst)) would return '$1 $2 $3 $4 $5 $6 $7 $8 $9 $10' – Wafture

adding a map actually helps with datatypes apart from string. Great! – Earwitness

In Python it is preferred to use generator expressions instead of map() and lambda. Updated example by @ScottMcC: ' '.join(f'${x}' for x in my_lst) evaluates to: '$1 $2 $3 $4 $5 $6 $7 $8 $9 $10' --- Note: The original example has unwanted space in ' $' – Courtnay

Answer 3

It's very useful for beginners to know why join is a string method.

It's very strange at the beginning, but very useful after this.

The result of join is always a string, but the object to be joined can be of many types (generators, list, tuples, etc).

.join is faster because it allocates memory only once. Better than classical concatenation (see, extended explanation).

Once you learn it, it's very comfortable and you can do tricks like this to add parentheses.

>>> ",".join("12345").join(("(",")"))
Out:
'(1,2,3,4,5)'

>>> list = ["(",")"]
>>> ",".join("12345").join(list)
Out:
'(1,2,3,4,5)'

Parley

can someone explain what the second join() i.e. join(("(",")")) does? its like magic – Kinin

("(",")") is a tuple of two strings - the open parenthesis "(" and the close parenthesis ")". So the second join() joins those two strings using the output of the first join() as the separator (i.e., '1,2,3,4,5'). – Aldershot

Is this correct? "The result of join is always a string, but the object to be joined can be of many types (generators, list, tuples, etc)." The answer by Aaron S suggests that this is not true, you have to use map(str, ) if there are other types in the list. – Rag

@O'Rooney the answer is saying you can .join lists, tuples or generators, which are different types of collection, but they should only contain strings. – Cavesson

Okay voting this up because it is cute as eff, but seriously don't do this for code you want humans to understand. I concede it may be useful for code golf or evil. – Bioastronautics

Answer 4

>>> list_abc = ['aaa', 'bbb', 'ccc']

>>> string = ''.join(list_abc)
>>> print(string)
aaabbbccc

>>> string = ','.join(list_abc)
>>> print(string)
aaa,bbb,ccc

>>> string = '-'.join(list_abc)
>>> print(string)
aaa-bbb-ccc

>>> string = '\n'.join(list_abc)
>>> print(string)
aaa
bbb
ccc

Ambush

The reverse use of the prompt sequence >>> on the output instead of commands is very confusing. – Courtnay

Answer 5

Edit from the future: Please don't use the answer below. This function was removed in Python 3 and Python 2 is dead. Even if you are still using Python 2 you should write Python 3 ready code to make the inevitable upgrade easier.

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Although @Burhan Khalid's answer is good, I think it's more understandable like this:

from str import join

sentence = ['this','is','a','sentence']

join(sentence, "-") 

The second argument to join() is optional and defaults to " ".

Geber

This function is deprecated in 2.x and going to be removed in 3.x. That being the case, I wouldn't recommend using this over the other answer provided. – Dynamometry

@Mathew Green this answer and Burhan Khalid's answer use the same function in different ways, so his answer is deprecated as well. But thanks for pointing that out, gota figure out how to join in 3.x. But if you're using 2.x, it's not going away. – Geber

His isn't deprecated though. Your recommendation uses a function from Lib/string.py which is wholly different and deprecated whereas @BurhanKhalid answer uses the built-in string method which is not deprecated. His answer is the most correct one now and going forward whereas this is only good for now on 2.x. – Dynamometry

All that to say that if you want to promote this alternative you should also include a warning that this is not recommended for the reasons outlined in my previous comments. – Dynamometry

It's scary that I still get up-votes on this. Seriously, use python 3. – Geber

Answer 6

We can also use Python's reduce function:

from functools import reduce

sentence = ['this','is','a','sentence']
out_str = str(reduce(lambda x,y: x+"-"+y, sentence))
print(out_str)

Bagging

Too much-unneeded complexity, why not just using join? – Donnetta

In my cace join doesn't make all job, i need more symbols added, so reduce works better – Melendez

Answer 7

We can specify how we join the string. Instead of '-', we can use ' ':

sentence = ['this','is','a','sentence']
s=(" ".join(sentence))
print(s)

Atwitter

Answer 8

If you have a mixed content list and want to stringify it, here is one way:

Consider this list:

>>> aa
[None, 10, 'hello']

Convert it to string:

>>> st = ', '.join(map(str, map(lambda x: f'"{x}"' if isinstance(x, str) else x, aa)))
>>> st = '[' + st + ']'
>>> st
'[None, 10, "hello"]'

If required, convert back to the list:

>>> ast.literal_eval(st)
[None, 10, 'hello']

Jeremiahjeremias

Why such overcomplication? str(aa) also gives "[None, 10, 'hello']"... – Aprilaprile

Answer 9

It's also possible to use str.format() to join values in a list by unpacking the list inside format() which inserts the values sequentially into the placeholders. It can handle non-strings as well.

lst1 = ['this', 'is', 'a', 'sentence']
lst2 = ['numbers', 1, 2, 3]
'{}-{}-{}-{}'.format(*lst1)       # 'this-is-a-sentence'
'{} {}, {} and {}'.format(*lst2)  # 'numbers 1, 2 and 3'

For a large list, we can use the list's length to initialize the appropriate number of placeholders. One of two ways could be used to do that:

• ', '.join(['{}']*len(lst)).format(*lst)
  

• ('{}, '*len(lst)).rstrip(', ').format(*lst)
  

A working example:

lst = [1.2345, 3.4567, 4.567, 5.6789, 7.8]
', '.join(['{}']*len(lst)).format(*lst)      # '1.2345, 3.4567, 4.567, 5.6789, 7.8'
('{}, '*len(lst)).format(*lst).rstrip(', ')

# with a float format specification
', '.join(['{:.2f}']*len(lst)).format(*lst)  # '1.23, 3.46, 4.57, 5.68, 7.80'

Etamine

Answer 10

If you want to generate a string of strings separated by commas in final result, you can use something like this:

sentence = ['this','is','a','sentence']
sentences_strings = "'" + "','".join(sentence) + "'"
print (sentences_strings) # you will get "'this','is','a','sentence'"

Outandout

It works but this code better follows the logic of things: sentences_strings = ','.join(f"'{word}'" for word in sentence) As an important bonus you have the characters to enclose the strings ' in one place, not scattered over the whole expression. – Courtnay

Answer 11

def eggs(someParameter):
    del spam[3]
    someParameter.insert(3, ' and cats.')


spam = ['apples', 'bananas', 'tofu', 'cats']
eggs(spam)
spam =(','.join(spam))
print(spam)

Celebration

While this code snippet may be the solution, including an explanation really helps to improve the quality of your post. Remember that you are answering the question for readers in the future, and those people might not know the reasons for your code suggestion. – Fatma

Answer 12

Without .join() method you can use this method:

my_list=["this","is","a","sentence"]

concenated_string=""
for string in range(len(my_list)):
    if string == len(my_list)-1:
        concenated_string+=my_list[string]
    else:
        concenated_string+=f'{my_list[string]}-'
print([concenated_string])
    >>> ['this-is-a-sentence']

So, range based for loop in this example , when the python reach the last word of your list, it should'nt add "-" to your concenated_string. If its not last word of your string always append "-" string to your concenated_string variable.

Chaldean