Python join method returning a list?

Question

I have a list of elements ("tokens") that I assume are acting like strings. I want to find words with an underscore in them and replace the underscore with a space. I have the following code:

for e in tokens:
      if '_' in e:
            cmpd = list(e)
            cmpd[e.find('_')] = ' '
            ''.join(cmpd)
            new_tokens[index] = cmpd

It's basically identical to what's here: Change one character in a string in Python?

Later I'm trying to concatenate all the list elements in a sentence, each separated by a space, but I get the following error:

TypeError: can only concatenate list (not "str") to list

And if I print out e and cmpd, I get this output:

e: my_string

cmpd: ['m', 'y', ' ', 's', 't', 'r', 'i', 'n', 'g']

Why is cmpd a list and not a string?

You want to replace the underscores? Or the entire word? – Willem Van Onsem Feb 07 '17 at 17:06 — Willem Van Onsem, Feb 07 '17 at 17:06
Perhaps you can do `string.replace('_', ' ')` – Mohammad Yusuf Feb 07 '17 at 17:08 — Mohammad Yusuf, Feb 07 '17 at 17:08
`join()` is not inline. you need to assign it to something – MooingRawr Feb 07 '17 at 17:08 — MooingRawr, Feb 07 '17 at 17:08
Can you include output of `print(tokens)`? – Mohammad Yusuf Feb 07 '17 at 17:10 — Mohammad Yusuf, Feb 07 '17 at 17:10

score 3 · Accepted Answer · answered Feb 07 '17 at 17:07

3

You need to assign ''.join(cmpd) to cmpd using =.:

cmpd=''.join(cmpd)

answered Feb 07 '17 at 17:07

Trelzevir

767
6
11

So obvious I completely missed it... Thank you! – eihe Feb 07 '17 at 17:09

score 2 · Answer 2 · answered Feb 07 '17 at 17:08

2

''.join(cmpd)

This line converts cmpd to a string, then throws that string away. You should keep the string:

cmpd = ''.join(cmpd)

answered Feb 07 '17 at 17:08

kindall

178,883
35
278
309

score 2 · Answer 3 · answered Feb 07 '17 at 17:08

2

There are a lot of problems here:

you use an index that is not defined,
.join does not work inplace;
if a token contains multiple underscores only one will be replaced;
if a token does not contain an underscore, it is not added; and
...

What about:

new_tokens = [token.replace('_',' ') for token in tokens]

This replaces the entire loop and makes it very clear that you are replacing underscores with spaces.

answered Feb 07 '17 at 17:08

Willem Van Onsem

443,496
30
428
555

The replace function is good to know, thanks for that! I don't have any words with more than one underscore, so I wasn't too bothered with finding multiple underscores. In this case, I suppose I could write `cmpd = e.replace('_',' ')` which is certainly neater! The for loop is necessary though, as I have multiple clauses for different cases. – eihe Feb 08 '17 at 14:48
1

@eihe: yeah in case the loop is more complex, you can indeed write `cmpd = ...`. If it is however a simple problem list comprehension is the way to go since it is faster and more elegant. – Willem Van Onsem Feb 08 '17 at 14:51

Ma0 · Answer 4 · 2017-02-07T17:15:14.853

1

no_underscores = [x.replace('_', ' ') for x in tokens]
print(' '.join(no_underscores))

So what happens is that you create a new list (you can also overwrite the previous one) that contains the elements of the old one but without the underscores (.replace works even if the string you are looking for is not there). On the second line all elements of the new list are joined together with a space in between them.

If you feel comfortable with the above, you can even combine them like so:

print(' '.join(x.replace('_', ' ') for x in tokens))

which has the advantage of not creating any extra lists.

edited Feb 07 '17 at 17:15

answered Feb 07 '17 at 17:09

Ma0

15,057
4
35
65

Here you join with a space? – Willem Van Onsem Feb 07 '17 at 17:10
@WillemVanOnsem "*concatenate all the list elements in a sentence, each separated by a space*". Am I reading\interpreting that wrong? – Ma0 Feb 07 '17 at 17:12
ah the join is on the tokens, not the characters. Missed that. – Willem Van Onsem Feb 07 '17 at 17:13

score 0 · Answer 5 · answered Feb 07 '17 at 17:07

0

when you get to this line cmpd = list(e) e is a string and you are converting it to a list this will make each character an item in a list

answered Feb 07 '17 at 17:07

davidejones

1,869
1
16
18

Python join method returning a list?

5 Answers5