Python list from csv line and write list to file

Question

I'm new to python and no prior experience at all.
I have a requirement to convert a file contains a single line as: abc,def,ghi,jkl, to another file which should have the values from the above line as below:

abc
def
ghi
jkl

I've tried as below but receives error say that TypeError: can only concatenate list (not "str") to list :

#!/usr/bin/python
inputFile = 'servers'
outputFile = 'cservers'
serverList = []
with open(inputFile) as fi, open(outputFile,'w') as fo:
  line = fi.readline()
  serverList.append(line.split(','))
for i in serverList:
  fo.write(i+'\n')

What is wrong with above code?

Thanks

i is a list of lists. If you want it to be a list of strings, probably do serverList.extend(line.split(',')). Think about what happens if split returns a list. (Hint: it always does.) — tripleee
– tripleee, Commented Oct 2, 2020 at 8:39

Roim · Accepted Answer · 2020-10-02 08:42:39Z

1

split returns a list, not a string. After the line serverList.append(..), the variable serverList is a list conataining a list that contains strings, looks something like that: [['abc', 'def', ...]].

So serverList is a list of lists, with one element. Then you iterate through serverList (remember, there is only one element) and the element is a list. You try to add '\n' to a list, which is an error: TypeError: can only concatenate list (not "str") to list. Generally, use the error you receive to see what wrong: it says you are tried to add string with a list.

To fix it, just use

serverList = line.split(',')

Then the whole list returns for split commands will be in serverList inside of an element within it

Edit: as mentioned in comments, this will handle only one line of input - but from what I understand, that is okay for you (as you also read only one line). If you want to iterate through lines in the future, you can use serverList = serverList + line.split(',') which adds two lists together

edited Oct 2, 2020 at 8:42

answered Oct 2, 2020 at 8:40

Roim

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3 Comments

tripleee Over a year ago

This will only handle a single line of input, and overwrite the serverList on each iteration over the file.

Roim Over a year ago

I know, but he only has one line anyway. I added an edit to clarify that, thanks

Vijesh Over a year ago

Yes, its clear now. Currently there is only one line. Thank you for the explanation!!

JPI93 · Accepted Answer · 2020-10-02 08:58:15Z

In your example the line

    serverList.append(line.split(','))

results in a serverList that looks like [['abc', 'def', 'ghi', 'jkl']]. So when you come to execute

    fo.write(i+'\n')

You will get a TypeError thrown for the reason given in the console output.

You can test this by running.

a = [['abc', 'def', 'ghi', 'jkl']]
print(a+'\n')

You could fix your code by changing your code so that serverList is a list of strings representing lines to be written to outputFile as opposed to a list containing a list of strings. See below for example solution.

#!/usr/bin/python
inputFile = 'servers'
outputFile = 'cservers'

with open(inputFile) as fi:
    serverList = fi.read().split(',') # Read file contents as list of strings delimited by comma in file

with open(outputFile,'w') as fo:
    fo.writelines(s + '\n' for s in serverList) # Write each string in serverList on own line

Iammrjude1 · Accepted Answer · 2020-10-02 09:03:50Z

1

When you use the split() function it returns a list

So your list named serverList now contains another list.

Now this code here:

for i in serverList:
  fo.write(i+'\n')

Means you're looping through the serverList and remember that each element of this list is another list.

So that is the reason for the error. Because here in your code:

+'\n'

You are trying to concatenate (join) a list with a string which is not possible.

answered Oct 2, 2020 at 9:03

Iammrjude1

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