Append lists from different csv files in Python

Question

I am trying to append several lists each from a different csv files in Python. Here is the code I am using:

from_ls = [1,2,3,4,5]
to_ls = [2,3,4,5,6]
mylists = []
for i in range(len(from_ls)):
    from_ = from_ls[i]
    to_ = to_ls[i]
    print(str(from_)+'-'+str(to_))
    f = 'data'+str(from_)+'to'+str(to_)+'.csv'
    if os.path.exists(f):
        with open('data'+str(from_)+'to'+str(to_)+'.csv') as f:
            reader = csv.reader(f)
            mylists.extend(reader)
        print('file '+f+' append successfully')
    else:
        print('file '+f+' not found')

It gives me the following error:

1-2
Traceback (most recent call last):
  File "<stdin>", line 12, in <module>
TypeError: can only concatenate str (not "_io.TextIOWrapper") to str

I know that f = 'data'+str(from_)+'to'+str(to_)+'.csv' creates an _io.TextIOWrapper but I don't know how to turn it into a string so that I can read the file.

I have tried to read the csv files individually as below and it works fine:

i=0
from_ = from_ls[i]
to_ = to_ls[i]
with open('data'+str(from_)+'to'+str(to_)+'.csv') as f:
    reader = csv.reader(f)
    mylists = list(reader)

What is text?

Red
– Red

2020-07-03 18:39:02 +00:00
Commented Jul 3, 2020 at 18:39 — Red
– Red, Commented Jul 3, 2020 at 18:39
I have corrected: it is actually mylists

Charlie
– Charlie

2020-07-03 18:44:49 +00:00
Commented Jul 3, 2020 at 18:44 — Charlie
– Charlie, Commented Jul 3, 2020 at 18:44

Carlos Bazilio · Accepted Answer · 2020-07-03 18:52:24Z

1

I've tried to reproduce your error, but I couldn't. However I've found some mistakes.

import os
import csv

from_ls = [1,2,3,4,5]
to_ls = [2,3,4,5,6]
mylists = []
for i in range(len(from_ls)):
    from_ = from_ls[i]
    to_ = to_ls[i]
    print(str(from_)+'-'+str(to_))
    fname = 'data'+str(from_)+'to'+str(to_)+'.csv'
    if os.path.exists(fname):
        with open(fname) as f:
            reader = csv.reader(f)
            mylists.extend(reader)
        print('file '+fname+' append successfully')
    else:
        print('file '+fname+' not found')

I do not have csv files here. Try to see if it corrects your code.

answered Jul 3, 2020 at 18:52

Carlos Bazilio

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Comments

Red · Accepted Answer · 2020-07-03 19:46:18Z

It's here:

    print('file '+f+' append successfully')
else:
    print('file '+f+' not found')

f is the variable you assigned open('data'+str(from_)+'to'+str(to_)+'.csv') to.

Although you have f = 'data'+str(from_)+'to'+str(to_)+'.csv', you have overwritten its value later in the code. Simply change the name of one of your variables.

Corrected version:

from_ls = [1,2,3,4,5]
to_ls = [2,3,4,5,6]
mylists = []
for i in range(len(from_ls)):
    from_ = from_ls[i]
    to_ = to_ls[i]
    print(str(from_)+'-'+str(to_))
    fn = 'data'+str(from_)+'to'+str(to_)+'.csv' # Note the change of variable name
    if os.path.exists(fn): # And here
        with open('data'+str(from_)+'to'+str(to_)+'.csv') as f:
            reader = csv.reader(f)
            mylists.extend(reader)
        print('file '+fn+' append successfully') # Here
    else:
        print('file '+fn+' not found') # Here also

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