Looping multiple files into a single csv file in python

Question

I am trying to process several files into a single, merged csv file using python. So far, I have

files = ["file1.txt", "file2.txt", "file3.txt"]

def doSomething(oldfile):
    content = []
    with open oldfile as file:
        content = file.read().splitlines()
    file.close()
    return content.reverse()

with open("newfile.txt", "w") as file:
    w = csv.writer(file, dialect = "excel-tab")
    for i in range(0, len(files)):
        w. writerows(doSomething(files[i])
file.close()

The new file is being created, but there is nothing in it. I am curious about what is going on.

Thanks!

Answer 1

For starters, list.reverse() reverses the list in place and doesn't return anything so you're essentially returning None from your doSomething() function. You'll actually want to split that into two lines:

content.reverse()
return content

If you want to streamline your code, here's a suggestion:

def doSomething(oldfile):
    with open(oldfile, "r") as f:
        return reversed(f.read().splitlines())

files = ["file1.txt", "file2.txt", "file3.txt"]

with open("newfile.txt", "wb") as file:
    w = csv.writer(file, dialect = "excel-tab")
    for current_file in files:
        w.writerows(doSomething(current_file))

Answer 2

I think your program crashes for several reasons:

1. open(..) is a function , so you cannot write:

    with open oldfile as file: 

2. a with statement for files is used to enforce closing of a file, so file.close() is actually not necessary.

3. .reverse() works inplace : it returns None , you can use reversed(..) for that.

You can fix it with:

files = ["file1.txt", "file2.txt", "file3.txt"]

def doSomething(oldfile):
    content = []
    with  as file:
        return 

with open("newfile.txt", "w") as file:
    w = csv.writer(file, dialect = "excel-tab")
    
        w.writerows(doSomething(oldfile))

I also used a for loop over the list, instead of the indices, since that is more " pythonic ". Furthermore a file is iterable over its rows. So one can use reversed(file) to obtain the lines of the file in reverse.