Writing to a file in a for loop only writes the last value

Question

text_file = open("new.txt", "r")
lines = text_file.readlines()

for line in lines:
        var1, var2 = line.split(",");
        myfile = open('xyz.txt', 'w')
        myfile.writelines(var1)
        myfile.close()

text_file.close()

I have 10 lines of text in new.txt like Adam:8154, George:5234, and so on. Now I want a text file which contains only the names. xyz.txt must contain Adam, George, and so on. The above code leaves me with the 10th name only.

How to have all the 10 names in a single text file?

pho · Accepted Answer · 2022-04-01 18:45:25Z

60

That is because you are opening , writing and closing the file 10 times inside your for loop. Opening a file in w mode erases whatever was in the file previously, so every time you open the file, the contents written to it in previous iterations get erased.

myfile = open('xyz.txt', 'w')
myfile.writelines(var1)
myfile.close()

You should open and close your file outside for loop.

myfile = open('xyz.txt', 'w')
for line in lines:
    var1, var2 = line.split(",");
    myfile.write("%s\n" % var1)
    
myfile.close()
text_file.close()

You should also notice to use write and not writelines.

writelines writes a list of lines to your file.

Also you should check out the answers posted by folks here that uses with statement. That is the elegant way to do file read/write operations in Python

edited Apr 1, 2022 at 18:45

pho

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answered Jun 25, 2012 at 23:16

pyfunc

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8 Comments

Ignacio Vazquez-Abrams Over a year ago

Or use with before the loop.

h4r5h4 Over a year ago

it didn't make any change to the result

pyfunc Over a year ago

@user1481274: I think you are using writelines

Levon Over a year ago

Won't you need the '\n' with your myfile.write(var1) statement?

pyfunc Over a year ago

@Levon: You are correct. I was not focusing on how but what he got wrong in his solution. file opening inside loop and incorrect usage of writeline. In fact usage of with is more elegant but that is another jump in understanding and did mention since solution was already provided for that :)

|

Levon · Accepted Answer · 2012-06-26 00:01:54Z

The main problem was that you were opening/closing files repeatedly inside your loop.

Try this approach:

with open('new.txt') as text_file, open('xyz.txt', 'w') as myfile:  
    for line in text_file:
        var1, var2 = line.split(",");
        myfile.write(var1+'\n')

We open both files at once and because we are using with they will be automatically closed when we are done (or an exception occurs). Previously your output file was repeatedly openend inside your loop.

We are also processing the file line-by-line, rather than reading all of it into memory at once (which can be a problem when you deal with really big files).

Note that write() doesn't append a newline ('\n') so you'll have to do that yourself if you need it (I replaced your writelines() with write() as you are writing a single item, not a list of items).

When opening a file for rread, the 'r' is optional since it's the default mode.

Kasun Siyambalapitiya · Accepted Answer · 2024-01-29 05:05:52Z

14

It's preferable to use context managers to close the files automatically

with open("new.txt", "r"), open('xyz.txt', 'w') as textfile, myfile:
    for line in textfile:
        var1, var2 = line.split(",");
        myfile.write(var1)

edited Jan 29, 2024 at 5:05

Kasun Siyambalapitiya

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answered Jun 25, 2012 at 23:19

John La Rooy

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3 Comments

Levon Over a year ago

I knew you could open multiple files with with (I do the same above) but I didn't know this format with the file variables at the end .. learned something new, neat.

andilabs Over a year ago

shouldn't be indeented for under with?

John La Rooy Over a year ago

@andilabs, Thanks. Sure should. Better late than never :)

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