How do I properly truncate a text file using Python?

Question

I want to truncate my file after reading the content of it, but it does not seem to do so and additions to the file are made after the existing content, instead of the beginning of a file.

My code is as follows:

from sys import argv
script, filename = argv
prompt= '??'

print("We're going to erase %r." %filename)
print("If you don't want that, hit CTRL-C.")
print("If you do want it, hit ENTER.")
input(prompt)

print("Opening the file...")
target = open(filename,'r+')
print(target.read())
print("I'm going to erase the file now!")


print("Truncating the file. Goodbye!")  
target.truncate()

print("Now I'm going to ask you 3 lines:")
line1 = input('Line 1: ')
line2 = input('Line 2: ')
line3 = input('Line 3: ')

print("I'm going to write these to the file now!")
target.write(line1)
target.write("\n")
target.write(line2)
target.write("\n")
target.write(line3)
target.write("\n")
print("And finally we close the file! Please check and see if the file 
    has been modified!")
target.close()

Doesn't .truncate remove everything after the current position? In your case, as you just .read everything, there's nothing to truncate. — jonrsharpe
– jonrsharpe, Commented Mar 2, 2018 at 8:12

cdarke · Accepted Answer · 2018-03-02 08:39:38Z

4

To truncate a file to zero bytes you can just open it with write access, no need to actually write anything. It can be done simply:

with open(filename, 'w'): pass

However, using your code you need to reset the current file position to beginning of file before the truncate:

....
print("Truncating the file. Goodbye!")  
target.seek(0)                            # <<< Add this line
target.truncate() 
....

Example run (script is gash.py):

$ echo -e 'x\ny\nz\n' > gash.txt
$ python3 gash.py gash.txt
We're going to erase 'gash.txt'.
If you don't want that, hit CTRL-C.
If you do want it, hit ENTER.
??
Opening the file...
x
y
z


I'm going to erase the file now!
Truncating the file. Goodbye!
Now I'm going to ask you 3 lines:
Line 1: one
Line 2: two
Line 3: three
I'm going to write these to the file now!
And finally we close the file! Please check and see if the file has been modified!
$ cat gash.txt
one
two
three
$

edited Mar 2, 2018 at 8:39

answered Mar 2, 2018 at 8:22

cdarke

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4 Comments

Rhagavan Over a year ago

It truncates the file now, but it doesn't print what was in the file initially after opening the file.

cdarke Over a year ago

Are you sure there was something in the file initially? It works for me. You should have only added that single target.seek(0) line, perhaps you added both methods?

Rhagavan Over a year ago

Yes. Checked to see that there was something in the file. That was the only line I added. Doesn't make sense to me too. :/

cdarke Over a year ago

Did you run the script to completion before checking the file contents? The file will appear to be empty until the buffers are flushed, usually when the file is closed.

Ankit Malik · Accepted Answer · 2018-03-02 13:29:41Z

0

To Truncate just write:

f = open('filename', 'w')
f.close()

edited Mar 2, 2018 at 13:29

answered Mar 2, 2018 at 8:12

Ankit Malik

909 bronze badges

2 Comments

cdarke Over a year ago

There is no need for the f.write() or the f.truncate(), the open with write access will truncate the file by itself.

Ankit Malik Over a year ago

Good Point @cdarke. Made the changes.

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