When a file that already exists is opened in append mode, the file’s existing contents are erased.

What is the best way to open a file as read/write if it exists, or if it does not, then create it and open it as read/write? From what I read, file = open('myfile.dat', 'rw') should do this, right?

It is not working for me (Python 2.6.2) and I'm wondering if it is a version problem, or not supposed to work like that or what.

The bottom line is, I just need a solution for the problem. I am curious about the other stuff, but all I need is a nice way to do the opening part.

The enclosing directory was writeable by user and group, not other (I'm on a Linux system... so permissions 775 in other words), and the exact error was:

IOError: no such file or directory.

Neuron

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asked Jun 3, 2010 at 15:05

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You should use open with the w+ mode:

file = open('myfile.dat', 'w+')

Igor Chubin

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answered Jun 3, 2010 at 15:12

muksiemuksie

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The advantage of the following approach is that the file is properly closed at the block's end, even if an exception is raised on the way. It's equivalent to try-finally, but much shorter.

with open("file.dat","a+") as f: f.write(...) ...

a+ Opens a file for both appending and reading. The file pointer is at the end of the file if the file exists. The file opens in the append mode. If the file does not exist, it creates a new file for reading and writing. -Python file modes

seek() method sets the file's current position.

f.seek(pos [, (0|1|2)]) pos .. position of the r/w pointer [] .. optionally () .. one of -> 0 .. absolute position 1 .. relative position to current 2 .. relative position from end

Only "rwab+" characters are allowed; there must be exactly one of "rwa" - see Stack Overflow question Python file modes detail.

answered Mar 12, 2013 at 11:06

QwertyQwerty

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''' w write mode r read mode a append mode w+ create file if it doesn't exist and open it in write mode r+ open for reading and writing. Does not create file. a+ create file if it doesn't exist and open it in append mode '''

example:

file_name = 'my_file.txt' f = open(file_name, 'w+') # open file in write mode f.write('python rules') f.close()

[FYI am using Python version 3.6.2]

bad_coder

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answered Dec 30, 2017 at 16:26

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Good practice is to use the following:

import os writepath = 'some/path/to/file.txt' mode = 'a' if os.path.exists(writepath) else 'w' with open(writepath, mode) as f: f.write('Hello, world!\n')

answered May 4, 2015 at 1:49

lollercoasterlollercoaster

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Change "rw" to "w+"

Or use 'a+' for appending (not erasing existing content)

answered Jun 3, 2010 at 15:12

baloobaloo

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Since python 3.4 you should use pathlib to "touch" files.
It is a much more elegant solution than the proposed ones in this thread.

from pathlib import Path filename = Path('myfile.txt') filename.touch(exist_ok=True) # will create file, if it exists will do nothing file = open(filename)

Same thing with directories:

filename.mkdir(parents=True, exist_ok=True)

answered Apr 23, 2018 at 6:29

GranitosaurusGranitosaurus

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>>> import os >>> if os.path.exists("myfile.dat"): ... f = file("myfile.dat", "r+") ... else: ... f = file("myfile.dat", "w")

r+ means read/write

answered Jun 3, 2010 at 15:18

KhorkrakKhorkrak

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My answer:

file_path = 'myfile.dat' try: fp = open(file_path) except IOError: # If not exists, create the file fp = open(file_path, 'w+')

answered May 27, 2014 at 6:20

Chien-Wei HuangChien-Wei Huang

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Use:

import os f_loc = r"C:\Users\Russell\Desktop\myfile.dat" # Create the file if it does not exist if not os.path.exists(f_loc): open(f_loc, 'w').close() # Open the file for appending and reading with open(f_loc, 'a+') as f: #Do stuff

Note: Files have to be closed after you open them, and the with context manager is a nice way of letting Python take care of this for you.

answered Feb 2, 2015 at 19:36

For Python 3+, I will do:

import os os.makedirs('path/to/the/directory', exist_ok=True) with open('path/to/the/directory/filename', 'w') as f: f.write(...)

So, the problem is with open cannot create a file before the target directory exists. We need to create it and then w mode is enough in this case.

answered Jan 30, 2021 at 0:27

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open('myfile.dat', 'a') works for me, just fine.

in py3k your code raises ValueError:

>>> open('myfile.dat', 'rw') Traceback (most recent call last): File "<pyshell#34>", line 1, in <module> open('myfile.dat', 'rw') ValueError: must have exactly one of read/write/append mode

in python-2.6 it raises IOError.

answered Jun 3, 2010 at 15:11

SilentGhostSilentGhost

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What do you want to do with file? Only writing to it or both read and write?

'w', 'a' will allow write and will create the file if it doesn't exist.

If you need to read from a file, the file has to be exist before open it. You can test its existence before opening it or use a try/except.

Neuron

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answered Jun 3, 2010 at 15:29

user49117user49117

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I think it's r+, not rw. I'm just a starter, and that's what I've seen in the documentation.

Neuron

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answered Jun 22, 2013 at 12:16

Put w+ for writing the file, truncating if it exist, r+ to read the file, creating one if it don't exist but not writing (and returning null) or a+ for creating a new file or appending to a existing one.

answered Oct 11, 2015 at 20:10

Gustavo6046Gustavo6046

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If you want to open it to read and write, I'm assuming you don't want to truncate it as you open it and you want to be able to read the file right after opening it. So this is the solution I'm using:

file = open('myfile.dat', 'a+') file.seek(0, 0)

answered Jan 12, 2018 at 13:26

So You want to write data to a file, but only if it doesn’t already exist?.

This problem is easily solved by using the little-known x mode to open() instead of the usual w mode. For example:

>>> with open('somefile', 'wt') as f: ... f.write('Hello\n') ... >>> with open('somefile', 'xt') as f: ... f.write('Hello\n') ... Traceback (most recent call last): File "<stdin>", line 1, in <module> FileExistsError: [Errno 17] File exists: 'somefile' >>>

If the file is binary mode, use mode xb instead of xt.

answered Dec 14, 2017 at 13:58

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import os, platform os.chdir('c:\\Users\\MS\\Desktop') try : file = open("Learn Python.txt","a") print('this file is exist') except: print('this file is not exist') file.write('\n''Hello Ashok') fhead = open('Learn Python.txt') for line in fhead: words = line.split() print(words)

answered Aug 8, 2018 at 5:45

When a file that already exists is opened in append mode?

What happens if you open an output file and the file already exists?

What will happen when a program opens a file in write mode if the file doesn't exist?

What type of file access jumps directly to any piece of data in the file without reading the data that came before it?