How to make sure a file exists or can be created before writing to it in Python?

Question

I'm writing a function and I want it to touch a file so that I can write to that file. If the file doesn't exist, I will get an error. How can I say that?

• user166390: The open() documentation may be interesting to look at.
• Daenyth: What you have described is not the purpose of touch
• Fosco: touch will update the timestamp of an existing file, or create a new file if it doesn't exist.
• magnetar: @Daenyth sorry. What's the purpose then?
• exhuma: Very misleading title! As others have suggested, this is not what touch does. You might want to consider to rephrase this!

Answer 1

Just open the file for writing and it will be created if it doesn't exist (assuming you have proper permission to write to that location).

f = open('some_file_that_might_not_exist.txt', 'w')
f.write(data)

You will get an IOError if you can't open the file for writing.

• Peter Gibson: Note, this is the correct answer to the question detailed, but this is not a "Python equivalent of touch" (as the question title may imply)
• Chris Johnson: For those innocently looking for a Python implementation of touch, note the above is not comparable to unix touch because USING THE w OPTION WILL DELETE THE CONTENTS OF THE FILE IF IT ALREADY EXISTS. touch only changes timestamps if the file already exists. For a Python implementation of touch, see stackoverflow.com/questions/1158076/….

Answer 2

Per the docs, os.utime() will function similar to touch if you give it None as the time argument, for example:

os.utime("test_file", None)

When I tested this (on Linux and later Windows), I found that test_file had to already exist. YMMV on other OS's.

Of course, this doesn't really address writing to the file. As other answers have said, you usually want open for that and try ... except for catching exceptions when the file does not exist.

• Peter Gibson: Here is a version that addresses the "file must exist" issue stackoverflow.com/questions/1158076/…

Answer 3

if you actually want to raise an error if the file doesn't exist, you can use

import os
if not os.access('file'):
 #raise error
f = open('file')
#etc.

• magnetar: I will definitely eventually use this. Thank you.
• Brian: Note that the file might become inaccessible between the call to os.access and the call to open.
• Tim McNamara: @Brian, so generally it's better to just use try & except IOError?
• Sam Dolan: @Tim McNamara: It's always better, as it can always happen no matter how many checks you do beforehand.