open() in Python does not create a file if it doesn't exist


Question

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.

UPDATE: 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.

1
576
8/23/2016 8:14:03 AM

Accepted Answer

You should use open with the w+ mode:

file = open('myfile.dat', 'w+')
726
6/29/2018 8:32:57 PM

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.


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