What is the difference between rw+ and r+

Question

I stumbled onto this stackoverflow question while doing some file IO: Confused by python file mode "w+"

• r for reading
• w for writing
• r+ opens for reading and writing (cannot truncate a file)
• w+ for writing and reading (can truncate a file)
• rb+ reading or writing a binary file
• wb+ writing a binary file
• a+ opens for appending

Note the r+ cannot truncate a file. So I was looking for something that could truncate the file after reading it which led me to another SO link: Python truncate lines as they are read

I saw that they used a different mode, rw+, which was not documented. From how it was used in the answer, I guessed it meant "open for reading, writing and truncating but do not truncate on open".

I later tested this mode and it seems that it was removed in Python 3, hence throwing a ValueError when used:

Python 2:

f = open("myfile.txt", "rw+")
text = f.read()
f.truncate(0)
f.close()

Python 3:

f = open("myfile.txt", "rw+")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: must have exactly one of create/read/write/append mode

However, I needed a file mode in Python 3 that can truncate as well as read but not truncate on open. So, after a bit more testing, I found that r+ can actually truncate in both Python 2 and 3.

Python 2:

f = open("myfile.txt", "r+")
text = f.read()
f.truncate(0)
f.seek(0, 0)
print f.read()
f.close()

Will print out nothing.

Python 3:

f = open("myfile.txt", "r+")
text = f.read()
f.truncate(0)
f.seek(0, 0)
print(f.read())
f.close()

Will also print out nothing.

My question is, if both r+ and rw+ can truncate, what are the differences between them in Python 2?

Solution

On Linux at least, there's no difference as far as I can tell. Here's a test script

f1 = open('f1', 'r+')
f2 = open('f2', 'rw+')
f3 = open('f3', 'w+')

and its corresponding OS syscalls (using strace); tested on python 2.7.9.

open("f1", O_RDWR|O_LARGEFILE)          = 3
open("f2", O_RDWR|O_LARGEFILE)          = 4
open("f3", O_RDWR|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 5

See http://man7.org/linux/man-pages/man2/open.2.html for more details on the file access and creation flags.

It's not accurate to say a file object opened with 'r+' cannot be used to truncate a file - it just doesn't do so at the point the file is opened.