python - open file in "w" mode: IOError: [Errno 2] No such file or directory

Question

When I try to open a file in write mode with the following code:

packetFile = open("%s/%s/%s/%s.mol2" % ("dir", "dir2", "dir3", "some_file"), "w")

Gives me the following error:

IOError: [Errno 2] No such file or directory: 'dir/dir2/dir3/some_file.mol2'

The w mode should create the file if it doesn't exist, right? So how can this error ever occur?

Answer 1

You'll see this error if the directory containing the file you're trying to open does not exist, even when trying to open the file in w mode.

Since you're opening the file with a relative path, it's possible that you're confused about exactly what that directory is. Try putting a quick print to check:

import os

curpath = os.path.abspath(os.curdir)
packet_file = "%s/%s/%s/%s.mol2" % ("dir", "dir2", "dir3", "some_file")
print "Current path is: %s" % (curpath)
print "Trying to open: %s" % (os.path.join(curpath, packet_file))

packetFile = open(packet_file, "w")