<s>[INST] Implement touch using Python? touch is a Unix utility that sets the modification and access times of files to the current time of day.  If the file doesn't exist, it is created with default permissions.
How would you implement it as a Python function? Try to be cross platform and complete.
(Current Google results for "python touch file" are not that great, but point to os.utime.)
 [/INST] This tries to be a little more race-free than the other solutions.  (The with keyword is new in Python 2.5.)
import os
def touch(fname, times=None):
    with open(fname, 'a'):
        os.utime(fname, times)

Roughly equivalent to this.
import os
def touch(fname, times=None):
    fhandle = open(fname, 'a')
    try:
        os.utime(fname, times)
    finally:
        fhandle.close()

Now, to really make it race-free, you need to use futimes and change the timestamp of the open filehandle, instead of opening the file and then changing the timestamp on the filename (which may have been renamed).  Unfortunately, Python doesn't seem to provide a way to call futimes without going through ctypes or similar...

EDIT
As noted by Nate Parsons, Python 3.3 will add specifying a file descriptor (when os.supports_fd) to functions such as os.utime, which will use the futimes syscall instead of the utimes syscall under the hood.  In other words:
import os
def touch(fname, mode=0o666, dir_fd=None, **kwargs):
    flags = os.O_CREAT | os.O_APPEND
    with os.fdopen(os.open(fname, flags=flags, mode=mode, dir_fd=dir_fd)) as f:
        os.utime(f.fileno() if os.utime in os.supports_fd else fname,
            dir_fd=None if os.supports_fd else dir_fd, **kwargs)

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