When to use os.name, sys.platform, or platform.system?

Dived a bit into the source code.

The output of sys.platform and os.name are determined at compile time. platform.system() determines the system type at run time.

• sys.platform is specified as a compiler define during the build configuration.
• os.name checks whether certain os specific modules are available (e.g. posix, nt, ...)
• platform.system() actually runs uname and potentially several other functions to determine the system type at run time.

My suggestion:

• Use os.name to check whether it's a posix-compliant system.
• Use sys.platform to check whether it's a linux, cygwin, darwin, atheos, etc.
• Use platform.system() if you don't believe the other sources.

There is a thin line difference between platform.system() and sys.platform and interestingly for most cases platform.system() degenerates to sys.platform

Here is what the Source Python2.7\Lib\Platform.py\system says

def system():

    """ Returns the system/OS name, e.g. 'Linux', 'Windows' or 'Java'.

        An empty string is returned if the value cannot be determined.

    """
    return uname()[0]

def uname():
    # Get some infos from the builtin os.uname API...
    try:
        system,node,release,version,machine = os.uname()
    except AttributeError:
        no_os_uname = 1

    if no_os_uname or not filter(None, (system, node, release, version, machine)):
        # Hmm, no there is either no uname or uname has returned
        #'unknowns'... we'll have to poke around the system then.
        if no_os_uname:
            system = sys.platform
            release = ''
            version = ''
            node = _node()
            machine = ''

Also per the documentation

os.uname()

Return a 5-tuple containing information identifying the current operating system. The tuple contains 5 strings: (sysname, nodename, release, version, machine). Some systems truncate the nodename to 8 characters or to the leading component; a better way to get the hostname is socket.gethostname() or even socket.gethostbyaddr(socket.gethostname()).

Availability: recent flavors of Unix.

From sys.platform docs:

• os.name has a coarser granularity
• os.uname() gives system-dependent version information
• The platform module provides detailed checks for the system’s identity

Often the "best" future-proof way to test whether some functionality is available is just to try to use it and use a fallback if it fails.

what about the difference between sys.platform and platform.system()?

platform.system() returns a normalized value that it might get from several sources: os.uname(), sys.platform, ver command (on Windows).