You shall always use sys.exit(exit_code)

return 0 will not be seen on system level.

Following (wrong) code:

if __name__ == "__main__":
    return 0

is wrong and complains on last line, that there is standalone return outside a function

Trying this will not complain, but will not be seen on system level:

def main():
    return 0

if __name__ == "__main__":
    main()

Correct is:

import sys

def main():
    sys.exit(0)

if __name__ == "__main__":
    main()

You could simply use

return

which does exactly the same as

return None

Your function will also return None if execution reaches the end of the function body without hitting a return statement. Returning nothing is the same as returning None in Python.

As you can read here, definition for contextmanager.__exit__ states:

"Exit the runtime context and return a Boolean flag indicating if any exception that occurred should be suppressed. (...) Returning a true value from this method will cause the with statement to suppress the exception and continue execution with the statement immediately following the with statement. Otherwise the exception continues propagating after this method has finished executing. Exceptions that occur during execution of this method will replace any exception that occurred in the body of the with statement.

The exception passed in should never be reraised explicitly - instead, this method should return a false value to indicate that the method completed successfully and does not want to suppress the raised exception. This allows context management code to easily detect whether or not an __exit__() method has actually failed."

So, if you only return False, it will be functionally the same as no explicit return at all and it is probably redundant. If you decide on adding cases with return True to suppress the exception, it might be helpful to add return False as well for clarity.

Your doing it in the past way is probably fine for quick& dirty scripts or for mini tools used by other developers. There’s often no need to be fancy. On the contrary, Python’s default traceback output can be an appropriate or even desired form of error reporting. A script falling off its end exits with code 0, an unhandled exception exits the script with a non-zero code, so you’re covered there, too:

def main() -> None:
    # here would be code raising exceptions on error
    # note the `None` return type

if __name__ == '__main__':
    main()

When you need something more fancy working with sys.exit() explicitly is a good idea. That’s basically your future approach. But don’t only think of scripts called directly, also think of installed packages. Setuptools has a cross-platform mechanism to define functions as entry points for scripts. If you have this in your setup.py:

setuptools.setup(
        ...
        entry_points={
            'console_scripts': ['your_script=your_package.your_module:main'],
    },
)

and install that package you can run your_script from the command line.

A console_scripts function must be callable without any arguments. So, as long as all parameters have default values you’re fine. For longer running scripts consider explicitly handling KeyboardInterrupt to print a nice message when the user aborts with Ctrl+C. In the end the relevant part of your your_package/your_module.py might look something like this:

def main(cli_args: List[str] = None) -> int:
    """
    `cli_args` makes it possible to call this function command-line-style
    from other Python code without touching sys.argv.
    """
    try:
        # Parsing with `argparse` and additional processing 
        # is usually lenghty enough to extract into separate functions.
        raw_config = _parse_cli(
            sys.argv[1:] if cli_args is None else cli_args)
        config = _validate_and_sanitize(raw_config)

        # Same exception raising idea as the simple approach
        do_real_work(config.foo, config.bar)

    except KeyboardInterrupt:
        print('Aborted manually.', file=sys.stderr)
        return 1

    except Exception as err:
        # (in real code the `except` would probably be less broad)
        # Turn exceptions into appropriate logs and/or console output.

        # non-zero return code to signal error
        # Can of course be more fine grained than this general
        # "something went wrong" code.
        return 1

    return 0  # success

# __main__ support is still here to make this file executable without
# installing the package first.
if __name__ == '__main__':
    sys.exit(main())

Yes, that return statement is redundant. Only when type is not None does the return value matter.

From the object.__exit__() documentation:

If an exception is supplied, and the method wishes to suppress the exception (i.e., prevent it from being propagated), it should return a true value. Otherwise, the exception will be processed normally upon exit from this method.

Note that a true value will suppress the exception; so 1 or "Handled!" would also work, not just True.

Removing that return line would result in None being returned instead, and the functionality would remain unchanged. Readability would be improved however, because that return type == None statement is just confusing on multiple levels (why not use type is None for example?).

0 and 1 are the exit codes.

exit(0) means a clean exit without any errors / problems

exit(1) means there was some issue / error / problem and that is why the program is exiting.

This is not Python specific and is pretty common. A non-zero exit code is treated as an abnormal exit, and at times, the error code indicates what the problem was. A zero error code means a successful exit.

This is useful for other programs, shell, caller etc. to know what happened with your program and proceed accordingly.

Using these magic methods (__enter__, __exit__) allows you to implement objects which can be used easily with the with statement.

The idea is that it makes it easy to build code which needs some 'cleandown' code executed (think of it as a try-finally block). Some more explanation here.

A useful example could be a database connection object (which then automagically closes the connection once the corresponding 'with'-statement goes out of scope):

class DatabaseConnection(object):

    def __enter__(self):
        # make a database connection and return it
        ...
        return self.dbconn

    def __exit__(self, exc_type, exc_val, exc_tb):
        # make sure the dbconnection gets closed
        self.dbconn.close()
        ...

As explained above, use this object with the with statement (you may need to do from __future__ import with_statement at the top of the file if you're on Python 2.5).

with DatabaseConnection() as mydbconn:
    # do stuff

PEP343 -- The 'with' statement' has a nice writeup as well.

Let me give some information on them:

1. quit() simply raises the SystemExit exception.

Furthermore, if you print it, it will give a message:

    >>> print (quit)
    Use quit() or Ctrl-Z plus Return to exit
    >>>

This functionality was included to help people who do not know Python. After all, one of the most likely things a newbie will try to exit Python is typing in quit.

Nevertheless, quit should not be used in production code. This is because it only works if the site module is loaded. Instead, this function should only be used in the interpreter.

2. exit() is an alias for quit (or vice-versa). They exist together simply to make Python more user-friendly.

Furthermore, it too gives a message when printed:

    >>> print (exit)
    Use exit() or Ctrl-Z plus Return to exit
    >>>

However, like quit, exit is considered bad to use in production code and should be reserved for use in the interpreter. This is because it too relies on the site module.

3. sys.exit() also raises the SystemExit exception. This means that it is the same as quit and exit in that respect.

Unlike those two however, sys.exit is considered good to use in production code. This is because the sys module will always be there.

4. os._exit() exits the program without calling cleanup handlers, flushing stdio buffers, etc. Thus, it is not a standard way to exit and should only be used in special cases. The most common of these is in the child process(es) created by os.fork.

Note that, of the four methods given, only this one is unique in what it does.

Summed up, all four methods exit the program. However, the first two are considered bad to use in production code and the last is a non-standard, dirty way that is only used in special scenarios. So, if you want to exit a program normally, go with the third method: sys.exit.

Or, even better in my opinion, you can just do directly what sys.exit does behind the scenes and run:

raise SystemExit

This way, you do not need to import sys first.

However, this choice is simply one on style and is purely up to you.