# 15.8 Built-in Module `MacOS`

This module provides access to MacOS specific functionality in the python interpreter, such as how the interpreter eventloop functions and the like. Use with care.

Note the capitalisation of the module name, this is a historical artefact.

`Error`
  This exception is raised on MacOS generated errors, either from functions in this module or from other mac-specific modules like the toolbox interfaces. The arguments are the integer error code (the OSErr value) and a textual description of the error code. Symbolic names for all known error codes are defined in the standard module macerrors.

`SetEventHandler` (handler)
  In the inner interpreter loop Python will occasionally check for events, unless disabled with ScheduleParams. With this function you can pass a Python event-handler function that will be called if an event is available. The event is passed as parameter and the function should return non-zero if the event has been fully processed, otherwise event processing continues (by passing the event to the console window package, for instance).

  Call SetEventHandler without parameter to clear the event handler. Setting an eventhandler while one is already set is an error.

`SchedParams` ([doint, evtmask, besocial, interval, bgyield])
  Influence the interpreter inner loop event handling. Interval specifies how often (in seconds, floating point) the interpreter should enter the event processing code. When true, doint causes interrupt (command-dot) checking to be done. Evtmask tells the interpreter to do event processing for events in the mask (redraws, mouseclicks to switch to other applications, etc). The besocial flag gives other processes a chance to run. They are granted minimal runtime when Python is in the foreground and bgyield seconds per interval when Python runs in the background.

  All parameters are optional, and default to the current value. The return value of this function is a tuple with the old values of these options. Initial defaults are that all processing is enabled, checking is done every quarter second and the CPU is given up for a quarter second when in the background.

`HandleEvent` (ev)
  Pass the event record `ev` back to the python event loop, or possibly to the handler for the `sys.stdout` window (based on the compiler used to build python). This allows python programs that do their own event handling to still have some command-period and window-switching capability.

  If you attempt to call this function from an event handler set through `SetEventHandler` you will get an exception.

`GetErrorString` (errno)
  Return the textual description of MacOS error code errno.

`splash` (resid)
  This function will put a splash window on-screen, with the contents of the DLOG resource specified by `resid`. Calling with a zero argument will remove the splash screen. This function is useful if you want an applet to post a splash screen early in initialization without first having to load numerous extension modules.

`DebugStr` (message [, object])
  Drop to the low-level debugger with message message. The optional object argument is not used, but can easily be inspected from the debugger.

  Note that you should use this function with extreme care: if no low-level debugger like MacsBug is installed this call will crash your system. It is intended mainly for developers of Python extension modules.

`openrf` (name [, mode])
  Open the resource fork of a file. Arguments are the same as for the builtin function `open`. The object returned has file-like semantics, but it is not a python file object, so there may be subtle differences.