This project is based on the (most excellent) EmbeddedInterpreter and CurrencyConvBinding projects shipped with PyObjC 1.4. It has been run and tested on Mac OS X 10.4.11 with PyObjC 1.4 installed into Python 2.5.

To that I've added a calls called InterpreterKeyController. This class runs a thread that has an event tap, which can act upon key presses "out of band". In other words, it can evaluate key presses as they arrive in the process, even if the main thread's run loop is busy doing something else.

It could be busy running the Python interpreter. InterpreterKeyController's event tap looks for control-C keypresses, and safely interrupts the Python interpreter. It's important to get Python's Global Interpreter Lock when doing this to avoid corrupting the interpreter. The Python interpreter gives up the lock every 100 bytecodes, allowing other threads to run, so the GIL will become available before too long.

KeyboardInterrupt is always taken on the main thread in Python. Due to the way EmbeddedInterpreter is written, it runs on the main thread. It does, in fact, run the main thread's runloop while Python is blocking on input, and in between I/O operations to the console.

The end result is a Python console that can be embedded into another running program with some level of safety: If you run something from the console that goes into an infinite loop, just press control-C and it will be interrupted. There is no need to force quit the application, as was necessary with the stock EmbeddedInterpreter.

This can be demonstrated by typing:

>>> while True: pass

which then places the interpreter in an infinite loop. Pressing control-C results in:

Traceback (most recent call last):
File "<console>", line 1, in <module>

KeyboardInterrupt >>>

One thing to note is that everything in the running program is available in Python. For instance, the application delegate is automatically included into the interpreter. Since the currency conversion uses Cocoa bindings, executing a statement like

>>> appDelegate.dollarsToConvert=5

directly affects the model, with the appropriate text field changing in the UI, and the calculation is triggered as well.

It is my hope that others will find this useful for more safely adding a console as a diagnostic tool and to aid in experimentation and diagnosis during application development.

More information on Quartz Event services can be found at:

The PyObjC project can be found at:

Kevin A. Mitchell February 2008