Summary Announcements

I lost Internet on the morning of June 18 and did not regain it until June 29. Since it was so close to the end of the month I decided to just combine the next summary into this month-long summary.

During my Internet blackout that reminded me of the days back when I spent my days playing video games and watching television, my future life as a graduate student began to form. Since I am going to have the joy of taking three upper-division undergraduate courses this upcoming fall semester I don't think I am going to have the time to do the summaries when school starts, at least in the capacity I am doing them currently.

At minimum I suspect I will have to cut back on the depth of them since I refuse to put the summaries before hacking on the core (hacking is more fun and I would like to think I can contribute more to Python that way). If you want this job I will be willing to give it up starting in September (I want a full year's worth of summaries behind me before I am willing to pass the torch). But if no one takes it I can probably continue to do them in a rather informal way by summarizing only big threads that I find interesting until someone steps forward to do a good, thorough job.

If you really want to do the summaries at the same level of depth as I am currently doing them and are interested in taking over starting in September, give me an email at brett@python.org .

Descriptor write-up [Draft: Please comment]

Related threads:

• Descriptor write-up [second draft]

Raymond Hettinger has written an paper on descriptors covering what they are to how to write your own. Since not only are properties class methods, static methods, and 'super' object descriptors (more on the last one later on in this summary) but descriptors are used throughout Python now to create bound objects for functions, methods, etc.

It would behoove all serious Python programmers to read http://users.rcn.com/python/download/Descriptor.htm .

Where to put the interrupt module?

Not earth-shattering, but to make sure that people know when new things are added to the stdlib I figured I should make this thread have a full-blown summary.

Thanks to the development work on IDLEfork, a new function in the thread module called interrupt_main has been added. It raises KeyboardInterrupt in the main thread of a program. The reason the exception is KeyboardInterrupt and not some new exception is because KeyboardInterrupt is the only asynchronous error in Python.

There was discussion of making it much more general, which led to the PyThreadState_SetAsyncState API that allows the raising of an exception in another thread.

Can't compile _tkinter.c with Redhat 9

Red Hat, apparently, modified their distribution of Tcl so as to support UCS-4 text encoding so that their distribution of Python 2.2 could be compiled with UCS-4 support. Problem is that this does not allow Python to compile with UCS-2 support; you have to compile with UCS-4 support and not UCS-2 under Red Hat 9 with their custom version of Tcl.

Thanks to Martin v. Löwis, 2.3 has been modified so that Python compiled in either UCS-4 or UCS-2 will work with standard Tcl which is UCS-2 natively.

PEP-317

Splintered threads:

• Exception masking/chaining

A discussion of PEP 317 broke out on python-dev. You can read the PEP for details but it advocated removing string exceptions (they are going to be gone in Python 3) and implicit exception instantiation (that is not changing). The PEP was eventually rejected and has the details of why it was rejected in the end.

Part of this discussion forked off to discuss exception masking. The idea came up that it might be nice for an exception to keep a reference to a previous exception that was still uncaught at that point. An example is an exception in the function passed to map; the function gets returned without knowing that it was in map. Masking it wouldn't work though since you then wouldn't know about the error in the function itself. But if there was a way to raise an exception from map that reference the original exception from the function then no information would be lost. Chaining the exceptions together would most likely just be sticking the previous exception in an attribute of the new one. This was all originally discussed in January and February of this year.

There was also discussion of adding the traceback for an exception as an attribute of the exception. This would make getting respective tracebacks easier period but also easier if exception chaining was implemented. The traceback would most likely be attached to the exception when it is caught and not at instantiation.

Both of these ideas are just that, ideas. They most likely won't occur until exceptions are moved over to new-style classes which probably won't be for a while.

towards a faster Python

Related threads:

• problem with assignment shadows builtin warning

A new warning has been added to Python for when you inject an attribute into a module to shadows a built-in:

import os
os.open = 42

This is in hopes of making this kind of thing illegal so as to allow the bytecode to be optimized for accessing built-ins. It also led to package imports, such as import A.B.C to work directly on the namespace dict instead of doing essentially setattr(A, 'B', A.B); setattr(A.B, 'C', A.B.C).

Sneaky 'super' instances

Splinter threads:

• PEP 246 and Protocols

Discovering that an instance to the 'super' type is both an instance of a class and a non-data descriptor was causing issues for pydoc and inspect led to the question of whether there was any definitive way to tell whether an object was an instance of a class defined by using the 'class' statement in Python. It turns out there is: if object.__class__.__flags__ >> 9 & 1 is 1 (which is the Py_TPFLAGS_HEAPTYPE in C) and its metaclass is or a subclass of type.

This then spawned a discussion about protocols and interfaces (which was discussed once way back when under the title type categories) since protocols could supposedly allow one to define a protocol for what makes up a 'super' instance. None of this will get into the language (if it ever gets in) until Guido can really think it through and that will be a while.

Details on Python shutting down

What happens while Python is shutting down? Well, it calls Py_Finalize in Python/pythonrun.c . This means that signals are turned off, gc.collect is called, calls PyImport_Cleanup in Python/import.c, and then calls. gc.collect one more time.

PyImport_Cleanup is where modules are torn down. __main__ is deleted, then everything but sys and __builtin__, and then sys and __builtin__ (in that order). Now "deletion" as mentioned he is setting the module to None in sys.modules, setting all names starting with a single underscore to None in the module, and then all others sans __builtins__ .

This is why when you define a __del__ method you need to make sure that all things referenced in it are local; global names in the module will have been set to None by the time the code is called if the object is collected during shutdown and raise an exception.

Re: [Python-checkins] python/dist/src/Objectslistobject.c,2.15

list.index now has optional start and end arguments.

RELEASED: Python 2.3b2

Related threads:

• 2.3b2 this weekend
• Python 2.3b2 branch tag created

Title says it all. Big thing with this beta release is the new version of IDLE. As with all betas, please download it and give it a whirl to try to shake out any bugs that need fixing ASAP; we are going to aim for an August 1 release date for final thanks to Apple planning to incorporate Python 2.3 into Panther.

Re: Activating `-i' from inside a script?

Related threads:

• Delayed `-i'! :-)

Thanks to Skip Montanaro and someone curious at comp.lang.python you can now set the PYTHONINSPECT environment variable to something and then be started into an interactive interpreter if an exception propagates all the way to the end of a program without being caught.

Quickies