|Title:||Reliable Acquisition/Release Pairs|
|Last-Modified:||2005-06-28 08:31:09 +0000 (Tue, 28 Jun 2005)|
|Author:||Michael Hudson <mwh at python.net>, Paul Moore <gustav at morpheus.demon.co.uk>|
It would be nice to have a less typing-intense way of writing: the_lock.acquire() try: .... finally: the_lock.release() This PEP proposes a piece of syntax (a 'with' block) and a "small-i" interface that generalizes the above.
This PEP is rejected in favor of PEP 343.
One of the advantages of Python's exception handling philosophy is that it makes it harder to do the "wrong" thing (e.g. failing to check the return value of some system call). Currently, this does not apply to resource cleanup. The current syntax for acquisition and release of a resource (for example, a lock) is the_lock.acquire() try: .... finally: the_lock.release() This syntax separates the acquisition and release by a (possibly large) block of code, which makes it difficult to confirm "at a glance" that the code manages the resource correctly. Another common error is to code the "acquire" call within the try block, which incorrectly releases the lock if the acquire fails.
Basic Syntax and Semantics
The syntax of a 'with' statement is as follows:: 'with' [ var '=' ] expr ':' suite This statement is defined as being equivalent to the following sequence of statements: var = expr if hasattr(var, "__enter__"): var.__enter__() try: suite finally: var.__exit__() (The presence of an __exit__ method is *not* checked like that of __enter__ to ensure that using inappropriate objects in with: statements gives an error). If the variable is omitted, an unnamed object is allocated on the stack. In that case, the suite has no access to the unnamed object.
A number of potential extensions to the basic syntax have been discussed on the Python Developers list. None of these extensions are included in the solution proposed by this PEP. In many cases, the arguments are nearly equally strong in both directions. In such cases, the PEP has always chosen simplicity, simply because where extra power is needed, the existing try block is available. Multiple expressions One proposal was for allowing multiple expressions within one 'with' statement. The __enter__ methods would be called left to right, and the __exit__ methods right to left. The advantage of doing so is that where more than one resource is being managed, nested 'with' statements will result in code drifting towards the right margin. The solution to this problem is the same as for any other deep nesting - factor out some of the code into a separate function. Furthermore, the question of what happens if one of the __exit__ methods raises an exception (should the other __exit__ methods be called?) needs to be addressed. Exception handling An extension to the protocol to include an optional __except__ handler, which is called when an exception is raised, and which can handle or re-raise the exception, has been suggested. It is not at all clear that the semantics of this extension can be made precise and understandable. For example, should the equivalent code be try ... except ... else if an exception handler is defined, and try ... finally if not? How can this be determined at compile time, in general? The alternative is to define the code as expanding to a try ... except inside a try ... finally. But this may not do the right thing in real life. The only use case identified for exception handling is with transactional processing (commit on a clean finish, and rollback on an exception). This is probably just as easy to handle with a conventional try ... except ... else block, and so the PEP does not include any support for exception handlers.
There is a potential race condition in the code specified as equivalent to the with statement. For example, if a KeyboardInterrupt exception is raised between the completion of the __enter__ method call and the start of the try block, the __exit__ method will not be called. This can lead to resource leaks, or to deadlocks. [XXX Guido has stated that he cares about this sort of race condition, and intends to write some C magic to handle them. The implementation of the 'with' statement should copy this.]
Should existing classes (for example, file-like objects and locks) gain appropriate __enter__ and __exit__ methods? The obvious reason in favour is convenience (no adapter needed). The argument against is that if built-in files have this but (say) StringIO does not, then code that uses "with" on a file object can't be reused with a StringIO object. So __exit__ = close becomes a part of the "file-like object" protocol, which user-defined classes may need to support. The __enter__ hook may be unnecessary - for many use cases, an adapter class is needed and in that case, the work done by the __enter__ hook can just as easily be done in the __init__ hook. If a way of controlling object lifetimes explicitly was available, the function of the __exit__ hook could be taken over by the existing __del__ hook. An email exchange with a proponent of this approach left one of the authors even more convinced that it isn't the right idea... It has been suggested that the "__exit__" method be called "close", or that a "close" method should be considered if no __exit__ method is found, to increase the "out-of-the-box utility" of the "with ..." construct. There are some similarities in concept between 'with ...' blocks and generators, which have led to proposals that for loops could implement the with block functionality. While neat on some levels, we think that for loops should stick to being loops.
IEXEC: Holger Krekel -- generalised approach with XML-like syntax (no URL found...) Holger has much more far-reaching ideas about "execution monitors" that are informed about details of control flow in the monitored block. While interesting, these ideas could change the language in deep and subtle ways and as such belong to a different PEP. Any Smalltalk/Ruby anonymous block style extension obviously subsumes this one. PEP 319 is in the same area, but did not win support when aired on python-dev.
This PEP proposes a new keyword, so the __future__ game will need to be played.
Cost of Adoption
Those who claim the language is getting larger and more complicated have something else to complain about. It's something else to teach. For the proposal to be useful, many file-like and lock-like classes in the standard library and other code will have to have __exit__ = close or similar added.
Cost of Non-Adoption
Writing correct code continues to be more effort than writing incorrect code.
There are various python-list and python-dev discussions that could be mentioned here.  Off-list conversation between Michael Hudson and Bill Soudan (made public with permission) http://starship.python.net/crew/mwh/pep310/  Samuele Pedroni on python-dev http://mail.python.org/pipermail/python-dev/2003-August/037795.html  Thread on python-dev with subject [Python-Dev] pre-PEP: Resource-Release Support for Generators starting at http://mail.python.org/pipermail/python-dev/2003-August/037803.html
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