PEP 310 – Reliable Acquisition/Release Pairs
- Reliable Acquisition/Release Pairs
- Michael Hudson <mwh at python.net>, Paul Moore <p.f.moore at gmail.com>
- Standards Track
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.
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
methods be called?) needs to be addressed.
An extension to the protocol to include an optional
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
try ... except ... else if an exception handler is
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
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
__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
Should existing classes (for example, file-like objects and locks)
__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)
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.
__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
If a way of controlling object lifetimes explicitly was available,
the function of the
__exit__ hook could be taken over by the
__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 https://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 https://mail.python.org/pipermail/python-dev/2003-August/037803.html
This document has been placed in the public domain.
Last modified: 2022-01-21 11:03:51 GMT