Add PEP 343: Anonymous Block Redux
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Guido van Rossum
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@ -120,6 +120,7 @@ Index by Category
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S 340 Anonymous Block Statements GvR
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S 341 Unifying try-except and try-finally Birkenfeld
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S 342 Enhanced Iterators GvR
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S 343 Anonymous Block Redux GvR
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S 754 IEEE 754 Floating Point Special Values Warnes
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Finished PEPs (done, implemented in CVS)
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@ -380,6 +381,7 @@ Numerical Index
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S 340 Anonymous Block Statements GvR
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S 341 Unifying try-except and try-finally Birkenfeld
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S 342 Enhanced Iterators GvR
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S 343 Anonymous Block Redux GvR
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SR 666 Reject Foolish Indentation Creighton
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S 754 IEEE 754 Floating Point Special Values Warnes
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I 3000 Python 3.0 Plans Kuchling, Cannon
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@ -0,0 +1,228 @@
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PEP: 343
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Title: Anonymous Block Redux
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Version: $Revision$
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Last-Modified: $Date$
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Author: Guido van Rossum
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Status: Draft
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Type: Standards Track
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Content-Type: text/plain
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Created: 13-May-2005
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Post-History:
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Introduction
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After a lot of discussion about PEP 340 and alternatives, I've
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decided to withdraw PEP 340 and propose a slight variant on PEP
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310.
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Motivation and Summary
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TBD.
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Use Cases
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See the Examples section near the end.
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Specification
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A new statement is proposed with the syntax
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do EXPR as VAR:
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BLOCK
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Here, 'do' and 'as' are new keywords; EXPR is an arbitrary
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expression (but not an expression-list) and VAR is an arbitrary
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assignment target (which may be a comma-separated list).
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The "as VAR" part is optional.
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The choice of the 'do' keyword is provisional; an alternative
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under consideration is 'with'.
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A yield-statement is illegal inside BLOCK. This is because the
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do-statement is translated into a try/finally statement, and yield
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is illegal in a try/finally statement.
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The translation of the above statement is:
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abc = EXPR
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try:
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VAR = abc.__enter__()
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BLOCK
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finally:
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abc.__exit__(*sys.exc_info()) # XXX See below
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If the "as VAR" part of the syntax is omitted, the "VAR =" part of
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the translation is omitted (but abc.__enter__() is still called).
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The call to abc.__exit__() is only approximated as written. The
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actual calling convention is: If the finally-suite was reached
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through normal completion of BLOCK or through a "non-local goto"
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(a break, continue or return statement in BLOCK), abc.__exit__()
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is called without arguments (or perhaps with three None
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arguments). If the finally-suite was reached through an exception
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raised in BLOCK, abc.__exit__() is called with three arguments
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representing the exception type, value, and traceback.
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Optional Generator Decorator
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It is possible to write a decorator that makes it possible to use
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a generator that yields exactly once to control a do-statement.
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Here's a sketch of such a decorator:
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class Wrapper(object):
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def __init__(self, gen):
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self.gen = gen
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self.state = "initial"
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def __enter__(self):
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assert self.state == "initial"
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self.state = "entered"
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try:
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return self.gen.next()
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except StopIteration:
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self.state = "error"
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raise RuntimeError("template generator didn't yield")
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def __exit__(self, *args):
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assert self.state == "entered"
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self.state = "exited"
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try:
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self.gen.next()
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except StopIteration:
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return
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else:
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self.state = "error"
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raise RuntimeError("template generator didn't stop")
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def do_template(func):
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def helper(*args, **kwds):
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return Wrapper(func(*args, **kwds))
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return helper
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This decorator could be used as follows:
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@do_template
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def opening(filename):
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f = open(filename) # IOError here is untouched by Wrapper
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yield f
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f.close() # Ditto for errors here (however unlikely)
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A robust implementation of such a decorator should be made part of
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the standard library.
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Examples
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Several of these examples contain "yield None". If PEP 342 is
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accepted, these can be changed to just "yield".
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1. A template for ensuring that a lock, acquired at the start of a
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block, is released when the block is left:
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@do_template
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def locking(lock):
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lock.acquire()
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yield None
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lock.release()
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Used as follows:
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do locking(myLock):
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# Code here executes with myLock held. The lock is
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# guaranteed to be released when the block is left (even
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# if via return or by an uncaught exception).
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2. A template for opening a file that ensures the file is closed
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when the block is left:
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@do_template
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def opening(filename, mode="r"):
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f = open(filename, mode)
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yield f
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f.close()
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Used as follows:
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do opening("/etc/passwd") as f:
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for line in f:
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print line.rstrip()
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3. A template for committing or rolling back a database
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transaction; this is written as a class rather than as a
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decorator since it requires access to the exception information:
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class transactional:
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def __init__(self, db):
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self.db = db
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def __enter__(self):
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pass
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def __exit__(self, *args):
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if args and args[0] is not None:
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self.db.rollback()
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else:
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self.db.commit()
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4. Example 1 rewritten without a generator:
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class locking:
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def __init__(self, lock):
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self.lock = lock
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def __enter__(self):
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self.lock.acquire()
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def __exit__(self, *args):
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self.lock.release()
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(This example is easily modified to implement the other
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examples; it shows how much simpler generators are for the same
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purpose.)
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5. Redirect stdout temporarily:
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@do_template
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def redirecting_stdout(new_stdout):
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save_stdout = sys.stdout
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try:
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sys.stdout = new_stdout
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yield None
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finally:
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sys.stdout = save_stdout
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Used as follows:
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do opening(filename, "w") as f:
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do redirecting_stdout(f):
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print "Hello world"
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6. A variant on opening() that also returns an error condition:
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@do_template
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def opening_w_error(filename, mode="r"):
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try:
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f = open(filename, mode)
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except IOError, err:
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yield None, err
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else:
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yield f, None
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f.close()
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Used as follows:
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do opening_w_error("/etc/passwd", "a") as f, err:
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if err:
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print "IOError:", err
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else:
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f.write("guido::0:0::/:/bin/sh\n")
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7. Another useful example would be an operation that blocks
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signals. The use could be like this:
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import signal
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do signal.blocking():
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# code executed without worrying about signals
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An optional argument might be a list of signals to be blocked;
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by default all signals are blocked. The implementation is left
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as an exercise to the reader.
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Copyright
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This document has been placed in the public domain.
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