PEP: 343 Title: Anonymous Block Redux and Generator Enhancements Version: $Revision$ Last-Modified: $Date$ Author: Guido van Rossum Status: Accepted Type: Standards Track Content-Type: text/plain Created: 13-May-2005 Post-History: 2-Jun-2005 Introduction After a lot of discussion about PEP 340 and alternatives, I decided to withdraw PEP 340 and proposed a slight variant on PEP 310. After more discussion, I have added back a mechanism for raising an exception in a suspended generator using a throw() method, and a close() method which throws a new GeneratorExit exception; these additions were first proposed on python-dev in [2] and universally approved of. I'm also changing the keyword to 'with'. On-line discussion of this PEP should take place in the Python Wiki [3]. If this PEP is approved, the following PEPs will be rejected due to overlap: - PEP 310, Reliable Acquisition/Release Pairs. This is the original with-statement proposal. - PEP 319, Python Synchronize/Asynchronize Block. Its use cases can be covered by the current PEP by providing suitable with-statement controllers: for 'synchronize' we can use the "locking" template from example 1; for 'asynchronize' we can use a similar "unlocking" template. I don't think having an "anonymous" lock associated with a code block is all that important; in fact it may be better to always be explicit about the mutex being used. (PEP 340 and PEP 346 have already been withdrawn.) Motivation and Summary PEP 340, Anonymous Block Statements, combined many powerful ideas: using generators as block templates, adding exception handling and finalization to generators, and more. Besides praise it received a lot of opposition from people who didn't like the fact that it was, under the covers, a (potential) looping construct. This meant that break and continue in a block-statement would break or continue the block-statement, even if it was used as a non-looping resource management tool. But the final blow came when I read Raymond Chen's rant about flow-control macros[1]. Raymond argues convincingly that hiding flow control in macros makes your code inscrutable, and I find that his argument applies to Python as well as to C. I realized that PEP 340 templates can hide all sorts of control flow; for example, its example 4 (auto_retry()) catches exceptions and repeats the block up to three times. However, the with-statement of PEP 310 does *not* hide control flow, in my view: while a finally-suite temporarily suspends the control flow, in the end, the control flow resumes as if the finally-suite wasn't there at all. Remember, PEP 310 proposes rougly this syntax (the "VAR =" part is optional): with VAR = EXPR: BLOCK which roughly translates into this: VAR = EXPR VAR.__enter__() try: BLOCK finally: VAR.__exit__() Now consider this example: with f = opening("/etc/passwd"): BLOCK1 BLOCK2 Here, just as if the first line was "if True" instead, we know that if BLOCK1 completes without an exception, BLOCK2 will be reached; and if BLOCK1 raises an exception or executes a non-local goto (a break, continue or return), BLOCK2 is *not* reached. The magic added by the with-statement at the end doesn't affect this. (You may ask, what if a bug in the __exit__() method causes an exception? Then all is lost -- but this is no worse than with other exceptions; the nature of exceptions is that they can happen *anywhere*, and you just have to live with that. Even if you write bug-free code, a KeyboardInterrupt exception can still cause it to exit between any two virtual machine opcodes.) This argument almost led me to endorse PEP 310, but I had one idea left from the PEP 340 euphoria that I wasn't ready to drop: using generators as "templates" for abstractions like acquiring and releasing a lock or opening and closing a file is a powerful idea, as can be seen by looking at the examples in that PEP. Inspired by a counter-proposal to PEP 340 by Phillip Eby I tried to create a decorator that would turn a suitable generator into an object with the necessary __enter__() and __exit__() methods. Here I ran into a snag: while it wasn't too hard for the locking example, it was impossible to do this for the opening example. The idea was to define the template like this: @contextmanager def opening(filename): f = open(filename) try: yield f finally: f.close() and used it like this: with f = opening(filename): ...read data from f... The problem is that in PEP 310, the result of calling EXPR is assigned directly to VAR, and then VAR's __exit__() method is called upon exit from BLOCK1. But here, VAR clearly needs to receive the opened file, and that would mean that __exit__() would have to be a method on the file. While this can be solved using a proxy class, this is awkward and made me realize that a slightly different translation would make writing the desired decorator a piece of cake: let VAR receive the result from calling the __enter__() method, and save the value of EXPR to call its __exit__() method later. Then the decorator can return an instance of a wrapper class whose __enter__() method calls the generator's next() method and returns whatever next() returns; the wrapper instance's __exit__() method calls next() again but expects it to raise StopIteration. (Details below in the section Optional Generator Decorator.) So now the final hurdle was that the PEP 310 syntax: with VAR = EXPR: BLOCK1 would be deceptive, since VAR does *not* receive the value of EXPR. Borrowing from PEP 340, it was an easy step to: with EXPR as VAR: BLOCK1 Additional discussion showed that people really liked being able to "see" the exception in the generator, even if it was only to log it; the generator is not allowed to yield another value, since the with-statement should not be usable as a loop (raising a different exception is marginally acceptable). To enable this, a new throw() method for generators is proposed, which takes one to three arguments representing an exception in the usual fashion (type, value, traceback) and raises it at the point where the generator is suspended. Once we have this, it is a small step to proposing another generator method, close(), which calls throw() with a special exception, GeneratorExit. This tells the generator to exit, and from there it's another small step to proposing that close() be called automatically when the generator is garbage-collected. Then, finally, we can allow a yield-statement inside a try-finally statement, since we can now guarantee that the finally-clause will (eventually) be executed. The usual cautions about finalization apply -- the process may be terminated abruptly without finalizing any objects, and objects may be kept alive forever by cycles or memory leaks in the application (as opposed to cycles or leaks in the Python implementation, which are taken care of by GC). Note that we're not guaranteeing that the finally-clause is executed immediately after the generator object becomes unused, even though this is how it will work in CPython. This is similar to auto-closing files: while a reference-counting implementation like CPython deallocates an object as soon as the last reference to it goes away, implementations that use other GC algorithms do not make the same guarantee. This applies to Jython, IronPython, and probably to Python running on Parrot. Use Cases See the Examples section near the end. Specification: The 'with' Statement A new statement is proposed with the syntax: with EXPR as VAR: BLOCK Here, 'with' and 'as' are new keywords; EXPR is an arbitrary expression (but not an expression-list) and VAR is a single assignment target. It can *not* be a comma-separated sequence of variables, but it *can* be a *parenthesized* comma-separated sequence of variables. (This restriction makes a future extension possible of the syntax to have multiple comma-separated resources, each with its own optional as-clause.) The "as VAR" part is optional. The translation of the above statement is: abc = EXPR exc = (None, None, None) VAR = abc.__enter__() try: try: BLOCK except: exc = sys.exc_info() raise finally: abc.__exit__(*exc) Here, the variables 'abc' and 'exc' are internal variables and not accessible to the user; they will most likely be implemented as special registers or stack positions. If the "as VAR" part of the syntax is omitted, the "VAR =" part of the translation is omitted (but abc.__enter__() is still called). The calling convention for abc.__exit__() is as follows. If the finally-suite was reached through normal completion of BLOCK or through a non-local goto (a break, continue or return statement in BLOCK), abc.__exit__() is called with three None arguments. If the finally-suite was reached through an exception raised in BLOCK, abc.__exit__() is called with three arguments representing the exception type, value, and traceback. The motivation for this API to __exit__(), as opposed to the argument-less __exit__() from PEP 310, was given by the transactional() use case, example 3 below. The template in that example must commit or roll back the transaction depending on whether an exception occurred or not. Rather than just having a boolean flag indicating whether an exception occurred, we pass the complete exception information, for the benefit of an exception-logging facility for example. Relying on sys.exc_info() to get at the exception information was rejected; sys.exc_info() has very complex semantics and it is perfectly possible that it returns the exception information for an exception that was caught ages ago. It was also proposed to add an additional boolean to distinguish between reaching the end of BLOCK and a non-local goto. This was rejected as too complex and unnecessary; a non-local goto should be considered unexceptional for the purposes of a database transaction roll-back decision. Generator Decorator If PEP 342 is accepted, it will be possible to write a decorator that makes it possible to use a generator that yields exactly once to control a with-statement. Here's a sketch of such a decorator: class ContextWrapper(object): def __init__(self, gen): self.gen = gen def __enter__(self): try: return self.gen.next() except StopIteration: raise RuntimeError("generator didn't yield") def __exit__(self, type, value, traceback): if type is None: try: self.gen.next() except StopIteration: return else: raise RuntimeError("generator didn't stop") else: try: self.gen.throw(type, value, traceback) except (type, StopIteration): return else: raise RuntimeError("generator caught exception") def contextmanager(func): def helper(*args, **kwds): return ContextWrapper(func(*args, **kwds)) return helper This decorator could be used as follows: @contextmanager def opening(filename): f = open(filename) # IOError is untouched by ContextWrapper try: yield f finally: f.close() # Ditto for errors here (however unlikely) A robust implementation of this decorator should be made part of the standard library, but not necessarily as a built-in function. (I'm not sure which exception it should raise for errors; RuntimeError is used above as an example only.) Optional Extensions It would be possible to endow certain objects, like files, sockets, and locks, with __enter__() and __exit__() methods so that instead of writing: with locking(myLock): BLOCK one could write simply: with myLock: BLOCK I think we should be careful with this; it could lead to mistakes like: f = open(filename) with f: BLOCK1 with f: BLOCK2 which does not do what one might think (f is closed before BLOCK2 is entered). OTOH such mistakes are easily diagnosed; for example, the contextmanager decorator above raises RuntimeError when the second with-statement calls f.__enter__() again. Resolved Open Issues Discussion on python-dev revealed some open issues. I list them here, with my preferred resolution and its motivation. The PEP has been accepted without these being challenged, so the issues are now resolved. 1. The __exit__() method of the contextmanager decorator class catches StopIteration and considers it equivalent to re-raising the exception passed to throw(). Is allowing StopIteration right here? This is so that a generator doing cleanup depending on the exception thrown (like the transactional() example below) can *catch* the exception thrown if it wants to and doesn't have to worry about re-raising it. I find this more convenient for the generator writer. Against this was brought in that the generator *appears* to suppress an exception that it cannot suppress: the transactional() example would be more clear according to this view if it re-raised the original exception after the call to db.rollback(). I personally would find the requirement to re-raise the exception an annoyance in a generator used as a with-template, since all the code after yield is used for is cleanup, and it is invoked from a finally-clause (the one implicit in the with-statement) which re-raises the original exception anyway. Examples (Note: several of these examples contain "yield None". If PEP 342 is accepted, these can be changed to just "yield".) 1. A template for ensuring that a lock, acquired at the start of a block, is released when the block is left: @contextmanager def locking(lock): lock.acquire() try: yield None finally: lock.release() Used as follows: with locking(myLock): # Code here executes with myLock held. The lock is # guaranteed to be released when the block is left (even # if via return or by an uncaught exception). PEP 319 gives a use case for also having an unlocking() template; this can be written very similarly (just swap the acquire() and release() calls). 2. A template for opening a file that ensures the file is closed when the block is left: @contextmanager def opening(filename, mode="r"): f = open(filename, mode) try: yield f finally: f.close() Used as follows: with opening("/etc/passwd") as f: for line in f: print line.rstrip() 3. A template for committing or rolling back a database transaction: @contextmanager def transactional(db): db.begin() try: yield None except: db.rollback() else: db.commit() 4. Example 1 rewritten without a generator: class locking: def __init__(self, lock): self.lock = lock def __enter__(self): self.lock.acquire() def __exit__(self, type, value, tb): self.lock.release() (This example is easily modified to implement the other examples; it shows the relative advantage of using a generator template.) 5. Redirect stdout temporarily: @contextmanager def redirecting_stdout(new_stdout): save_stdout = sys.stdout sys.stdout = new_stdout try: yield None finally: sys.stdout = save_stdout Used as follows: with opening(filename, "w") as f: with redirecting_stdout(f): print "Hello world" This isn't thread-safe, of course, but neither is doing this same dance manually. In single-threaded programs (for example, in scripts) it is a popular way of doing things. 6. A variant on opening() that also returns an error condition: @contextmanager def opening_w_error(filename, mode="r"): try: f = open(filename, mode) except IOError, err: yield None, err else: try: yield f, None finally: f.close() Used as follows: with opening_w_error("/etc/passwd", "a") as (f, err): if err: print "IOError:", err else: f.write("guido::0:0::/:/bin/sh\n") 7. Another useful example would be an operation that blocks signals. The use could be like this: import signal with signal.blocking(): # code executed without worrying about signals An optional argument might be a list of signals to be blocked; by default all signals are blocked. The implementation is left as an exercise to the reader. 8. Another use for this feature is the Decimal context. Here's a simple example, after one posted by Michael Chermside: import decimal @contextmanager def extra_precision(places=2): c = decimal.getcontext() saved_prec = c.prec c.prec += places try: yield None finally: c.prec = saved_prec Sample usage (adapted from the Python Library Reference): def sin(x): "Return the sine of x as measured in radians." with extra_precision(): i, lasts, s, fact, num, sign = 1, 0, x, 1, x, 1 while s != lasts: lasts = s i += 2 fact *= i * (i-1) num *= x * x sign *= -1 s += num / fact * sign # The "+s" rounds back to the original precision, # so this must be outside the with-statement: return +s 9. Here's a more general Decimal-context-switching template: @contextmanager def decimal_context(newctx=None): oldctx = decimal.getcontext() if newctx is None: newctx = oldctx.copy() decimal.setcontext(newctx) try: yield newctx finally: decimal.setcontext(oldctx) Sample usage: def sin(x): with decimal_context() as ctx: ctx.prec += 2 # Rest of algorithm the same as above return +s (Nick Coghlan has proposed to add __enter__() and __exit__() methods to the decimal.Context class so that this example can be simplified to "with decimal.getcontext() as ctx: ...".) 10. A generic "object-closing" template: @contextmanager def closing(obj): try: yield obj finally: obj.close() This can be used to deterministically close anything with a close method, be it file, generator, or something else: # emulate opening(): with closing(open("argument.txt")) as contradiction: for line in contradiction: print line # deterministically finalize a generator: with closing(some_gen()) as data: for datum in data: process(datum) References [1] http://blogs.msdn.com/oldnewthing/archive/2005/01/06/347666.aspx [2] http://mail.python.org/pipermail/python-dev/2005-May/053885.html [3] http://wiki.python.org/moin/WithStatement Copyright This document has been placed in the public domain.