this is something we discussed with Guido, and also Moshe Zadka at Europython.
Guido thought it seems reasonable enough, if the details can be nailed.
I have written it down so the idea doesn't get lost, for the moment is more a matter of whether it can get a number, and then it can go dormant for a while.
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PEP: XXX Title: Resource-Release Support for Generators Version: $Revision$ Last-Modified: $Date$ Author: Samuele Pedroni firstname.lastname@example.org Status: Draft Type: Standards Track Content-Type: text/plain Created: 25-Aug-2003 Python-Version: 2.4 Post-History:
Generators allow for natural coding and abstraction of traversal over data. Currently if external resources needing proper timely release are involved, generators are unfortunately not adequate. The typical idiom for timely release is not supported, a yield statement is not allowed in the try clause of a try-finally statement inside a generator. The finally clause execution cannot be either guaranteed or enforced.
This PEP proposes that generators support a close method and destruction semantics, such that the restriction can be lifted, expanding the applicability of generators.
Python generators allow for natural coding of many data traversal scenarios Their instantiation produces iterators, i.e. first-class objects abstracting traversal (with all the advantages of first- classness). In this respect they match in power and offer some advantages over the approach using iterator methods taking a (smalltalkish) block. On the other hand, given current limitations (no yield allowed in a try clause of a try-finally inside a generator) the latter approach seems better suited at encapsulating not only traversal but also exception handling and proper resource acquisition and release.
Let's consider an example (for simplicity, files in read-mode are used):
def all_lines(index_path): for path in file(index_path,"r"): for line in file(path.strip(),"r"): yield line
this is short and to the point, but the try-finally for timely closing of the files cannot be added. (While instead of a path, a file, whose closing then would be responsibility of the caller, could be passed in as argument, the same is not applicable for the files opened depending on the contents of the index).
If we want timely release, we have to sacrifice the simplicity and directness of the generator-only approach: (e.g.)
class AllLines: def __init__(self,index_path): self.index_path = index_path self.index = None self.document = None
def __iter__(self): self.index = file(self.index_path,"r") for path in self.index: self.document = file(path.strip(),"r") for line in self.document: yield line self.document.close() self.document = None
def close(self): if self.index: self.index.close() if self.document: self.document.close()
to be used as:
all_lines = AllLines("index.txt") try: for line in all_lines: ... finally: all_lines.close()
The more convoluted solution implementing timely release, seems to offer a precious hint. What we have done is encapsulating our traversal in an object (iterator) with a close method.
This PEP proposes that generators should grow such a close method with such semantics that the example could be rewritten as:
def all_lines(index_path): index = file(index_path,"r") try: for path in file(index_path,"r"): document = file(path.strip(),"r") try: for line in document: yield line finally: document.close() finally: index.close()
all = all_lines("index.txt") try: for line in all: ... finally: all.close()
PEP 255  disallows yield inside a try clause of a try-finally statement, because the execution of the finally clause cannot be guaranteed as required by try-finally semantics. The semantics of the proposed close method should be such, that while the finally clause execution still cannot be guaranteed, it can be enforced when required. The semantics of generator destruction on the other hand should be extended in order to implement a best-effort policy for the general case. This strikes as a reasonable compromise, the resulting global behavior being similar to that of files and closing.
A close() method should be implemented for generator objects.
1) If a generator is already terminated, close should be a no-op.
Otherwise: (two alternative solutions)
(Return Semantics) The generator should be resumed, generator execution should continue as if the instruction at re-entry point is a return. Consequently finally clauses surrounding the re-entry point would be executed, in the case of a then allowed try-yield-finally pattern.
Issues: is it important to be able to distinguish forced termination by close, normal termination, exception propagation from generator or generator-called code? In the normal case it seems not, finally clauses should be there to work the same in all these cases, still this semantics could make such a distinction hard.
Except-clauses, like by a normal return, are not executed, such clauses in legacy generators expect to be executed for exceptions raised by the generator or by code called from it. Not executing them in the close case seems correct.
OR (Exception Semantics) The generator should be resumed and execution should continue as if a special-purpose exception (e.g. CloseGenerator) has been raised at re-entry point. Close implementation should consume and not propagate further this exception.
Issues: should StopIteration be reused for this purpose? Probably not. We would like close to be a harmless operation for legacy generators, which could contain code catching StopIteration to deal with other generators/iterators.
In general, with exception semantics, it is unclear what to do if the generator does not terminate or we do not receive the special exception propagated back. Other different exceptions should probably be propagated, but consider this possible legacy generator code:
try: ... yield ... ... except: # or except Exception:, etc raise Exception("boom")
If close is invoked with the generator suspended after the yield, the except clause would catch our special purpose exception, so we would get a different exception propagated back, which in this case ought to be reasonably consumed and ignored but in general should be propagated, but separating these scenarios seem hard.
The exception approach has the advantage to let the generator distinguish between termination cases and have more control. On the other hand clear-cut semantics seem harder to define.
2) Generator destruction should invoke close method behavior.
If this proposal is accepted, it should become common practice to document whether a generator acquires resources, so that its close method ought to be called. If a generator is no longer used, calling close should be harmless.
On the other hand, in the typical scenario the code that instantiated the generator should call close if required by it, generic code dealing with iterators/generators instantiated elsewhere should typically not be littered with close calls.
The rare case of code that has acquired ownership of and need to properly deal with all of iterators, generators and generators acquiring resources that need timely release, is easily solved:
if hasattr(iterator,'close'): iterator.close()
Definitive semantics ought to be chosen, implementation issues should be explored.
The idea that the yield placement limitation should be removed and that generator destruction should trigger execution of finally clauses has been proposed more than once. Alone it cannot guarantee that timely release of resources acquired by a generator can be enforced.
PEP 288  proposes a more general solution, allowing custom exception passing to generators.
 PEP 255 Simple Generators http://www.python.org/peps/pep-0255.html
 PEP 288 Generators Attributes and Exceptions http://www.python.org/peps/pep-0288.html
This document has been placed in the public domain.
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