Abstract

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 can be neither guaranteed nor enforced.

This PEP proposes that the built-in generator type implement a close method and destruction semantics, such that the restriction on yield placement can be lifted, expanding the applicability of generators.

Pronouncement

Rejected in favor of PEP 342 which includes substantially all of the requested behavior in a more refined form.

Rationale

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 to 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 encapsulate 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:

# Today this is not valid Python: yield is not allowed between
# try and finally, and generator type instances support no
# close method.

def all_lines(index_path):
    index = file(index_path, "r")
    try:
        for path in index:
            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() # close on generator

Currently 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. Specifically, the close method behavior should trigger the execution of the finally clauses inside the generator, either by forcing a return in the generator frame or by throwing an exception in it. In situations requiring timely resource release, close could then be explicitly invoked.

The semantics of generator destruction on the other hand should be extended in order to implement a best-effort policy for the general case. Specifically, destruction should invoke close(). The best-effort limitation comes from the fact that the destructor’s execution is not guaranteed in the first place.

This seems to be a reasonable compromise, the resulting global behavior being similar to that of files and closing.

Possible Semantics

The built-in generator type should have a close method implemented, which can then be invoked as:

gen.close()

where gen is an instance of the built-in generator type. Generator destruction should also invoke close method behavior.

If a generator is already terminated, close should be a no-op.

Otherwise, there are two alternative solutions, Return or Exception Semantics:

A - Return Semantics: The generator should be resumed, generator execution should continue as if the instruction at the 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.

B - 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 seems 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.

Remarks

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()

Open Issues

Definitive semantics ought to be chosen. Currently Guido favors Exception Semantics. If the generator yields a value instead of terminating, or propagating back the special exception, a special exception should be raised again on the generator side.

It is still unclear whether spuriously converted special exceptions (as discussed in Possible Semantics) are a problem and what to do about them.

Implementation issues should be explored.

Alternative Ideas

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. The proposal in this PEP addresses more directly the problem of resource release. Were PEP 288 implemented, Exceptions Semantics for close could be layered on top of it, on the other hand PEP 288 should make a separate case for the more general functionality.

Copyright

This document has been placed in the public domain.