[Python-Dev] pre-PEP: Resource-Release Support for Generators
pedronis at bluewin.ch
Tue Aug 26 16:36:46 EDT 2003
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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Title: Resource-Release Support for Generators
Author: Samuele Pedroni <pedronis at python.org>
Type: Standards Track
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
for path in file(index_path,"r"):
for line in file(path.strip(),"r"):
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.)
self.index_path = index_path
self.index = None
self.document = None
self.index = file(self.index_path,"r")
for path in self.index:
self.document = file(path.strip(),"r")
for line in self.document:
self.document = None
to be used as:
all_lines = AllLines("index.txt")
for line in all_lines:
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:
index = file(index_path,"r")
for path in file(index_path,"r"):
document = file(path.strip(),"r")
for line in document:
all = all_lines("index.txt")
for line in all:
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
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
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
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
except: # or except Exception:, etc
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:
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
 PEP 288 Generators Attributes and Exceptions
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