python-peps/peps/pep-0340.rst

PEP: 340
Title: Anonymous Block Statements
Version: $Revision$
Last-Modified: $Date$
Author: Guido van Rossum
Status: Rejected
Type: Standards Track
Content-Type: text/x-rst
Created: 27-Apr-2005
Post-History:

Introduction
============

This PEP proposes a new type of compound statement which can be
used for resource management purposes.  The new statement type
is provisionally called the block-statement because the keyword
to be used has not yet been chosen.

This PEP competes with several other PEPs: :pep:`288` (Generators
Attributes and Exceptions; only the second part), :pep:`310`
(Reliable Acquisition/Release Pairs), and :pep:`325`
(Resource-Release Support for Generators).

I should clarify that using a generator to "drive" a block
statement is really a separable proposal; with just the definition
of the block statement from the PEP you could implement all the
examples using a class (similar to example 6, which is easily
turned into a template).  But the key idea is using a generator to
drive a block statement; the rest is elaboration, so I'd like to
keep these two parts together.

(:pep:`342`, Enhanced Iterators, was originally a part of this PEP;
but the two proposals are really independent and with Steven
Bethard's help I have moved it to a separate PEP.)

Rejection Notice
================

I am rejecting this PEP in favor of :pep:`343`.  See the motivational
section in that PEP for the reasoning behind this rejection.  GvR.

Motivation and Summary
======================

(Thanks to Shane Hathaway -- Hi Shane!)

Good programmers move commonly used code into reusable functions.
Sometimes, however, patterns arise in the structure of the
functions rather than the actual sequence of statements.  For
example, many functions acquire a lock, execute some code specific
to that function, and unconditionally release the lock.  Repeating
the locking code in every function that uses it is error prone and
makes refactoring difficult.

Block statements provide a mechanism for encapsulating patterns of
structure.  Code inside the block statement runs under the control
of an object called a block iterator.  Simple block iterators
execute code before and after the code inside the block statement.
Block iterators also have the opportunity to execute the
controlled code more than once (or not at all), catch exceptions,
or receive data from the body of the block statement.

A convenient way to write block iterators is to write a generator
(:pep:`255`).  A generator looks a lot like a Python function, but
instead of returning a value immediately, generators pause their
execution at "yield" statements.  When a generator is used as a
block iterator, the yield statement tells the Python interpreter
to suspend the block iterator, execute the block statement body,
and resume the block iterator when the body has executed.

The Python interpreter behaves as follows when it encounters a
block statement based on a generator.  First, the interpreter
instantiates the generator and begins executing it.  The generator
does setup work appropriate to the pattern it encapsulates, such
as acquiring a lock, opening a file, starting a database
transaction, or starting a loop.  Then the generator yields
execution to the body of the block statement using a yield
statement.  When the block statement body completes, raises an
uncaught exception, or sends data back to the generator using a
continue statement, the generator resumes.  At this point, the
generator can either clean up and stop or yield again, causing the
block statement body to execute again.  When the generator
finishes, the interpreter leaves the block statement.

Use Cases
=========

See the Examples section near the end.

Specification: the __exit__() Method
====================================

An optional new method for iterators is proposed, called
``__exit__()``.  It takes up to three arguments which correspond to
the three "arguments" to the raise-statement: type, value, and
traceback.  If all three arguments are ``None``, ``sys.exc_info()`` may be
consulted to provide suitable default values.

Specification: the Anonymous Block Statement
============================================

A new statement is proposed with the syntax::

    block EXPR1 as VAR1:
        BLOCK1

Here, 'block' and 'as' are new keywords; ``EXPR1`` is an arbitrary
expression (but not an expression-list) and ``VAR1`` is an arbitrary
assignment target (which may be a comma-separated list).

The "as VAR1" part is optional; if omitted, the assignments to
VAR1 in the translation below are omitted (but the expressions
assigned are still evaluated!).

The choice of the 'block' keyword is contentious; many
alternatives have been proposed, including not to use a keyword at
all (which I actually like).  :pep:`310` uses 'with' for similar
semantics, but I would like to reserve that for a with-statement
similar to the one found in Pascal and VB.  (Though I just found
that the C# designers don't like 'with' [2]_, and I have to agree
with their reasoning.)  To sidestep this issue momentarily I'm
using 'block' until we can agree on the right keyword, if any.

Note that the 'as' keyword is not contentious (it will finally be
elevated to proper keyword status).

Note that it is up to the iterator to decide whether a
block-statement represents a loop with multiple iterations; in the
most common use case ``BLOCK1`` is executed exactly once.  To the
parser, however, it is always a loop; break and continue return
transfer to the block's iterator (see below for details).

The translation is subtly different from a for-loop: ``iter()`` is
not called, so ``EXPR1`` should already be an iterator (not just an
iterable); and the iterator is guaranteed to be notified when
the block-statement is left, regardless if this is due to a
break, return or exception::

    itr = EXPR1  # The iterator
    ret = False  # True if a return statement is active
    val = None   # Return value, if ret == True
    exc = None   # sys.exc_info() tuple if an exception is active
    while True:
        try:
            if exc:
                ext = getattr(itr, "__exit__", None)
                if ext is not None:
                    VAR1 = ext(*exc)   # May re-raise *exc
                else:
                    raise exc[0], exc[1], exc[2]
            else:
                VAR1 = itr.next()  # May raise StopIteration
        except StopIteration:
            if ret:
                return val
            break
        try:
            ret = False
            val = exc = None
            BLOCK1
        except:
            exc = sys.exc_info()

(However, the variables 'itr' etc. are not user-visible and the
built-in names used cannot be overridden by the user.)

Inside ``BLOCK1``, the following special translations apply:

- "break" is always legal; it is translated into::

    exc = (StopIteration, None, None)
    continue

- "return EXPR3" is only legal when the block-statement is
  contained in a function definition; it is translated into::

    exc = (StopIteration, None, None)
    ret = True
    val = EXPR3
    continue

The net effect is that break and return behave much the same as
if the block-statement were a for-loop, except that the iterator
gets a chance at resource cleanup before the block-statement is
left, through the optional ``__exit__()`` method. The iterator also
gets a chance if the block-statement is left through raising an
exception.  If the iterator doesn't have an ``__exit__()`` method,
there is no difference with a for-loop (except that a for-loop
calls ``iter()`` on ``EXPR1``).

Note that a yield-statement in a block-statement is not treated
differently.  It suspends the function containing the block
**without** notifying the block's iterator.  The block's iterator is
entirely unaware of this yield, since the local control flow
doesn't actually leave the block.  In other words, it is **not**
like a break or return statement.  When the loop that was resumed
by the yield calls ``next()``, the block is resumed right after the
yield.  (See example 7 below.)  The generator finalization
semantics described below guarantee (within the limitations of all
finalization semantics) that the block will be resumed eventually.

Unlike the for-loop, the block-statement does not have an
else-clause.  I think it would be confusing, and emphasize the
"loopiness" of the block-statement, while I want to emphasize its
**difference** from a for-loop.  In addition, there are several
possible semantics for an else-clause, and only a very weak use
case.

Specification: Generator Exit Handling
======================================

Generators will implement the new ``__exit__()`` method API.

Generators will be allowed to have a ``yield`` statement inside a
try-finally statement.

The expression argument to the yield-statement will become
optional (defaulting to None).

When ``__exit__()`` is called, the generator is resumed but at the
point of the yield-statement the exception represented by the
``__exit__`` argument(s) is raised.  The generator may re-raise this
exception, raise another exception, or yield another value,
except that if the exception passed in to ``__exit__()`` was
StopIteration, it ought to raise StopIteration (otherwise the
effect would be that a break is turned into continue, which is
unexpected at least).  When the **initial** call resuming the
generator is an ``__exit__()`` call instead of a ``next()`` call, the
generator's execution is aborted and the exception is re-raised
without passing control to the generator's body.

When a generator that has not yet terminated is garbage-collected
(either through reference counting or by the cyclical garbage
collector), its ``__exit__()`` method is called once with
StopIteration as its first argument.  Together with the
requirement that a generator ought to raise StopIteration when
``__exit__()`` is called with StopIteration, this guarantees the
eventual activation of any finally-clauses that were active when
the generator was last suspended.  Of course, under certain
circumstances the generator may never be garbage-collected.  This
is no different than the guarantees that are made about finalizers
(``__del__()`` methods) of other objects.

Alternatives Considered and Rejected
====================================

- Many alternatives have been proposed for 'block'.  I haven't
  seen a proposal for another keyword that I like better than
  'block' yet.  Alas, 'block' is also not a good choice; it is a
  rather popular name for variables, arguments and methods.
  Perhaps 'with' is the best choice after all?

- Instead of trying to pick the ideal keyword, the block-statement
  could simply have the form::

    EXPR1 as VAR1:
        BLOCK1

  This is at first attractive because, together with a good choice
  of function names (like those in the Examples section below)
  used in ``EXPR1``, it reads well, and feels like a "user-defined
  statement".  And yet, it makes me (and many others)
  uncomfortable; without a keyword the syntax is very "bland",
  difficult to look up in a manual (remember that 'as' is
  optional), and it makes the meaning of break and continue in the
  block-statement even more confusing.

- Phillip Eby has proposed to have the block-statement use
  an entirely different API than the for-loop, to differentiate
  between the two.  A generator would have to be wrapped in a
  decorator to make it support the block API.  IMO this adds more
  complexity with very little benefit; and we can't really deny
  that the block-statement is conceptually a loop -- it supports
  break and continue, after all.

- This keeps getting proposed: "block VAR1 = EXPR1" instead of
  "block EXPR1 as VAR1".  That would be very misleading, since
  VAR1 does **not** get assigned the value of EXPR1; EXPR1 results
  in a generator which is assigned to an internal variable, and
  VAR1 is the value returned by successive calls to the ``__next__()``
  method of that iterator.

- Why not change the translation to apply ``iter(EXPR1)``?  All the
  examples would continue to work.  But this makes the
  block-statement **more** like a for-loop, while the emphasis ought
  to be on the **difference** between the two.  Not calling ``iter()``
  catches a bunch of misunderstandings, like using a sequence as
  ``EXPR1``.

Comparison to Thunks
====================

Alternative semantics proposed for the block-statement turn the
block into a thunk (an anonymous function that blends into the
containing scope).

The main advantage of thunks that I can see is that you can save
the thunk for later, like a callback for a button widget (the
thunk then becomes a closure).  You can't use a yield-based block
for that (except in Ruby, which uses yield syntax with a
thunk-based implementation).  But I have to say that I almost see
this as an advantage: I think I'd be slightly uncomfortable seeing
a block and not knowing whether it will be executed in the normal
control flow or later.  Defining an explicit nested function for
that purpose doesn't have this problem for me, because I already
know that the 'def' keyword means its body is executed later.

The other problem with thunks is that once we think of them as the
anonymous functions they are, we're pretty much forced to say that
a return statement in a thunk returns from the thunk rather than
from the containing function.  Doing it any other way would cause
major weirdness when the thunk were to survive its containing
function as a closure (perhaps continuations would help, but I'm
not about to go there :-).

But then an IMO important use case for the resource cleanup
template pattern is lost.  I routinely write code like this::

   def findSomething(self, key, default=None):
       self.lock.acquire()
       try:
            for item in self.elements:
                if item.matches(key):
                    return item
            return default
       finally:
          self.lock.release()

and I'd be bummed if I couldn't write this as::

   def findSomething(self, key, default=None):
       block locking(self.lock):
            for item in self.elements:
                if item.matches(key):
                    return item
            return default

This particular example can be rewritten using a break::

   def findSomething(self, key, default=None):
       block locking(self.lock):
            for item in self.elements:
                if item.matches(key):
                    break
            else:
                item = default
        return item

but it looks forced and the transformation isn't always that easy;
you'd be forced to rewrite your code in a single-return style
which feels too restrictive.

Also note the semantic conundrum of a yield in a thunk -- the only
reasonable interpretation is that this turns the thunk into a
generator!

Greg Ewing believes that thunks "would be a lot simpler, doing
just what is required without any jiggery pokery with exceptions
and break/continue/return statements.  It would be easy to explain
what it does and why it's useful."

But in order to obtain the required local variable sharing between
the thunk and the containing function, every local variable used
or set in the thunk would have to become a 'cell' (our mechanism
for sharing variables between nested scopes).  Cells slow down
access compared to regular local variables: access involves an
extra C function call (``PyCell_Get()`` or ``PyCell_Set()``).

Perhaps not entirely coincidentally, the last example above
(``findSomething()`` rewritten to avoid a return inside the block)
shows that, unlike for regular nested functions, we'll want
variables **assigned to** by the thunk also to be shared with the
containing function, even if they are not assigned to outside the
thunk.

Greg Ewing again: "generators have turned out to be more powerful,
because you can have more than one of them on the go at once. Is
there a use for that capability here?"

I believe there are definitely uses for this; several people have
already shown how to do asynchronous light-weight threads using
generators (e.g. David Mertz quoted in :pep:`288`, and Fredrik
Lundh [3]_).

And finally, Greg says: "a thunk implementation has the potential
to easily handle multiple block arguments, if a suitable syntax
could ever be devised. It's hard to see how that could be done in
a general way with the generator implementation."

However, the use cases for multiple blocks seem elusive.

(Proposals have since been made to change the implementation of
thunks to remove most of these objections, but the resulting
semantics are fairly complex to explain and to implement, so IMO
that defeats the purpose of using thunks in the first place.)

Examples
========

(Several of these examples contain "yield None".  If :pep:`342` is
accepted, these can be changed to just "yield" of course.)

1. A template for ensuring that a lock, acquired at the start of a
   block, is released when the block is left::

        def locking(lock):
            lock.acquire()
            try:
                yield None
            finally:
                lock.release()

   Used as follows::

        block 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).

2. A template for opening a file that ensures the file is closed
   when the block is left::

        def opening(filename, mode="r"):
            f = open(filename, mode)
            try:
                yield f
            finally:
                f.close()

   Used as follows::

        block opening("/etc/passwd") as f:
            for line in f:
                print line.rstrip()

3. A template for committing or rolling back a database
   transaction::

    def transactional(db):
        try:
            yield None
        except:
            db.rollback()
            raise
        else:
            db.commit()

4. A template that tries something up to n times::

        def auto_retry(n=3, exc=Exception):
            for i in range(n):
                try:
                    yield None
                    return
                except exc, err:
                    # perhaps log exception here
                    continue
            raise # re-raise the exception we caught earlier

   Used as follows::

        block auto_retry(3, IOError):
            f = urllib.urlopen("https://www.example.com/")
            print f.read()

5. It is possible to nest blocks and combine templates::

        def locking_opening(lock, filename, mode="r"):
            block locking(lock):
                block opening(filename) as f:
                    yield f

   Used as follows::

        block locking_opening(myLock, "/etc/passwd") as f:
            for line in f:
                print line.rstrip()

   (If this example confuses you, consider that it is equivalent
   to using a for-loop with a yield in its body in a regular
   generator which is invoking another iterator or generator
   recursively; see for example the source code for ``os.walk()``.)

6. It is possible to write a regular iterator with the
   semantics of example 1::

    class locking:
       def __init__(self, lock):
           self.lock = lock
           self.state = 0
       def __next__(self, arg=None):
           # ignores arg
           if self.state:
               assert self.state == 1
               self.lock.release()
               self.state += 1
               raise StopIteration
           else:
               self.lock.acquire()
               self.state += 1
               return None
       def __exit__(self, type, value=None, traceback=None):
           assert self.state in (0, 1, 2)
           if self.state == 1:
               self.lock.release()
           raise type, value, traceback

   (This example is easily modified to implement the other
   examples; it shows how much simpler generators are for the same
   purpose.)

7. Redirect stdout temporarily::

        def redirecting_stdout(new_stdout):
            save_stdout = sys.stdout
            try:
                sys.stdout = new_stdout
                yield None
            finally:
                sys.stdout = save_stdout

   Used as follows::

        block opening(filename, "w") as f:
            block redirecting_stdout(f):
                print "Hello world"

8. A variant on ``opening()`` that also returns an error condition::

        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::

        block opening_w_error("/etc/passwd", "a") as f, err:
            if err:
                print "IOError:", err
            else:
                f.write("guido::0:0::/:/bin/sh\n")

Acknowledgements
================

In no useful order: Alex Martelli, Barry Warsaw, Bob Ippolito,
Brett Cannon, Brian Sabbey, Chris Ryland, Doug Landauer, Duncan
Booth, Fredrik Lundh, Greg Ewing, Holger Krekel, Jason Diamond,
Jim Jewett, Josiah Carlson, Ka-Ping Yee, Michael Chermside,
Michael Hudson, Neil Schemenauer, Nick Coghlan, Paul Moore,
Phillip Eby, Raymond Hettinger, Georg Brandl, Samuele
Pedroni, Shannon Behrens, Skip Montanaro, Steven Bethard, Terry
Reedy, Tim Delaney, Aahz, and others.  Thanks all for the valuable
contributions!

References
==========

[1] https://mail.python.org/pipermail/python-dev/2005-April/052821.html

.. [2] https://web.archive.org/web/20060719195933/http://msdn.microsoft.com/vcsharp/programming/language/ask/withstatement/

.. [3] https://web.archive.org/web/20050204062901/http://effbot.org/zone/asyncore-generators.htm

Copyright
=========

This document has been placed in the public domain.