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PEP:377
Title:Allow __enter__() methods to skip the statement body
Version:a2a1039f532c
Last-Modified:2009-03-21 08:40:19 +0000 (Sat, 21 Mar 2009)
Author:Nick Coghlan <ncoghlan at gmail.com>
Status:Rejected
Type:Standards Track
Content-Type:text/x-rst
Created:8-Mar-2009
Python-Version:2.7, 3.1
Post-History:8-Mar-2009

Abstract

This PEP proposes a backwards compatible mechanism that allows __enter__() methods to skip the body of the associated with statement. The lack of this ability currently means the contextlib.contextmanager decorator is unable to fulfil its specification of being able to turn arbitrary code into a context manager by moving it into a generator function with a yield in the appropriate location. One symptom of this is that contextlib.nested will currently raise RuntimeError in situations where writing out the corresponding nested with statements would not [1].

The proposed change is to introduce a new flow control exception SkipStatement, and skip the execution of the with statement body if __enter__() raises this exception.

PEP Rejection

This PEP was rejected by Guido [2] as it imposes too great an increase in complexity without a proportional increase in expressiveness and correctness. In the absence of compelling use cases that need the more complex semantics proposed by this PEP the existing behaviour is considered acceptable.

Proposed Change

The semantics of the with statement will be changed to include a new try/except/else block around the call to __enter__(). If SkipStatement is raised by the __enter__() method, then the main section of the with statement (now located in the else clause) will not be executed. To avoid leaving the names in any as clause unbound in this case, a new StatementSkipped singleton (similar to the existing NotImplemented singleton) will be assigned to all names that appear in the as clause.

The components of the with statement remain as described in PEP 343 [2]:

with EXPR as VAR:
    BLOCK

After the modification, the with statement semantics would be as follows:

mgr = (EXPR)
exit = mgr.__exit__  # Not calling it yet
try:
    value = mgr.__enter__()
except SkipStatement:
    VAR = StatementSkipped
    # Only if "as VAR" is present and
    # VAR is a single name
    # If VAR is a tuple of names, then StatementSkipped
    # will be assigned to each name in the tuple
else:
    exc = True
    try:
        try:
            VAR = value  # Only if "as VAR" is present
            BLOCK
        except:
            # The exceptional case is handled here
            exc = False
            if not exit(*sys.exc_info()):
                raise
            # The exception is swallowed if exit() returns true
    finally:
        # The normal and non-local-goto cases are handled here
        if exc:
            exit(None, None, None)

With the above change in place for the with statement semantics, contextlib.contextmanager() will then be modified to raise SkipStatement instead of RuntimeError when the underlying generator doesn't yield.

Rationale for Change

Currently, some apparently innocuous context managers may raise RuntimeError when executed. This occurs when the context manager's __enter__() method encounters a situation where the written out version of the code corresponding to the context manager would skip the code that is now the body of the with statement. Since the __enter__() method has no mechanism available to signal this to the interpreter, it is instead forced to raise an exception that not only skips the body of the with statement, but also jumps over all code until the nearest exception handler. This goes against one of the design goals of the with statement, which was to be able to factor out arbitrary common exception handling code into a single context manager by putting into a generator function and replacing the variant part of the code with a yield statement.

Specifically, the following examples behave differently if cmB().__enter__() raises an exception which cmA().__exit__() then handles and suppresses:

with cmA():
  with cmB():
    do_stuff()
# This will resume here without executing "do_stuff()"

@contextlib.contextmanager
def combined():
  with cmA():
    with cmB():
      yield

with combined():
  do_stuff()
# This will raise a RuntimeError complaining that the context
# manager's underlying generator didn't yield

with contextlib.nested(cmA(), cmB()):
  do_stuff()
# This will raise the same RuntimeError as the contextmanager()
# example (unsurprising, given that the nested() implementation
# uses contextmanager())

# The following class based version shows that the issue isn't
# specific to contextlib.contextmanager() (it also shows how
# much simpler it is to write context managers as generators
# instead of as classes!)
class CM(object):
  def __init__(self):
    self.cmA = None
    self.cmB = None

  def __enter__(self):
    if self.cmA is not None:
      raise RuntimeError("Can't re-use this CM")
    self.cmA = cmA()
    self.cmA.__enter__()
    try:
      self.cmB = cmB()
      self.cmB.__enter__()
    except:
      self.cmA.__exit__(*sys.exc_info())
      # Can't suppress in __enter__(), so must raise
      raise

  def __exit__(self, *args):
    suppress = False
    try:
      if self.cmB is not None:
        suppress = self.cmB.__exit__(*args)
    except:
      suppress = self.cmA.__exit__(*sys.exc_info()):
      if not suppress:
        # Exception has changed, so reraise explicitly
        raise
    else:
      if suppress:
        # cmB already suppressed the exception,
        # so don't pass it to cmA
        suppress = self.cmA.__exit__(None, None, None):
      else:
        suppress = self.cmA.__exit__(*args):
    return suppress

With the proposed semantic change in place, the contextlib based examples above would then "just work", but the class based version would need a small adjustment to take advantage of the new semantics:

class CM(object):
  def __init__(self):
    self.cmA = None
    self.cmB = None

  def __enter__(self):
    if self.cmA is not None:
      raise RuntimeError("Can't re-use this CM")
    self.cmA = cmA()
    self.cmA.__enter__()
    try:
      self.cmB = cmB()
      self.cmB.__enter__()
    except:
      if self.cmA.__exit__(*sys.exc_info()):
        # Suppress the exception, but don't run
        # the body of the with statement either
        raise SkipStatement
      raise

  def __exit__(self, *args):
    suppress = False
    try:
      if self.cmB is not None:
        suppress = self.cmB.__exit__(*args)
    except:
      suppress = self.cmA.__exit__(*sys.exc_info()):
      if not suppress:
        # Exception has changed, so reraise explicitly
        raise
    else:
      if suppress:
        # cmB already suppressed the exception,
        # so don't pass it to cmA
        suppress = self.cmA.__exit__(None, None, None):
      else:
        suppress = self.cmA.__exit__(*args):
    return suppress

There is currently a tentative suggestion [3] to add import-style syntax to the with statement to allow multiple context managers to be included in a single with statement without needing to use contextlib.nested. In that case the compiler has the option of simply emitting multiple with statements at the AST level, thus allowing the semantics of actual nested with statements to be reproduced accurately. However, such a change would highlight rather than alleviate the problem the current PEP aims to address: it would not be possible to use contextlib.contextmanager to reliably factor out such with statements, as they would exhibit exactly the same semantic differences as are seen with the combined() context manager in the above example.

Performance Impact

Implementing the new semantics makes it necessary to store the references to the __enter__ and __exit__ methods in temporary variables instead of on the stack. This results in a slight regression in with statement speed relative to Python 2.6/3.1. However, implementing a custom SETUP_WITH opcode would negate any differences between the two approaches (as well as dramatically improving speed by eliminating more than a dozen unnecessary trips around the eval loop).

Reference Implementation

Patch attached to Issue 5251 [1]. That patch uses only existing opcodes (i.e. no SETUP_WITH).

Acknowledgements

James William Pye both raised the issue and suggested the basic outline of the solution described in this PEP.

References

[1]Issue 5251: contextlib.nested inconsistent with nested with statements (http://bugs.python.org/issue5251)
[2]PEP 343: The "with" Statement (http://www.python.org/dev/peps/pep-0343/)
[3]Import-style syntax to reduce indentation of nested with statements (http://mail.python.org/pipermail/python-ideas/2009-March/003188.html)
[4]Guido's rejection of the PEP (http://mail.python.org/pipermail/python-dev/2009-March/087263.html)