PEP: 264
Title: Future statements in simulated shells
Author: Michael Hudson <mwh@python.net>
Status: Final
Type: Standards Track
Requires: 236
Created: 30-Jul-2001
Python-Version: 2.2
Post-History: 30-Jul-2001


Abstract
========

As noted in :pep:`236`, there is no clear way for "simulated
interactive shells" to simulate the behaviour of ``__future__``
statements in "real" interactive shells, i.e. have ``__future__``
statements' effects last the life of the shell.

The PEP also takes the opportunity to clean up the other
unresolved issue mentioned in :pep:`236`, the inability to stop
``compile()`` inheriting the effect of future statements affecting the
code calling ``compile()``.

This PEP proposes to address the first problem by adding an
optional fourth argument to the builtin function "compile", adding
information to the ``_Feature`` instances defined in ``__future__.py`` and
adding machinery to the standard library modules "codeop" and
"code" to make the construction of such shells easy.

The second problem is dealt with by simply adding *another*
optional argument to ``compile()``, which if non-zero suppresses the
inheriting of future statements' effects.


Specification
=============

I propose adding a fourth, optional, "flags" argument to the
builtin "compile" function.  If this argument is omitted,
there will be no change in behaviour from that of Python 2.1.

If it is present it is expected to be an integer, representing
various possible compile time options as a bitfield.  The
bitfields will have the same values as the ``CO_*`` flags already used
by the C part of Python interpreter to refer to future statements.

``compile()`` shall raise a ``ValueError`` exception if it does not
recognize any of the bits set in the supplied flags.

The flags supplied will be bitwise-"or"ed with the flags that
would be set anyway, unless the new fifth optional argument is a
non-zero integer, in which case the flags supplied will be exactly
the set used.

The above-mentioned flags are not currently exposed to Python.  I
propose adding ``.compiler_flag`` attributes to the ``_Feature`` objects
in ``__future__.py`` that contain the necessary bits, so one might
write code such as::

    import __future__
    def compile_generator(func_def):
        return compile(func_def, "<input>", "suite",
                    __future__.generators.compiler_flag)

A recent change means that these same bits can be used to tell if
a code object was compiled with a given feature; for instance ::

   codeob.co_flags & __future__.generators.compiler_flag``

will be non-zero if and only if the code object "codeob" was
compiled in an environment where generators were allowed.

I will also add a ``.all_feature_flags`` attribute to the ``__future__``
module, giving a low-effort way of enumerating all the ``__future__``
options supported by the running interpreter.

I also propose adding a pair of classes to the standard library
module codeop.

One - ``Compile`` - will sport a ``__call__`` method which will act much
like the builtin "compile" of 2.1 with the difference that after
it has compiled a ``__future__`` statement, it "remembers" it and
compiles all subsequent code with the ``__future__`` option in effect.

It will do this by using the new features of the ``__future__`` module
mentioned above.

Objects of the other class added to codeop - ``CommandCompiler`` -
will do the job of the existing ``codeop.compile_command`` function,
but in a ``__future__``-aware way.

Finally, I propose to modify the class ``InteractiveInterpreter`` in
the standard library module code to use a ``CommandCompiler`` to
emulate still more closely the behaviour of the default Python
shell.


Backward Compatibility
======================

Should be very few or none; the changes to compile will make no
difference to existing code, nor will adding new functions or
classes to codeop.  Existing code using
``code.InteractiveInterpreter`` may change in behaviour, but only for
the better in that the "real" Python shell will be being better
impersonated.


Forward Compatibility
=====================

The fiddling that needs to be done to ``Lib/__future__.py`` when
adding a ``__future__`` feature will be a touch more complicated.
Everything else should just work.


Issues
======

I hope the above interface is not too disruptive to implement for
Jython.


Implementation
==============

A series of preliminary implementations are at [1]_.

After light massaging by Tim Peters, they have now been checked in.


References
==========

.. [1] http://sourceforge.net/tracker/?func=detail&atid=305470&aid=449043&group_id=5470

Copyright
=========

This document has been placed in the public domain.
