PEP: 264 Title: Future statements in simulated shells Version: $Revision$ Last-Modified: $Date$ Author: Michael Hudson Status: Final Type: Standards Track Content-Type: text/x-rst 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, "", "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. .. Local Variables: mode: indented-text indent-tabs-mode: nil End: