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PEP 264 -- Future statements in simulated shells

PEP: 264
Title: Future statements in simulated shells
Author: Michael Hudson <mwh at 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.

Source: https://github.com/python/peps/blob/master/pep-0264.txt