mercurial/thirdparty/zope/interface/advice.py

##############################################################################
#
# Copyright (c) 2003 Zope Foundation and Contributors.
# All Rights Reserved.
#
# This software is subject to the provisions of the Zope Public License,
# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
# FOR A PARTICULAR PURPOSE.
#
##############################################################################
"""Class advice.

This module was adapted from 'protocols.advice', part of the Python
Enterprise Application Kit (PEAK).  Please notify the PEAK authors
([e-mail] and [e-mail]) if bugs are found or
Zope-specific changes are required, so that the PEAK version of this module
can be kept in sync.

PEAK is a Python application framework that interoperates with (but does
not require) Zope 3 and Twisted.  It provides tools for manipulating UML
models, object-relational persistence, aspect-oriented programming, and more.
Visit the PEAK home page at http://peak.telecommunity.com for more information.
"""

from __future__ import absolute_import

from types import FunctionType
try:
    from types import ClassType
except ImportError:
    __python3 = True
else:
    __python3 = False

import sys

def getFrameInfo(frame):
    """Return (kind,module,locals,globals) for a frame

    'kind' is one of "exec", "module", "class", "function call", or "unknown".
    """

    f_locals = frame.f_locals
    f_globals = frame.f_globals

    sameNamespace = f_locals is f_globals
    hasModule = '__module__' in f_locals
    hasName = '__name__' in f_globals

    sameName = hasModule and hasName
    sameName = sameName and f_globals['__name__']==f_locals['__module__']

    module = hasName and sys.modules.get(f_globals['__name__']) or None

    namespaceIsModule = module and module.__dict__ is f_globals

    if not namespaceIsModule:
        # some kind of funky exec
        kind = "exec"
    elif sameNamespace and not hasModule:
        kind = "module"
    elif sameName and not sameNamespace:
        kind = "class"
    elif not sameNamespace:
        kind = "function call"
    else: # pragma: no cover
        # How can you have f_locals is f_globals, and have '__module__' set?
        # This is probably module-level code, but with a '__module__' variable.
        kind = "unknown"
    return kind, module, f_locals, f_globals


def addClassAdvisor(callback, depth=2):
    """Set up 'callback' to be passed the containing class upon creation

    This function is designed to be called by an "advising" function executed
    in a class suite.  The "advising" function supplies a callback that it
    wishes to have executed when the containing class is created.  The
    callback will be given one argument: the newly created containing class.
    The return value of the callback will be used in place of the class, so
    the callback should return the input if it does not wish to replace the
    class.

    The optional 'depth' argument to this function determines the number of
    frames between this function and the targeted class suite.  'depth'
    defaults to 2, since this skips this function's frame and one calling
    function frame.  If you use this function from a function called directly
    in the class suite, the default will be correct, otherwise you will need
    to determine the correct depth yourself.

    This function works by installing a special class factory function in
    place of the '__metaclass__' of the containing class.  Therefore, only
    callbacks *after* the last '__metaclass__' assignment in the containing
    class will be executed.  Be sure that classes using "advising" functions
    declare any '__metaclass__' *first*, to ensure all callbacks are run."""
    # This entire approach is invalid under Py3K.  Don't even try to fix
    # the coverage for this block there. :(
    if __python3: # pragma: no cover
        raise TypeError('Class advice impossible in Python3')

    frame = sys._getframe(depth)
    kind, module, caller_locals, caller_globals = getFrameInfo(frame)

    # This causes a problem when zope interfaces are used from doctest.
    # In these cases, kind == "exec".
    #
    #if kind != "class":
    #    raise SyntaxError(
    #        "Advice must be in the body of a class statement"
    #    )

    previousMetaclass = caller_locals.get('__metaclass__')
    if __python3:   # pragma: no cover
        defaultMetaclass  = caller_globals.get('__metaclass__', type)
    else:
        defaultMetaclass  = caller_globals.get('__metaclass__', ClassType)


    def advise(name, bases, cdict):

        if '__metaclass__' in cdict:
            del cdict['__metaclass__']

        if previousMetaclass is None:
            if bases:
                # find best metaclass or use global __metaclass__ if no bases
                meta = determineMetaclass(bases)
            else:
                meta = defaultMetaclass

        elif isClassAdvisor(previousMetaclass):
            # special case: we can't compute the "true" metaclass here,
            # so we need to invoke the previous metaclass and let it
            # figure it out for us (and apply its own advice in the process)
            meta = previousMetaclass

        else:
            meta = determineMetaclass(bases, previousMetaclass)

        newClass = meta(name,bases,cdict)

        # this lets the callback replace the class completely, if it wants to
        return callback(newClass)

    # introspection data only, not used by inner function
    advise.previousMetaclass = previousMetaclass
    advise.callback = callback

    # install the advisor
    caller_locals['__metaclass__'] = advise


def isClassAdvisor(ob):
    """True if 'ob' is a class advisor function"""
    return isinstance(ob,FunctionType) and hasattr(ob,'previousMetaclass')


def determineMetaclass(bases, explicit_mc=None):
    """Determine metaclass from 1+ bases and optional explicit __metaclass__"""

    meta = [getattr(b,'__class__',type(b)) for b in bases]

    if explicit_mc is not None:
        # The explicit metaclass needs to be verified for compatibility
        # as well, and allowed to resolve the incompatible bases, if any
        meta.append(explicit_mc)

    if len(meta)==1:
        # easy case
        return meta[0]

    candidates = minimalBases(meta) # minimal set of metaclasses

    if not candidates: # pragma: no cover
        # they're all "classic" classes
        assert(not __python3) # This should not happen under Python 3
        return ClassType

    elif len(candidates)>1:
        # We could auto-combine, but for now we won't...
        raise TypeError("Incompatible metatypes",bases)

    # Just one, return it
    return candidates[0]


def minimalBases(classes):
    """Reduce a list of base classes to its ordered minimum equivalent"""

    if not __python3: # pragma: no cover
        classes = [c for c in classes if c is not ClassType]
    candidates = []

    for m in classes:
        for n in classes:
            if issubclass(n,m) and m is not n:
                break
        else:
            # m has no subclasses in 'classes'
            if m in candidates:
                candidates.remove(m)    # ensure that we're later in the list
            candidates.append(m)

    return candidates