src/nmutil/noconflict.py

   1 import inspect, types
   2
   3 ############## preliminary: two utility functions #####################
   4
   5 def skip_redundant(iterable, skipset=None):
   6    "Redundant items are repeated items or items in the original skipset."
   7    if skipset is None: skipset = set()
   8    for item in iterable:
   9        if item not in skipset:
  10            skipset.add(item)
  11            yield item
  12
  13
  14 def remove_redundant(metaclasses):
  15    skipset = set([type])
  16    for meta in metaclasses: # determines the metaclasses to be skipped
  17        skipset.update(inspect.getmro(meta)[1:])
  18    return tuple(skip_redundant(metaclasses, skipset))
  19
  20 ##################################################################
  21 ## now the core of the module: two mutually recursive functions ##
  22 ##################################################################
  23
  24 memoized_metaclasses_map = {}
  25
  26 def get_noconflict_metaclass(bases, left_metas, right_metas):
  27     """Not intended to be used outside of this module, unless you know
  28     what you are doing."""
  29     # make tuple of needed metaclasses in specified priority order
  30     metas = left_metas + tuple(map(type, bases)) + right_metas
  31     needed_metas = remove_redundant(metas)
  32
  33     # return existing confict-solving meta, if any
  34     if needed_metas in memoized_metaclasses_map:
  35       return memoized_metaclasses_map[needed_metas]
  36     # nope: compute, memoize and return needed conflict-solving meta
  37     elif not needed_metas:         # wee, a trivial case, happy us
  38         meta = type
  39     elif len(needed_metas) == 1: # another trivial case
  40        meta = needed_metas[0]
  41     # check for recursion, can happen i.e. for Zope ExtensionClasses
  42     elif needed_metas == bases:
  43         raise TypeError("Incompatible root metatypes", needed_metas)
  44     else: # gotta work ...
  45         metaname = '_' + ''.join([m.__name__ for m in needed_metas])
  46         meta = classmaker()(metaname, needed_metas, {})
  47     memoized_metaclasses_map[needed_metas] = meta
  48     return meta
  49
  50 def classmaker(left_metas=(), right_metas=()):
  51     def make_class(name, bases, adict):
  52         print ("make_class", name)
  53         metaclass = get_noconflict_metaclass(bases, left_metas, right_metas)
  54         return metaclass(name, bases, adict)
  55     return make_class