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1"""Registration facilities for DOM. This module should not be used
2directly. Instead, the functions getDOMImplementation and
3registerDOMImplementation should be imported from xml.dom."""
4
5# This is a list of well-known implementations.  Well-known names
6# should be published by posting to xml-sig@python.org, and are
7# subsequently recorded in this file.
8
9well_known_implementations = {
10    'minidom':'xml.dom.minidom',
11    '4DOM': 'xml.dom.DOMImplementation',
12    }
13
14# DOM implementations not officially registered should register
15# themselves with their
16
17registered = {}
18
19def registerDOMImplementation(name, factory):
20    """registerDOMImplementation(name, factory)
21
22    Register the factory function with the name. The factory function
23    should return an object which implements the DOMImplementation
24    interface. The factory function can either return the same object,
25    or a new one (e.g. if that implementation supports some
26    customization)."""
27
28    registered[name] = factory
29
30def _good_enough(dom, features):
31    "_good_enough(dom, features) -> Return 1 if the dom offers the features"
32    for f,v in features:
33        if not dom.hasFeature(f,v):
34            return 0
35    return 1
36
37def getDOMImplementation(name=None, features=()):
38    """getDOMImplementation(name = None, features = ()) -> DOM implementation.
39
40    Return a suitable DOM implementation. The name is either
41    well-known, the module name of a DOM implementation, or None. If
42    it is not None, imports the corresponding module and returns
43    DOMImplementation object if the import succeeds.
44
45    If name is not given, consider the available implementations to
46    find one with the required feature set. If no implementation can
47    be found, raise an ImportError. The features list must be a sequence
48    of (feature, version) pairs which are passed to hasFeature."""
49
50    import os
51    creator = None
52    mod = well_known_implementations.get(name)
53    if mod:
54        mod = __import__(mod, {}, {}, ['getDOMImplementation'])
55        return mod.getDOMImplementation()
56    elif name:
57        return registered[name]()
58    elif "PYTHON_DOM" in os.environ:
59        return getDOMImplementation(name = os.environ["PYTHON_DOM"])
60
61    # User did not specify a name, try implementations in arbitrary
62    # order, returning the one that has the required features
63    if isinstance(features, str):
64        features = _parse_feature_string(features)
65    for creator in registered.values():
66        dom = creator()
67        if _good_enough(dom, features):
68            return dom
69
70    for creator in well_known_implementations.keys():
71        try:
72            dom = getDOMImplementation(name = creator)
73        except Exception: # typically ImportError, or AttributeError
74            continue
75        if _good_enough(dom, features):
76            return dom
77
78    raise ImportError("no suitable DOM implementation found")
79
80def _parse_feature_string(s):
81    features = []
82    parts = s.split()
83    i = 0
84    length = len(parts)
85    while i < length:
86        feature = parts[i]
87        if feature[0] in "0123456789":
88            raise ValueError("bad feature name: %r" % (feature,))
89        i = i + 1
90        version = None
91        if i < length:
92            v = parts[i]
93            if v[0] in "0123456789":
94                i = i + 1
95                version = v
96        features.append((feature, version))
97    return tuple(features)
98