shared.py

# encoding: utf-8

"""
Objects shared by opc modules.
"""

from __future__ import absolute_import, print_function, unicode_literals


class CaseInsensitiveDict(dict):
    """
    Mapping type that behaves like dict except that it matches without respect
    to the case of the key. E.g. cid['A'] == cid['a']. Note this is not
    general-purpose, just complete enough to satisfy opc package needs. It
    assumes str keys, and that it is created empty; keys passed in constructor
    are not accounted for
    """
    def __contains__(self, key):
        return super(CaseInsensitiveDict, self).__contains__(key.lower())

    def __getitem__(self, key):
        return super(CaseInsensitiveDict, self).__getitem__(key.lower())

    def __setitem__(self, key, value):
        return super(CaseInsensitiveDict, self).__setitem__(
            key.lower(), value
        )


def lazyproperty(f):
    """
    @lazyprop decorator. Decorated method will be called only on first access
    to calculate a cached property value. After that, the cached value is
    returned.
    """
    cache_attr_name = '_%s' % f.__name__  # like '_foobar' for prop 'foobar'
    docstring = f.__doc__

    def get_prop_value(obj):
        try:
            return getattr(obj, cache_attr_name)
        except AttributeError:
            value = f(obj)
            setattr(obj, cache_attr_name, value)
            return value

    return property(get_prop_value, doc=docstring)