3. Dictionaries and Sets

1. Generic Mapping Types

• The collections.abc module provides the Mapping and MutableMapping ABCs to formalize the interfaces of dict and similar types (in Python 2.6 to 3.2, these classes are imported from the collections module, and not from collections.abc).
• All mapping types in the standard library use the basic dict in their implementation, so they share the limitation that the keys must be hashable.
  • An object is hashable if it has a hash value which never changes during its lifetime (it needs a __hash__() method), and can be compared to other objects (it needs an __eq__() method). Hashable objects which compare equal must have the same hash value. (atomic immutable types / frozen set / tuple when all its items are hashable)
  • User-defined types are hashable by default because their hash value is their id() and they all compare not equal. If an object implements a custom __eq__ that takes into account its internal state, it may be hashable only if all its attributes are immutable.

a = dict(one=1, two=2, three=3)
b = {'one': 1, 'two': 2, 'three': 3}
c = dict(zip(['one', 'two', 'three'], [1, 2, 3]))
d = dict([('two', 2), ('one', 1), ('three', 3)])
e = dict({'three': 3, 'one': 1, 'two': 2})
print(a == b == c == d == e)  # True


from collections import abc
print(isinstance(a, abc.Mapping))  # True
# 1. The main value of the ABCs is documenting and formalizing the 
# minimal interfaces for mappings, and serving as criteria for isinstance
# tests in code that needs to support mappings in a broad sense.


print(hash((1, 2, (3, 4))))  # -2725224101759650258
# 1. An object is hashable if it has a hash value which never changes
# during its lifetime (it needs a __hash__() method), and can be compared
# to other objects (it needs an __eq__() method). 
# 2. Hashable objects which compare equal must have the same hash value.
# 3. (atomic immutable types / frozen set / tuple when all its items are hashable)
# 4. User-defined types are hashable by default because their hash value is their
# id() and they all compare not equal. If an object implements a custom __eq__
# that takes into account its internal state, it may be hashable only if all its
# attributes are immutable.