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.
Hashability makes an object usable as a dictionary key and a set member, because these data structures use the hash value internally.
All of Python’s immutable built-in objects are hashable; mutable containers (such as lists or dictionaries) are not. Objects which are instances of user-defined classes are hashable by default. They all compare unequal (except with themselves), and their hash value is derived from their id().
>>> a = 1
>>> a.__hash__()
1
>>> hash(a)
1
>>> a = 1.2
>>> hash(a)
461168601842738689
>>> import collections
>>> isinstance([2,3], collections.Hashable)
False
>>> isinstance(2, collections.Hashable)
True
>>> a = {[2,3]:1}
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
immutable
An object with a fixed value. Immutable objects include numbers, strings and tuples. Such an object cannot be altered. A new object has to be created if a different value has to be stored. They play an important role in places where a constant hash value is needed, for example as a key in a dictionary.
mutable
Mutable objects can change their value but keep their id().
Return the “identity” of an object. This is an integer which is guaranteed to be unique and constant for this object during its lifetime. Two objects with non-overlapping lifetimes may have the same value.(因为内存re-use)
CPython implementation detail: This is the address of the object in memory.
