Glossary — Python 3.6.5 documentation https://docs.python.org/3/glossary.html?highlight=equal
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().
不变性对象可以哈希化，可变容器：list、dict不可哈希化
可哈希化的仅仅和自己equal，
immutable 不可变性
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.