Can the keys of a python dictionary be a list?

Keys of python dictionary

The values ​​in the dictionary have no restrictions and can be any Python object, from standard objects to user-defined Any object is acceptable, but the keys in the dictionary are type-restricted.

Does not allow one key to correspond to multiple values

One principle must be made clear: each key can only correspond to one item. In other words: multiple values ​​corresponding to one key are not allowed (container objects like lists, tuples, and other dictionaries are allowed). When there is a key conflict (that is, a dictionary key is assigned a value repeatedly), the last (most recent) assignment is taken. Python does not generate an error due to conflicting keys in the dictionary. It does not check for key conflicts because if it did, it would be checked every time a key-value pair is assigned, which would take up a certain amount of time. amount of memory.

>>> dict1 = {'foo':789, 'foo': 'xyz'} 
>>> dict1 
{'foo': 'xyz'} 
>>> dict1['foo'] = 123
>>> dict1 
{'foo': 123}

Keys must be hashable

Most Python objects can serve as keys, but they must be hashable objects. Mutable types like lists and dictionaries cannot be used as keys since they are not hashable. All immutable types are hashable, so they can be used as dictionary keys. It should be noted that numbers with equal values ​​represent the same key, that is, the hash values ​​of the integer number 1 and the floating point number 1.0 are the same, and they are the same key.

At the same time, there are also some mutable objects (rarely) that are hashable, and they can be used as dictionary keys, but they are rare. Take, for example, a class that implements the __hash__() special method. Because the __hash__() method returns an integer, an immutable value is still used (as a dictionary key). Why do keys have to be hashable? The interpreter calls a hash function to calculate where to store your data based on the value of the key in the dictionary. If the key is a mutable object, its value can change. If the key changes, the hash function maps to a different address to store the data. If this happens, the hash function cannot reliably store or retrieve the relevant data. The reason for choosing hashable keys is because their values ​​cannot be changed.

Numbers and strings can be used as dictionary keys. Tuples are immutable but may not be immutable. Therefore, using tuples as valid keys must be restricted: if the tuple only contains Only immutable parameters like numbers and strings can be used as valid keys in the dictionary.

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