What does hashable mean in Python

What does hashable mean in Python

Questions : What does hashable mean in Python


I tried searching internet but could not in4codes_python find the meaning of hashable.

When they say objects are hashable or in4codes_python hashable objects what does it mean?

Total Answers 10

Answers 1 : of What does hashable mean in Python

From the Python glossary:

An object is hashable if it has a hash in4codes_python value which never changes during its in4codes_python lifetime (it needs a __hash__() method), in4codes_python and can be compared to other objects (it in4codes_python needs an __eq__() or __cmp__() method). in4codes_python Hashable objects which compare equal in4codes_python must have the same hash value.

Hashability makes an object usable as a in4codes_python dictionary key and a set member, because in4codes_python these data structures use the hash value in4codes_python internally.

All of Python’s immutable in4codes_python built-in objects are hashable, while no in4codes_python mutable containers (such as lists or in4codes_python dictionaries) are. Objects which are in4codes_python instances of user-defined classes are in4codes_python hashable by default; they all compare in4codes_python unequal, and their hash value is their in4codes_python id().



Answers 2 : of What does hashable mean in Python

All the answers here have good working in4codes_python explanation of hashable objects in in4codes_python python, but I believe one needs to in4codes_python understand the term Hashing first.

Hashing is a concept in computer science in4codes_python which is used to create high in4codes_python performance, pseudo random access data in4codes_python structures where large amount of data is in4codes_python to be stored and accessed quickly.

For example, if you have 10,000 phone in4codes_python numbers, and you want to store them in in4codes_python an array (which is a sequential data in4codes_python structure that stores data in contiguous in4codes_python memory locations, and provides random in4codes_python access), but you might not have the in4codes_python required amount of contiguous memory in4codes_python locations.

So, you can instead use an array of size in4codes_python 100, and use a hash function to map a in4codes_python set of values to same indices, and these in4codes_python values can be stored in a linked list. in4codes_python This provides a performance similar to in4codes_python an array.

Now, a hash function can be as simple as in4codes_python dividing the number with the size of in4codes_python the array and taking the remainder as in4codes_python the index.

For more detail refer to in4codes_python https://en.wikipedia.org/wiki/Hash_function

Here is another good reference: in4codes_python http://interactivepython.org/runestone/static/pythonds/SortSearch/Hashing.html



Answers 3 : of What does hashable mean in Python

Anything that is not mutable (mutable in4codes_python means, likely to change) can be hashed. in4codes_python Besides the hash function to look for, in4codes_python if a class has it, by eg. dir(tuple) and in4codes_python looking for the __hash__ method, here in4codes_python are some examples

#x = hash(set([1,2])) #set unhashable x = hash(frozenset([1,2])) #hashable #x = hash(([1,2], [2,3])) #tuple of mutable objects, unhashable x = hash((1,2,3)) #tuple of immutable objects, hashable #x = hash() #x = hash({1,2}) #list of mutable objects, unhashable #x = hash([1,2,3]) #list of immutable objects, unhashable 

List of immutable types:

int, float, decimal, complex, bool, string, tuple, range, frozenset, bytes 

List of mutable types:

list, dict, set, bytearray, user-defined classes 



Answers 4 : of What does hashable mean in Python

In my understanding according to Python in4codes_python glossary, when you create an instance of in4codes_python objects that are hashable, an in4codes_python unchangeable value is also calculated in4codes_python according to the members or values of in4codes_python the instance. For example, that value in4codes_python could then be used as a key in a in4codes_python dictionary as below:

>>> tuple_a = (1, 2, 3) >>> tuple_a.__hash__() 2528502973977326415 >>> tuple_b = (2, 3, 4) >>> tuple_b.__hash__() 3789705017596477050 >>> tuple_c = (1, 2, 3) >>> tuple_c.__hash__() 2528502973977326415 >>> id(tuple_a) == id(tuple_c) # tuple_a and tuple_c same object? False >>> tuple_a.__hash__() == tuple_c.__hash__() # hash of tuple_a and tuple_c same value? True >>> dict_a = {} >>> dict_a[tuple_a] = 'hiahia' >>> dict_a[tuple_c] 'hiahia' 

We can find that the hash value of in4codes_python tuple_a and tuple_c are the same since in4codes_python they have the same members. When we use in4codes_python tuple_a as the key in dict_a, we can in4codes_python find that the value for dict_a[tuple_c] in4codes_python is the same, which means that, when they in4codes_python are used as the key in a dictionary, in4codes_python they return the same value because the in4codes_python hash values are the same. For those in4codes_python objects that are not hashable, the in4codes_python method __hash__ is defined as None:

>>> type(dict.__hash__) <class 'NoneType'> 

I guess this hash value is calculated in4codes_python upon the initialization of the instance, in4codes_python not in a dynamic way, that’s why only in4codes_python immutable objects are hashable. Hope in4codes_python this helps.



Answers 5 : of What does hashable mean in Python

Hashable = capable of being hashed.

Ok, what is hashing? A hashing function in4codes_python is a function which takes an object, say in4codes_python a string such as in4codes_python “Python,” and in4codes_python returns a fixed-size code. For in4codes_python simplicity, assume the return value is in4codes_python an integer.

When I run in4codes_python hash(‘Python’) in in4codes_python Python 3, I get 5952713340227947791 as in4codes_python the result. Different versions of Python in4codes_python are free to change the underlying hash in4codes_python function, so you will likely get a in4codes_python different value. The important thing is in4codes_python that no matter now many times I run in4codes_python hash(‘Python’), in4codes_python I’ll always get the same in4codes_python result with the same version of Python.

But hash(‘Java’) in4codes_python returns 1753925553814008565. So if the in4codes_python object I am hashing changes, so does the in4codes_python result. On the other hand, if the object in4codes_python I am hashing does not change, then the in4codes_python result stays the same.

Why does this matter?

Well, Python dictionaries, for example, in4codes_python require the keys to be immutable. That in4codes_python is, keys must be objects which do not in4codes_python change. Strings are immutable in Python, in4codes_python as are the other basic types (int, in4codes_python float, bool). Tuples and frozensets are in4codes_python also immutable. Lists, on the other in4codes_python hand, are not immutable (i.e., they are in4codes_python mutable) because you can change them. in4codes_python Similarly, dicts are mutable.

So when we say something is hashable, we in4codes_python mean it is immutable. If I try to pass a in4codes_python mutable type to the hash() function, it in4codes_python will fail:

>>> hash('Python') 1687380313081734297 >>> hash('Java') 1753925553814008565 >>> >>> hash([1, 2]) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unhashable type: 'list' >>> hash({1, 2}) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unhashable type: 'set' >>> hash({1 : 2}) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unhashable type: 'dict' >>> >>> hash(frozenset({1, 2})) -1834016341293975159 >>> hash((1, 2)) 3713081631934410656 



Answers 6 : of What does hashable mean in Python

In Python, any immutable object (such as in4codes_python an integer, boolean, string, tuple) is in4codes_python hashable, meaning its value does not in4codes_python change during its lifetime. This allows in4codes_python Python to create a unique hash value to in4codes_python identify it, which can be used by in4codes_python dictionaries to track unique keys and in4codes_python sets to track unique values.

This is why Python requires us to use in4codes_python immutable datatypes for the keys in a in4codes_python dictionary.



Answers 7 : of What does hashable mean in Python

Let me give you a working example to in4codes_python understand the hashable objects in in4codes_python python. I am taking 2 Tuples for this in4codes_python example.Each value in a tuple has a in4codes_python unique Hash Value which never changes in4codes_python during its lifetime. So based on this in4codes_python has value, the comparison between two in4codes_python tuples is done. We can get the hash in4codes_python value of a tuple element using the Id().



Answers 8 : of What does hashable mean in Python

In python it means that the object can in4codes_python be members of sets in order to return a in4codes_python index. That is, they have unique in4codes_python identity/ id.

for example, in python 3.3:

the data structure Lists are not in4codes_python hashable but the data structure Tuples in4codes_python are hashable.



Answers 9 : of What does hashable mean in Python

As an addition to the other answers: The in4codes_python Python glossary says that “Objects which in4codes_python are instances of user-defined classes in4codes_python are hashable by default.”. But it also in4codes_python says “Hashable objects which compare in4codes_python equal must have the same hash value.”.

By this, if you implement the in4codes_python __eq__-Method without Implementing the in4codes_python __hash__-Method your object will not be in4codes_python hashable. Otherwise deriving the hash in4codes_python from id() would not guarantee that for in4codes_python two object which compare equal the same in4codes_python hash is generated (id(a) != id(b) but a in4codes_python == b)

>>> class Foo(object): ...  def __eq__(self, other): pass ... >>> >>> class Bar(object): ...  pass ... >>> f = Foo() >>> b = Bar() >>> >>> hash(f) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unhashable type: 'Foo' >>> hash(b) 8758325844794 



Answers 10 : of What does hashable mean in Python

For creating a hashing table from in4codes_python scratch, all the values has to set to in4codes_python “None” and modified once a requirement in4codes_python arises. Hashable objects refers to the in4codes_python modifiable datatypes(Dictionary,lists in4codes_python etc). Sets on the other hand cannot be in4codes_python reinitialized once assigned, so sets are in4codes_python non hashable. Whereas, The variant of in4codes_python set() — frozenset() — is hashable.


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