What does hashable mean in Python

Question

I tried searching internet but could not find the meaning of hashable   When they say objects are hashable or hashable objects what does it mean

User · Answer

In my understanding according to Python glossary, when you create a instance of objects that are hashable, an unchangeable value is also calculated according to the members or values of the instance. For example, that value could then be used as a key in a dict 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(a) == id(c)  # a and c same object?
False
>>> a.__hash__() == c.__hash__()  # a and c same value?
True
>>> dict_a = {}
>>> dict_a[tuple_a] = 'hiahia'
>>> dict_a[tuple_c]
'hiahia'

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

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

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

User · Answer

All the answers here have good working explanation of hashable objects in python  but I believe one needs to understand the term Hashing first   Hashing is a concept in computer science which is used to create high performance  pseudo random access data structures where large amount of data is to be stored and accessed quickly   For example  if you have 10 000 phone numbers  and you want to store them in an array  which is a sequential data structure that stores data in contiguous memory locations  and provides random access   but you might not have the required amount of contiguous memory locations   So  you can instead use an array of size 100  and use a hash function to map a set of values to same indices  and these values can be stored in a linked list  This provides a performance similar to an array    Now  a hash function can be as simple as dividing the number with the size  of the array and taking the remainder as the index   For more detail refer to https   en wikipedia org wiki Hash function  Here is another good reference  http   interactivepython org runestone static pythonds SortSearch Hashing html

User · Answer

Hashable   capable of being hashed   Ok  what is hashing  A hashing function is a function which takes an object  say a string such as    Python     and returns a fixed-size code  For simplicity  assume the return value is an integer   When I run hash    Python     in Python 3  I get 5952713340227947791 as the result  Different versions of Python are free to change the underlying hash function  so you will likely get a different value  The important thing is that no matter now many times I run hash    Python      I   ll always get the same result with the same version of Python   But hash    Java     returns 1753925553814008565  So if the object I am hashing changes  so does the result  On the other hand  if the object I am hashing does not change  then the result stays the same   Why does this matter   Well  Python dictionaries  for example  require the keys to be immutable  That is  keys must be objects which do not change  Strings are immutable in Python  as are the other basic types  int  float  bool   Tuples and frozensets are also immutable  Lists  on the other hand  are not immutable  i e   they are mutable  because you can change them  Similarly  dicts are mutable   So when we say something is hashable  we mean it is immutable  If I try to pass a mutable type to the hash   function  it will fail    gt  gt  gt  hash  Python   1687380313081734297  gt  gt  gt  hash  Java   1753925553814008565  gt  gt  gt   gt  gt  gt  hash  1  2   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  unhashable type   list   gt  gt  gt  hash  1  2   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  unhashable type   set   gt  gt  gt  hash  1   2   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  unhashable type   dict   gt  gt  gt   gt  gt  gt  hash frozenset  1  2    -1834016341293975159  gt  gt  gt  hash  1  2   3713081631934410656

User · Answer

In python it means that the object can be members of sets in order to return a index  That is  they have unique identity  id   for example  in python 3 3   the data structure Lists are not hashable but the data structure Tuples are hashable

User · Answer

From the Python glossary      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     or   cmp     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  while no mutable containers  such as lists or dictionaries  are  Objects which are instances of user-defined classes are hashable by default  they all compare unequal  and their hash value is their id

User · Answer

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

User · Answer

Anything that is not mutable  mutable means  likely to change  can be hashed  Besides the hash function to look for  if a class has it  by eg  dir tuple  and looking for the   hash   method  here 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

User · Answer

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

User · Answer

In Python  any immutable object  such as an integer  boolean  string  tuple  is hashable  meaning its value does not change during its lifetime  This allows Python to create a unique hash value to identify it  which can be used by dictionaries to track unique keys and sets to track unique values   This is why Python requires us to use immutable datatypes for the keys in a dictionary

[python] What does "hashable" mean in Python?

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