Why and how are Python functions hashable?
Last Updated :
31 Aug, 2020
So start with the question i.e. Why and how are Python functions hashable? First, one should know what actually hashable means in Python. So, hashable is a feature of Python objects that tells if the object has a hash value or not. If the object has a hash value then it can be used as a key for a dictionary or as an element in a set.
An object is hashable if it has a hash value that does not change during its entire lifetime. Python has a built-in hash method ( __hash__() ) that can be compared to other objects. For comparing it needs __eq__() or __cmp__() method and if the hashable objects are equal then they have the same hash value. All immutable built-in objects in Python are hashable like tuples while the mutable containers like lists and dictionaries are not hashable.
Objects which are instances of the user-defined class are hashable by default, they all compare unequal, and their hash value is their id().
Example: Consider two tuples t1, t2 with the same values, and see the differences:
Python3
t1 = ( 1 , 5 , 6 )
t2 = ( 1 , 5 , 6 )
print ( id (t1))
print ( id (t2))
|
Output:
140040984150664
140040984150880
In the above example, two objects are different as for immutable types the hash value depends on the data stored not on their id.
Example: Let’s see lambda functions are hashable or not.
Python3
l = lambda x : 1
print ( hash (l))
print ( id (l))
print (l.__hash__())
|
Output:
-9223363246992694337
140637793303544
-9223363246992694337
Hence, lambda functions are hashable.
Example: Let’s see user defined def based function are hashable or not.
Python3
def fun():
pass
print ( type (fun))
print (fun.__hash__())
print ( hash (fun))
|
Output:
<class 'function'>
-9223363242199589441
-9223363242199589441
Therefore, any user defined function is hashable as its hash value remains same during its lifetime.
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