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pythonimmutabilityhashable

What the difference between hash-able and immutable?

  • What's the difference between immutable and hash-able ?
  • What does this mean "An object is hashable if it has a hash value which never changes during its lifetime?"
  • Is a tuple immutable and hashable or only immutable ? why ?
Solution

    1. Immutable means that the object, the top-level container for the item, cannot be changed. Note that this applies only to the top level; it may contain references to sub-objects that are mutable.

    2. Hashable has a functional definition: Python's built-in hash function returns a value. This generally means that the object's closure (following all references to their leaf-node values) consists of immutable objects.

    3. Your premise is incorrect: a tuple can contain mutable items. Any such reference renders the tuple un-hashable.

    For instance:

    >>> b = [7]
>>> a = (b, 5)
>>> a
([7], 5)
>>> type(a)
<class 'tuple'>
>>> set(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
>>> b[0] = 'b'
>>> a
(['b'], 5)

    b is a mutable list. The reference to b can be held in the tuple a. We can change the value of b[0] (not it's object handle), and the display value of a changes. However, we cannot make a set including a, because the mutability of b renders a unhashable.

    Continuing the example:

    >>> b = False
>>> a
(['b'], 5)
>>> b = [14]
>>> a
(['b'], 5)

    a is immutable. Thus, when we change b, only b gets the reference to the new object. a retains the original object handle, still pointing to ['b'].