On Tue, Apr 16, 2019 at 5:44 PM Arup Rakshit <a...@zeit.io> wrote:
>
> Hi,
>
> I am reading a sweet introduction about Python object from [3.1. Objects,
> values and
> types](https://docs.python.org/3/reference/datamodel.html#objects-values-and-types).
> And here the author is said about the Object values nice way:
>
> > The value of some objects can change. Objects whose value can change are
> > said to be mutable; objects whose value is unchangeable once they are
> > created are called immutable. (The value of an immutable container object
> > that contains a reference to a mutable object can change when the latter’s
> > value is changed; however the container is still considered immutable,
> > because the collection of objects it contains cannot be changed. So,
> > immutability is not strictly the same as having an unchangeable value, it
> > is more subtle.) An object’s mutability is determined by its type; for
> > instance, numbers, strings and tuples are immutable, while dictionaries and
> > lists are mutable.
>
> But what I don’t understand here is that what he said about the immutable
> container objects. Why after changing the value of internal mutable objects
> we still say the container object as mutable? Any examples to define what the
> author meant will be helpful.
>
A tuple is immutable.
>>> x = (1, 2, 3)
>>> x[1] = 4
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
This is true even if the tuple contains mutable objects:
>>> x = (1, [], 3)
>>> x[1] = ["test"]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
But since those objects are themselves mutable, you can change the
overall value of the tuple:
>>> x = (1, [], 3)
>>> x[1].append("test")
>>> x
(1, ['test'], 3)
Since equality is defined by the values of the tuple's contents, that
means that two equal tuples can become unequal:
>>> y = (1, ["test"], 3)
>>> x == y
True
>>> y[1][0] = "other"
>>> x == y
False
Thus, a tuple that contains mutable objects will always contain the
same objects (defined by *identity*), but may not always represent the
same value (defined by nested values). A tuple that contains any
mutable objects is going to be unhashable, even though tuples in
general are hashable:
>>> stuff = {}
>>> stuff[(1, 2, 3)] = "ok"
>>> stuff[(1, [], 3)] = "not ok"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
Mutability (defined by identity) will usually correspond to
hashability (defined by value), but this is the subtler case that the
author was referring to.
ChrisA
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