donkopotamus - 4 months ago 16

Python Question

In python, a value

`x`

`NaN`

`>>> x = float("NaN")`

>>> x == x

False

Now consider a list of exactly one item. We might consider two such lists to be

`>>> ["hello"] == ["hello"]`

True

But this does not appear to be the case with

`NaN`

`>>> x = float("NaN")`

>>> x == x

False

>>> [x] == [x]

True

So these lists of items that are "not equal", are "equal". But only sometimes ... in particular:

- two lists consisting of the same instance of are considered equal; while
`NaN`

- two separate lists consisting of different instances of are not equal
`NaN`

Observe:

`>>> x = float("NaN")`

>>> [x] == [x]

True

>>> [x] == [float("NaN")]

False

This general behaviour also applies to other collection types such as tuples and sets. Is there a good rationale for this?

Answer

Per the docs,

In enforcing reflexivity of elements,

the comparison of collections assumes that for a collection element x, x == x is always true. Based on that assumption, element identity is compared first, and element comparison is performed only for distinct elements. This approach yields the same result as a strict element comparison would, if the compared elements are reflexive. For non-reflexive elements, the result is different than for strict element comparison, and may be surprising: The non-reflexive not-a-number values for example result in the following comparison behavior when used in a list:`>>> nan = float('NaN') >>> nan is nan True >>> nan == nan False <-- the defined non-reflexive behavior of NaN >>> [nan] == [nan] True <-- list enforces reflexivity and tests identity first`