dorafmon - 1 year ago 115

Python Question

I just want to figure out the logic behind these results:

`>>>nan = float('nan')`

>>>nan == nan

False

# I understand that this is because the __eq__ method is defined this way

>>>nan in [nan]

True

# This is because the __contains__ method for list is defined to compare the identity first then the content?

But in both cases I think behind the scene the function

`PyObject_RichCompareBool`

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Answer Source

But in both cases I think behind the scene the function

`PyObject_RichCompareBool`

is called right? Why there is a difference? Shouldn't they have the same behaviour?

`==`

never calls `PyObject_RichCompareBool`

on the float objects directly, floats have their own `rich_compare`

method(called for `__eq__`

) that may or may not call `PyObject_RichCompareBool`

depending on the the arguments passed to it.

```
/* Comparison is pretty much a nightmare. When comparing float to float,
* we do it as straightforwardly (and long-windedly) as conceivable, so
* that, e.g., Python x == y delivers the same result as the platform
* C x == y when x and/or y is a NaN.
* When mixing float with an integer type, there's no good *uniform* approach.
* Converting the double to an integer obviously doesn't work, since we
* may lose info from fractional bits. Converting the integer to a double
* also has two failure modes: (1) a long int may trigger overflow (too
* large to fit in the dynamic range of a C double); (2) even a C long may have
* more bits than fit in a C double (e.g., on a a 64-bit box long may have
* 63 bits of precision, but a C double probably has only 53), and then
* we can falsely claim equality when low-order integer bits are lost by
* coercion to double. So this part is painful too.
*/
static PyObject*
float_richcompare(PyObject *v, PyObject *w, int op)
{
double i, j;
int r = 0;
assert(PyFloat_Check(v));
i = PyFloat_AS_DOUBLE(v);
/* Switch on the type of w. Set i and j to doubles to be compared,
* and op to the richcomp to use.
*/
if (PyFloat_Check(w))
j = PyFloat_AS_DOUBLE(w);
else if (!Py_IS_FINITE(i)) {
if (PyInt_Check(w) || PyLong_Check(w))
/* If i is an infinity, its magnitude exceeds any
* finite integer, so it doesn't matter which int we
* compare i with. If i is a NaN, similarly.
*/
j = 0.0;
else
goto Unimplemented;
}
...
```

On the other hand the `list_contains`

directly calls `PyObject_RichCompareBool`

on the items hence you get True in the second case.

Note that this is true only for CPython, PyPy's `list.__contains__`

method only seems to be comparing the items by calling their `__eq__`

method:

```
$~/pypy-2.4.0-linux64/bin# ./pypy
Python 2.7.8 (f5dcc2477b97, Sep 18 2014, 11:33:30)
[PyPy 2.4.0 with GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>> nan = float('nan')
>>>> nan == nan
False
>>>> nan is nan
True
>>>> nan in [nan]
False
```

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