[issue36934] C API Function PyLong_AsDouble Returning Wrong Value

2019-05-15 Thread Farhan Sajjad 


New submission from Farhan Sajjad :

Found this rather obscure behavior where certain 64 bit numbers are changing 
(probably losing precision somewhere down the call chain) if converted from 
PyLong to double using the PyLong_AsDouble C API function.

TO REPRODUCE:

#define __SIZEOF_STRS__ 512

static PyObject*
test_pylong(PyObject* self, PyObject* args)
{
char rBuffer[__SIZEOF_STRS__];
char* strValue;
if (!PyArg_ParseTuple(args, "s", &strValue))
return NULL;

{
printf("%s AS INGRESS\n", strValue);
double dblValue = PyLong_AsDouble(
PyLong_FromString(strValue, NULL, 10));

snprintf(rBuffer, __SIZEOF_STRS__, "%.0f",
 PyLong_AsDouble(PyLong_FromString(strValue, NULL, 10)));

printf("CONVERT 1: %.0f\nCONVERT 2: %s\n", dblValue, rBuffer);

}

Py_RETURN_NONE;
}

Test:
>>> test_pylong("1639873214337061279")
1639873214337061279 AS INGRESS
CONVERT 1: 1639873214337061376
CONVERT 2: 1639873214337061376

--
messages: 342619
nosy: sajjadfx
priority: normal
severity: normal
status: open
title: C API Function PyLong_AsDouble Returning Wrong Value
type: behavior
versions: Python 3.7

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[issue36934] C API Function PyLong_AsDouble Returning Wrong Value

2019-05-15 Thread Farhan Sajjad 


Farhan Sajjad  added the comment:

Thanks for your input Tim. Here is what I understand:
1. In Python 3, int can be arbitrarily large.
2. C double data type can hold very large numbers, and the number tested here 
is quite small compared to the max. It even fits fine in a long long int.
3. Quite interestingly, this function/conversion works in Python 2.

>>> sys.float_info
sys.float_info(max=1.7976931348623157e+308, max_exp=1024, max_10_exp=308, 
min=2.2250738585072014e-308, min_exp=-1021, min_10_exp=-307, dig=15, 
mant_dig=53, epsilon=2.220446049250313e-16, radix=2, rounds=1)

--

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[issue36934] C API Function PyLong_AsDouble Returning Wrong Value

2019-05-15 Thread Farhan Sajjad 


Farhan Sajjad  added the comment:

Maybe I need to go back and understand why this loss of precision is taking 
place for the int->float conversion, and why for certain numbers.

Also, it does not work in Python 2. Please disregard the previous message.

--
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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