Meaning of error numbers in Python exceptions

Question

Catching Python's OverflowError after some dumb calculation, I checked the error's args and saw it's a tuple containing an integer as its first coordinate. I assume this is some kind of error number (errno). However, I could not find any documentation or reference for it.

Example:

try:
    1e4**100
except OverflowError as ofe:
    print ofe.args

## prints '(34, 'Numerical result out of range')'

Do you know what 34 means in this context? Do you know other possible error numbers for this exception?

For the record, 1E400 is not representable as a double which is the usual internal representation of Python's float. — Gassa
– Gassa, Commented Apr 9, 2014 at 7:40
For another record, 1e400 equals inf in python 2.7 (as math.isinf(1e400) shows). — Bach
– Bach, Commented Apr 9, 2014 at 7:51

vaultah · Accepted Answer · 2015-05-19 18:01:56Z

6

There is a module in the standard library called errno:

This module makes available standard errno system symbols. The value of each symbol is the corresponding integer value. The names and descriptions are borrowed from linux/include/errno.h, which should be pretty all-inclusive.

/usr/include/linux/errno.h includes /usr/include/asm/errno.h that includes /usr/include/asm-generic/errno-base.h.

me@my_pc:~$ cat /usr/include/asm-generic/errno-base.h | grep 34
#define ERANGE      34  /* Math result not representable */

Now we know that the 34 error code stands for ERANGE.

1e4**100 is processed with float_pow function from Object/floatobject.c. Partial source code of that function:

static PyObject *
float_pow(PyObject *v, PyObject *w, PyObject *z)
{
    // 107 lines omitted

    if (errno != 0) {
        /* We do not expect any errno value other than ERANGE, but
         * the range of libm bugs appears unbounded.
         */
        PyErr_SetFromErrno(errno == ERANGE ? PyExc_OverflowError :
                             PyExc_ValueError);
        return NULL;
    }
    return PyFloat_FromDouble(ix);
}

So, 1e4**100 causes ERANGE error (resulting in PyExc_OverflowError) and then the higher level OverflowError exception raises.

edited May 19, 2015 at 18:01

answered Apr 9, 2014 at 7:39

vaultah

46.9k13 gold badges120 silver badges145 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Bach Over a year ago

Oh, that's nice. I believe I have the exact same output as you have.

Collectives™ on Stack Overflow

Meaning of error numbers in Python exceptions

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related