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From: MRAB <python@mrabarnett.plus.com>
Newsgroups: comp.lang.python
Subject: Re: Convert from unsigned long long to PyLong
Date: Fri, 22 Jul 2016 15:46:20 +0100
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On 2016-07-22 07:01, Tian JiaLin wrote:
> HI There,
>
> I'm using MySQLdb as the MySQL client. Recently I got a weird problem of this library. After looking into it, I suspect the problem may related to the conversion from unsigned long to PyLongObject.
>
> Here is the detail, If you are familiar with MySQLdb, the following snippet is a way to query the data from MySQL:
>
>
> connection = MySQLdb.connect(...)
>
> connection.autocommit(True)
> try:
>     cursor = connection.cursor()
>     if not cursor.execute(sql, values) > 0:
>             return None
>     row = cursor.fetchone()
> finally:
>     connection.close()
> return row[0]
>
>
> Sometimes the return value of execute method would be 18446744073709552000 even there is no matched data available. I checked the source code of the library, the underlying implementation is https://github.com/farcepest/MySQLdb1/blob/master/_mysql.c#L835,
>
> static PyObject *
> _mysql_ConnectionObject_affected_rows(
> 	_mysql_ConnectionObject *self,
> 	PyObject *args)
> {
> 	if (!PyArg_ParseTuple(args, "")) return NULL;
> 	check_connection(self);
> 	return PyLong_FromUnsignedLongLong(mysql_affected_rows(&(self->connection)));
> }
>
> And here is the official doc for mysql_affected_rows http://dev.mysql.com/doc/refman/5.7/en/mysql-affected-rows.html.
>
> Let me give a superficial understanding, please correct me if I were wrong.
>
> In a 64-bit system, the mysql_affected_rows is supposed to return a number of unsigned long, which means the range should be 0 ~ 2^64 (18446744073709551616), How could it be possible the function PyLong_FromUnsignedLongLong return a converted value larger than 2^64, that's what I don't understand.
>
> Does anyone have some ideas of it?
>
>
> The versions of the components I used:
>
> Python: 2.7.6
> MySQL 5.7.11
> MySQLdb 1.2.5
>
>
The function returns an unsigned value, but it will return -1 (i.e. ~0) 
if there's an error, so check for an error with ~result == 0.