Question

I am new to Python, and I’m working on writing some database code using the cx_Oracle module. In the cx_Oracle documentation they have a code example like this:

import sys import cx_Oracle  connection = cx_Oracle.Connection('user/pw@tns') cursor = connection.cursor()  try:     cursor.execute('select 1 / 0 from dual') except cx_Oracle.DatabaseError, exc:     error, = exc.args     print >> sys.stderr, 'Oracle-Error-Code:', error.code     print >> sys.stderr, 'Oracle-Error-Message:', error.message 

My question has to do with where the ‘error’ object is created. What does the ‘, =‘ do? I tried searching Python documentation, and search engines don’t work very well when you’re searching for operators. 🙂

I know that the exc.args is a singleton tuple, but I just don’t understand the ‘, =‘ syntax. If I remove the comma, I get the error message, ‘AttributeError: 'tuple' object has no attribute 'code'‘.

Can someone point me to where this is documented? Thanks!

EDIT:

This works without having to unpack the tuple:

import sys import cx_Oracle  connection = cx_Oracle.Connection('user/pw@tns') cursor = connection.cursor()  try:     cursor.execute('select 1 / 0 from dual') except cx_Oracle.DatabaseError, exc:     print >> sys.stderr, 'Oracle-Error-Code:', exc.args[0].code     print >> sys.stderr, 'Oracle-Error-Message:', exc.args[0].message 

Answer 1

error, = exc.args 

This is a case of sequence unpacking.

A more readable way to write the same, and the style I personally favor, is:

[error] = exc.args 

There are two bits required to understand the previous example:

1. When the left hand side of an assignment is a recursive sequence of names, the value of the right hand side must be a sequence with the same length, and each item of the RHS value is assigned to the corresponding name in the LHS.
2. A one-item tuple in python is written (foo,). In most contexts, the parenthesis can be ommitted. In particular, they can be omitted next to the assignment operator.