Newbie Python Question about tuples

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.