Pizza.py comes with no warranty of any kind. As each source file states in its header, it is a copyrighted code that is distributed free-of-charge, under the terms of the GNU Public License (GPL). This is often referred to as open-source distribution - see www.gnu.org or www.opensource.org for more information. The legal text of the GPL is in the LICENSE file that is included in the Pizza.py distribution.
Here is a summary of what the GPL means for Pizza.py users:
(1) Anyone is free to use, modify, or extend Pizza.py in any way they choose, including for commercial purposes.
(2) If you distribute a modified version of Pizza.py, it must remain open-source, meaning you distribute it under the terms of the GPL. You should clearly annotate such a code as a derivative version of Pizza.py.
(3) If you release any code that includes Pizza.py source code, then it must also be open-sourced, meaning you distribute it under the terms of the GPL.
(4) If you give Pizza.py files to someone else, the GPL LICENSE file and source file headers (including the copyright and GPL notices) should remain part of the code.
This page describes how you can contribute to an open-source code like Pizza.py.