Explore a comprehensive walkthrough of a Python application for demand and purchase order quantity calculations in the retail and mail order industry. Discover how various Python packages, both community-provided and custom-written, are combined to create flexible solutions for real-world problems. Learn about testing, configuring, parallelizing, and deploying code while examining the practical use of packages such as pytest, voluptuous, redis, SciPy, scikit-learn, and pandas. Gain valuable insights from hands-on experience with retail and mail order customers in this informative EuroPython 2015 conference talk.