TO avoid circumlocutions, we will often use the terminology of the operator product expansion when discussing factorization theorems. In particular, we will use the term "Wilson coefficient" to denote the short-distance coefficient in the standard factorization theorem for deep-inelastic scattering etc.3 102

During the last decade, Python (an interpreted, high-level programming language) has arguably become the de facto standard for exploratory, interactive, and computational driven scientific research. This issue discusses the advantages of Python for scientific research and presents several of the core Python libraries and tools used in scientific research. While the articles in the present issue are self-contained, they nicely compliment the articles in the May/June 2007 special issue of CiSE titled "Python: Batteries Included." 1

Existing calculations of heavy quark production in charged-current and neutral current lepton-hadron scattering are formulated differently because of the artificial distinction of "light" and "heavy" quarks made in the traditional approach. A proper QCD formalism valid for a wide kinematic range from near threshold to energies much higher then the quark mass should treat these processes in a uniform way. We formulate a unified approach to both types of leptoproduction processes based on the conventional factorization theorem. In this paper, we present the general framework with complete kinematics appropriate for arbitrary masses, emphasizing the simplifications provided by the helicity formalism. We illustrate this approach with an explicit calculation of the leading order contribution to the quark structure functions with general masses. This provides the basis for a complete QCD analysis of charged current and neutral current leptoproduction of charm and bottom quarks to be presented in subsequent papers.