Python Programming On Win32

Executing Commands and Evaluating Expressions

Python has built-in functions and statements to facilitate this dynamic code evaluation: eval( expression, [globals[, locals]] ) is a built-in function that evaluates a string, and exec expression, [globals[, locals]] is a statement (not a function: no parentheses needed) that executes a string. The following clarifies how it works:

>>> exec "print 'this expression was compiled on the fly' "
this expression was compiled on the fly
>>> exec "x = 3.14"
>>> eval("x + 1")
4.14

Let’s pause for a moment and consider the implications of this code. You could pass a chunk of text to a running Python application, and it’s parsed, compiled, and executed on the fly. In Python, the interpreter is always available. Few languages offer this capability, and it’s what makes Python a good macro language.

Case Study: Formula Evaluation

In 1997, one of the authors was consulting for a large packaging company building a database of its packaging designs. A cardboard box has a certain geometry, which may be thought of as a collection of flat panels joined at the edges, with a series of folding instructions. The overwhelming majority of designs were for cuboid boxes. This allowed the length of a certain edge of a panel to be expressed as a formula such as “2L + 3.5W + T,” where L is the length of the product to be packed, W is the width, and T is the thickness of the board. Packaging designers all over Europe had been patiently entering formulae for each panel ...