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")

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.

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