Scientific Computing with Python 3

Interpreter

The Python interpreter executes the following steps:

• First, run the syntax.
• Then execute the code line by line.
• Code inside a function or class declaration is not executed (but checked for syntax).

      def f(x):
          return y**2  
      a = 3   # here both a and f are defined

You can run the preceding program because there are no syntactical errors. You get an error only when you call the function f.

f(2) # error, y is not defined