Debugging

At this point the syntax error you are most likely to make is an illegal variable name, like class and yield, which are keywords, or odd job and GBP£, which contain illegal characters (remember, variable names cannot contain a blank space).

If you put a space in a variable name, Python thinks it is two operands without an operator:

>>> bad name = 5 
SyntaxError: invalid syntax 

For syntax errors, the error messages don't help much. The most common messages are SyntaxError: invalid syntax and SyntaxError: invalid token, neither of which is very informative.

The runtime error you are most likely to make is a NameError that is, trying to use a variable before you have assigned a value. This can happen if you spell a variable name wrong:

>>> principal = 327.68 
>>> interest = principle * rate
 Traceback (most recent call last): 
     File "<pyshell#36>", line 1, 
        in interest = principle * rate 
  NameError: name 'principle' is not defined 

Variables names are case sensitive, so Principal is not the same as principal.

At this point the most likely cause of a semantic error is the order of operations. For example, to evaluate $\frac{1}{2 \pi}$, you might be tempted to write:

1.0 / 2.0 * pi

But the division happens first, so you would get $\pi / 2$, which is not the same thing! There is no way for Python to know what you meant to write, so in this case you don't get an error message; you just get the wrong answer.