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2.10. Debugging 17

2.10 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 US$, which contain illegal charac-
ters.

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 “use before def;” 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
NameError: name 'principle ' is not defined
Variables names are case sensitive, so LaTeX is not the same as latex .

At this point the most likely cause of a semantic error is the order of operations. For exam-
ple, to evaluate 1 , you might be tempted to write
2π
>>> 1.0 / 2.0 * pi
But the division happens first, so you would get π/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.

2.11 Glossary

value: One of the basic units of data, like a number or string, that a program manipulates.

type: A category of values. The types we have seen so far are integers (type int), floating-
point numbers (type float ), and strings (type str).
integer: A type that represents whole numbers.

floating-point: A type that represents numbers with fractional parts.
string: A type that represents sequences of characters.

variable: A name that refers to a value.
statement: A section of code that represents a command or action. So far, the statements
we have seen are assignments and print statements.
assignment: A statement that assigns a value to a variable.

state diagram: A graphical representation of a set of variables and the values they refer to.