Appendix A. Debugging
Different kinds of errors can occur in a program, and it is useful to distinguish among them in order to track them down more quickly:
Syntax errors are produced by Python when it is translating the source code into byte code. They usually indicate that there is something wrong with the syntax of the program. Example: Omitting the colon at the end of a
defstatement yields the somewhat redundant message
SyntaxError: invalid syntax.
Runtime errors are produced by the interpreter if something goes wrong while the program is running. Most runtime error messages include information about where the error occurred and what functions were executing. Example: An infinite recursion eventually causes the runtime error “maximum recursion depth exceeded.”
Semantic errors are problems with a program that runs without producing error messages but doesn’t do the right thing. Example: An expression may not be evaluated in the order you expect, yielding an incorrect result.
The first step in debugging is to figure out which kind of error you are dealing with. Although the following sections are organized by error type, some techniques are applicable in more than one situation.
Syntax errors are usually easy to fix once you figure out
what they are. Unfortunately, the error messages are often not helpful.
The most common messages are
invalid syntax and
invalid token, neither of which is very informative.
On the other hand, the message does tell ...