errors and exceptions

Let’s start with a standard error message:

>>> '2' + 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't convert 'int' object to str implicitly

The last line of this little piece of info states the type of the error, and the rest of the line provides the details. TypeError here is a built-in exception class.

The preceding part of the error message shows the context where the exception happened, in the form of a stack traceback. In general it contains a stack traceback listing source lines; however, it will not display lines read from standard input.

The piece above shows a ‘normal’ error message we might often face. When we handle these errors by our own, we should notice that

A class in an except clause is compatible with an exception if it is the same class or a base class thereof (but not the other way around — an except clause listing a derived class is not compatible with a base class).

that means, if you raises an error of the derived class, an ‘except clause’ with a base class error will be triggered. And.. the first matching one is invoked. See examples here.

Use except only to perform a wildcard-like task, but it should be used with extreme caution, at times it is used to print error message and re-raise the exception (to allow a caller to handle the exception as well).

If an exception has arguments, they are printed as the last part (‘detail’) of the message for unhandled exceptions.

The try … except has an optional else cause so that it avoids accidentally catching an exception that wasn’t raised by the code being protected by the try … except statement.

Usually, the finally statement serves as a clean-up action, but we also have those pre-defined one using with statement.