Errors and Exceptions
Until now error messages haven’t been more than mentioned, but if you have tried out the examples you have probably seen some. There are (at least) two distinguishable kinds of errors: syntax errors and exceptions.
8.1. Syntax Errors
Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while you are still learning Python:
The parser repeats the offending line and displays a little ‘arrow’ pointing at the earliest point in the line where the error was detected. The error is caused by (or at least detected at) the token preceding the arrow: in the example, the error is detected at the keyword
':') is missing before it. File name and line number are printed so you know where to look in case the input came from a script.
Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions and are not unconditionally fatal: you will soon learn how to handle them in Python programs. Most exceptions are not handled by programs, however, and result in error messages as shown here:
The last line of the error message indicates what happened. Exceptions come in different types, and the type is printed as part of the message: the types in the example are
TypeError. The string printed as the exception type is the name of the built-in exception that occurred. This is true for all built-in exceptions, but need not be true for user-defined exceptions (although it is a useful convention). Standard exception names are built-in identifiers (not reserved keywords).
The rest of the line provides detail based on the type of exception and what caused it.
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.
Built-in Exceptions lists the built-in exceptions and their meanings.
8.3. Handling Exceptions
It is possible to write programs that handle selected exceptions. Look at the following example, which asks the user for input until a valid integer has been entered, but allows the user to interrupt the program (using
Control-Cor whatever the operating system supports); note that a user-generated interruption is signalled by raising the
trystatement works as follows.
- First, the try clause (the statement(s) between the
exceptkeywords) is executed.
- If no exception occurs, the except clause is skipped and execution of the
trystatement is finished.
- If an exception occurs during execution of the try clause, the rest of the clause is skipped. Then if its type matches the exception named after the
exceptkeyword, the except clause is executed, and then execution continues after the
- If an exception occurs which does not match the exception named in the except clause, it is passed on to outer
trystatements; if no handler is found, it is an unhandled exception and execution stops with a message as shown above.
trystatement may have more than one except clause, to specify handlers for different exceptions. At most one handler will be executed. Handlers only handle exceptions that occur in the corresponding try clause, not in other handlers of the same
trystatement. An except clause may name multiple exceptions as a parenthesized tuple, for example:
Note that the parentheses around this tuple are required, because
except ValueError, e:was the syntax used for what is normally written as
except ValueError as e:in modern Python (described below). The old syntax is still supported for backwards compatibility. This means
exceptRuntimeError, TypeErroris not equivalent to
except (RuntimeError, TypeError):but to
except RuntimeError as TypeError:which is not what you want.
The last except clause may omit the exception name(s), to serve as a wildcard. Use this with extreme caution, since it is easy to mask a real programming error in this way! It can also be used to print an error message and then re-raise the exception (allowing a caller to handle the exception as well):
exceptstatement has an optional else clause, which, when present, must follow all except clauses. It is useful for code that must be executed if the try clause does not raise an exception. For example:
The use of the
elseclause is better than adding additional code to the
tryclause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the
When an exception occurs, it may have an associated value, also known as the exception’s argument. The presence and type of the argument depend on the exception type.
The except clause may specify a variable after the exception name (or tuple). The variable is bound to an exception instance with the arguments stored in
instance.args. For convenience, the exception instance defines
__str__()so the arguments can be printed directly without having to reference
One may also instantiate an exception first before raising it and add any attributes to it as desired.
If an exception has an argument, it is printed as the last part (‘detail’) of the message for unhandled exceptions.
Exception handlers don’t just handle exceptions if they occur immediately in the try clause, but also if they occur inside functions that are called (even indirectly) in the try clause. For example:
8.4. Raising Exceptions
raisestatement allows the programmer to force a specified exception to occur. For example:
The sole argument to
raiseindicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from
If you need to determine whether an exception was raised but don’t intend to handle it, a simpler form of the
raisestatement allows you to re-raise the exception:
8.5. User-defined Exceptions
Programs may name their own exceptions by creating a new exception class (see Classes for more about Python classes). Exceptions should typically be derived from the
Exceptionclass, either directly or indirectly. For example:
In this example, the default
Exceptionhas been overridden. The new behavior simply creates the value attribute. This replaces the default behavior of creating the args attribute.
Exception classes can be defined which do anything any other class can do, but are usually kept simple, often only offering a number of attributes that allow information about the error to be extracted by handlers for the exception. When creating a module that can raise several distinct errors, a common practice is to create a base class for exceptions defined by that module, and subclass that to create specific exception classes for different error conditions:
Most exceptions are defined with names that end in “Error,” similar to the naming of the standard exceptions.
Many standard modules define their own exceptions to report errors that may occur in functions they define. More information on classes is presented in chapter Classes.
8.6. Defining Clean-up Actions
trystatement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. For example:
A finally clause is always executed before leaving the
trystatement, whether an exception has occurred or not. When an exception has occurred in the
tryclause and has not been handled by an
exceptclause (or it has occurred in a
elseclause), it is re-raised after the
finallyclause has been executed. The
finallyclause is also executed “on the way out” when any other clause of the
trystatement is left via a
returnstatement. A more complicated example (having
finallyclauses in the same
trystatement works as of Python 2.5):
As you can see, the
finallyclause is executed in any event. The
TypeErrorraised by dividing two strings is not handled by the
exceptclause and therefore re-raised after the
finallyclause has been executed.
In real world applications, the
finallyclause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.
8.7. Predefined Clean-up Actions
Some objects define standard clean-up actions to be undertaken when the object is no longer needed, regardless of whether or not the operation using the object succeeded or failed. Look at the following example, which tries to open a file and print its contents to the screen.
The problem with this code is that it leaves the file open for an indeterminate amount of time after the code has finished executing. This is not an issue in simple scripts, but can be a problem for larger applications. The
withstatement allows objects like files to be used in a way that ensures they are always cleaned up promptly and correctly.
After the statement is executed, the file f is always closed, even if a problem was encountered while processing the lines. Other objects which provide predefined clean-up actions will indicate this in their documentation.