Errors & Exceptions
Exceptions
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 ZeroDivisionError
, NameError
and 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.
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-C or whatever the operating system supports); note that a user-generated interruption is signalled by raising the KeyboardInterrupt
exception.
The try
statement works as follows.
If no exception occurs, the except clause is skipped and execution of the
try
statement is finished.If an exception occurs which does not match the exception named in the except clause, it is passed on to outer
try
statements; if no handler is found, it is an unhandled exception and execution stops with a message as shown above.
A try
statement 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 try
statement. 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 except RuntimeError, TypeError
is 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):
The try
âĶ except
statement 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 else
clause is better than adding additional code to the try
clause because it avoids accidentally catching an exception that wasnât raised by the code being protected by the try
âĶ except
statement.
When an exception occurs, it may have an associated value, also known as the exceptionâs 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 .args
.
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 (âdetailâ) of the message for unhandled exceptions.
Exception handlers donât 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:
Raising Exceptions
The raise
statement allows the programmer to force a specified exception to occur. For example:
The sole argument to raise
indicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from Exception
).
If you need to determine whether an exception was raised but donât intend to handle it, a simpler form of the raise
statement allows you to re-raise the exception:
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 Exception
class, either directly or indirectly. For example:
In this example, the default __init__()
of Exception
has 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 âErrorâ, 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.
Defining Clean-up Actions
The try
statement 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 try
statement, whether an exception has occurred or not. When an exception has occurred in the try
clause and has not been handled by an except
clause (or it has occurred in an except
or else
clause), it is re-raised after the finally
clause has been executed. The finally
clause is also executed âon the way outâ when any other clause of the try
statement is left via a break
, continue
or return
statement. A more complicated example (having except
and finally
clauses in the same try
statement works as of Python 2.5):
As you can see, the finally
clause is executed in any event. The TypeError
raised by dividing two strings is not handled by the except
clause and therefore re-raised after the finally
clause has been executed.
In real world applications, the finally
clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.
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 with
statement 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.
Reference : https://docs.python.org/2/tutorial/errors.html
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