Exceptions¶
This section is about exceptions, that is, language functions that specifically handle unusual circumstances during the execution of a programme. The most common exception is to handle errors, but they can also be used effectively for many other purposes. Python provides a comprehensive set of exceptions, and you can define new exceptions for your own purposes.
The entire exception mechanism in Python is object-oriented: An exception is an
object that is automatically created by Python functions with a raise
statement. This raise
statement causes the Python programme to be executed
in a different way than usually intended: The current call chain is searched for
a handler that can handle the generated exception. If such a handler is found,
it is called and can access the exception object to obtain further information.
If no suitable exception handler is found, the programme terminates with an
error message.
It is possible to create different types of exceptions to reflect the actual
cause of the reported error or unusual circumstance. For an overview of the
class hierarchy of built-in exceptions, see Exception hierarchy in the
Python documentation. Each exception type is a Python class that inherits from
its parent exception type. For example, a ZeroDivisionError
is also an
ArithmeticError
, an Exception
and also a BaseException
by
inheritance. This hierarchy is intentional: most exceptions inherit from
Exception
, and it is strongly recommended that all user-defined exceptions
also subclass Exception
, and not BaseException
:
class EmptyFileError(Exception):
pass
This defines your own exception type, which inherits from the Exception
base
type.
filenames = ["myFile1.py", "nonExistent.py", "emptyFile.py", "myFile2.py"]
A list of different file types is defined.
Finally, exceptions or errors are caught and handled using the compound
statement try
-except
-else
-finally
. Any exception that is not
caught will cause the programme to terminate.
7 try:
8 f = open(file, "r")
9 line = f.readline()
10 if line == "":
11 f.close()
12 raise EmptyFileError(f"{file} is empty")
13 except IOError as error:
14 print(f"Cannot open file {file}: {error.strerror}")
15 except EmptyFileError as error:
16 print(error)
17 else:
18 print(f"{file}: {f.readline()}")
19 finally:
20 print("File", file, "processed")
- Line 7
If an
IOError
orEmptyFileError
occurs during the execution of the instructions in thetry
block, the correspondingexcept
block is executed.- Line 9
An
IOError
could be triggered here.- Line 12
Here you trigger the
EmptyFileError
.- Line 17
The
else
clause is optional; it is executed if no exception occurs in thetry
block.Note
In this example,
continue
statements could have been used in theexcept
blocks instead.- Line 19
The
finally
clause is optional; it is executed at the end of the block, regardless of whether an exception was thrown or not.