52 lines
1.3 KiB
Markdown
52 lines
1.3 KiB
Markdown
# 4.4 Defining Exceptions
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User defined exceptions are defined by classes.
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```python
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class NetworkError(Exception):
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pass
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```
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**Exceptions always inherit from `Exception`.**
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Usually they are empty classes. Use `pass` for the body.
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You can also make a hierarchy of your exceptions.
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```python
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class AuthenticationError(NetworkError):
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pass
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class ProtocolError(NetworkError):
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pass
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```
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## Exercises
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### Exercise 4.11: Defining a custom exception
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It is often good practice for libraries to define their own exceptions.
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This makes it easier to distinguish between Python exceptions raised
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in response to common programming errors versus exceptions
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intentionally raised by a library to a signal a specific usage
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problem.
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Modify the `create_formatter()` function from the last exercise so
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that it raises a custom `FormatError` exception when the user provides
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a bad format name.
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For example:
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```python
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>>> from tableformat import create_formatter
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>>> formatter = create_formatter('xls')
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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File "tableformat.py", line 71, in create_formatter
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raise FormatError('Unknown table format %s' % name)
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FormatError: Unknown table format xls
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>>>
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```
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[Contents](../Contents) \| [Previous (4.3 Special methods)](03_Special_methods) \| [Next (5 Object Model)](../05_Object_model/00_Overview)
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