Custom exceptions are implemented in Python by inheriting the Exception class or its subclasses, which are used to improve code readability and targeted error handling. When built-in exceptions cannot meet specific business needs, for example, you need to distinguish between multiple user input error scenarios, you can create different exception types such as InvalidEmailError and PasswordTooShortError. The creation method is simple. You only need to define a new class. You can also add initialization parameters such as message and error_code to provide more information to assist in debugging. Scenarios suitable for using custom exceptions include modular projects, business rule verification and third-party library development. For example, the data parsing module can define ParseError and its subclasses FileFormatError and MissingDataError, which facilitates callers to granularly capture and process. Notes include avoiding overuse, clear and concise naming, keeping exception classes lightweight and managing them uniformly to special modules (such as errors.py), thereby improving code structure and maintenance.
Creating custom exceptions in Python is not difficult. The core is to inherit Exception class or its subclass. The advantage of this is that it makes your code clearer and more readable, especially when dealing with complex logic or large projects, where specific errors can be caught and processed more targetedly.
Why do you need to customize exceptions?
Python has many built-in exception types, such as ValueError , TypeError , etc., but they do not always meet our needs.
For example: You wrote a function that handles user input. When the input does not comply with business rules, it is possible to throw a common ValueError , but if you have multiple different verification rules, it will be easier to distinguish the source of the problem with different exception types.
At this time, you can clearly express different types of errors through custom exceptions, such as:
class InvalidEmailError(ValueError):
pass
class PasswordTooShortError(Exception):
passIn this way, corresponding exceptions can be thrown according to specific problems when calling, which also facilitates the upper-level code to perform refined error handling.
How to create a basic custom exception?
This step is very simple, you just need to define a new class inherited from Exception or its subclass:
class MyCustomError(Exception):
passYou can add initialization methods as needed, such as passing in error messages:
class MyCustomError(Exception):
def __init__(self, message, error_code):
super().__init__(message)
self.error_code = error_codeThen throw and capture when used:
try:
raise MyCustomError("Something went wrong", 400)
except MyCustomError as e:
print(f"Error: {e}, Code: {e.error_code}")This approach is especially useful when developing APIs or server programs, and can carry more information through exceptions to help debug or return to the front-end.
In which scenario is suitable for using custom exceptions?
- Modular project : When you encapsulate some functional modules, you want to expose clear error types.
- Business rule verification : For example, when registering a user, different exceptions can be defined.
- Third-party library development : Defining exclusive exceptions for your library can allow users to know more clearly where something went wrong.
For example, a data parsing module may have the following exceptions:
class ParseError(Exception):
pass
class FileFormatError(ParseError):
pass
class MissingDataError(ParseError):
passIn this way, when catching exceptions, users can choose to only handle errors at a certain level:
try:
parse_data()
except FileFormatError:
print("The file format is wrong")
except ParseError:
print("Other parsing errors")Tips: Some precautions when defining exceptions
- Don't overuse custom exceptions unless it does help with misclassification or improve code maintainability.
- The naming is as clear as possible, usually ending with
ErrororException. - The exception class should not contain too much logic, keep it light and concise.
- It can be organized into a special module (such as
errors.py) for unified management.
Basically that's it. Creating custom exceptions is a small feature, but using them well will make the code structure clearer and it will be easier to locate problems when errors occur.
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