User-defined exception
A user-defined exception is a programmer-defined exception type representing an application-specific error or abnormal condition.
The Quick Python Book, Fourth Edition
A custom exception is a programmer-defined exception class representing an application-specific abnormal condition.
- How it is used
- Represents application-specific exceptional conditions that code can raise and handle.
- How deep it goes
- Works through definition, raising, catching, and arguments; defers detailed design of exception subhierarchies.
- Examples
- Uses MyError with a printed message and multiple arguments, plus EmptyStringError in text cleaning.
- What you can do after
- Build it from scratch: define an Exception subclass, raise it, catch it, and decide whether a dedicated type is worthwhile.
Read how The Quick Python Book, Fourth Edition explains user-defined exception →
The Art of Code
A custom exception is an application-defined exception type whose name and structure represent a specific failure condition.
- How it is used
- Translates technical failures into meaningful domain failures at business, API, and user-facing boundaries.
- How deep it goes
- Explains naming, immutable context, wrapping, and cause preservation, without defining exception runtime internals.
- Examples
- Uses data parsing to wrap NumberFormatException as DataProcessingException, with a Python exception-chaining counterpart.
- What you can do after
- Apply it to name domain failures and retain original causes for useful diagnostics.
Python How-To
A custom exception is a programmer-defined exception type used to represent application-specific error conditions.
- How it is used
- Uses named, package-specific failures to communicate application conditions and support callers handling library errors.
- How deep it goes
- Explains inheritance, package hierarchies, custom messages, __str__, raising, catching, and documentation, without detailing exception internals.
- Examples
- Demonstrates TaskierError, FileExtensionError, CSV validation, upload_file, and custom handling for tasks.csv and tasks.docx.
- What you can do after
- Build package-specific exception hierarchies, raise informative failures, catch named conditions, and document possible exceptions.
Related concepts
- exceptionkind of · An exception is a runtime signal or object representing an abnormal condition that interrupts ordinary program flow and may be handled or propagated.
- classkind of · A class is a programming-language construct that defines a type’s structure or behavior and provides a basis for creating or representing objects.
- packageused by · A software package is a named grouping of modules or components, often with related resources and metadata, organized for import, reuse, distribution, or management.
- exception chainingimplemented by · Exception chaining is the preservation of an underlying exception as the cause of a newly raised exception.
- base exception classincludes · A base exception class is a Python class that defines or serves as a superclass for one or more exception types.
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