---
source: https://iinventedlight.com/library/notes/python-exceptions-hierarchy.md
title: "Built-in exceptions and BaseException"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "PSF License Version 2 (Python docs); prose restates cited source lines in own words"
license_family: python-psf
attribution: "Summary by iinventedlight (AI-assisted). Sources: Python Software Foundation, Built-in Exceptions, PSF License Version 2; Python Software Foundation, concurrent.futures — Launching parallel tasks, PSF License Version 2."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-06
status: published
checked_on: 2026-11-06
as_of: 2026-11-06
confidence: medium
untested: "No Python interpreter was run for this note; only the fetched documentation pages were read."
category: coding-references
topic_area: "code-python: exceptions"
tags: [python, exceptions]
client: Librarian
sources:
  - id: S1
    title: "Built-in Exceptions"
    publisher: Python Software Foundation
    url: https://docs.python.org/3/library/exceptions.html
    accessed: 2026-11-06
    license: "PSF License Version 2"
  - id: S2
    title: "concurrent.futures — Launching parallel tasks"
    publisher: Python Software Foundation
    url: https://docs.python.org/3/library/concurrent.futures.html
    accessed: 2026-11-06
    license: "PSF License Version 2"
---
# Built-in exceptions and BaseException

In Python, all exceptions must be instances of a class that derives from BaseException. [S1]

## Hierarchy

all exceptions must be instances of a class that derives from BaseException. [S1]

that clause also handles any exception classes derived from that class. [S1]

## Parallel callables still raise

high-level interface for asynchronously executing callables. [S2]

## Claims

- id: C1
  claim: Exceptions derive from BaseException.
  source: S1
  evidence: "all exceptions must be instances of a class that derives from BaseException"

- id: C2
  claim: except also catches subclasses.
  source: S1
  evidence: "that clause also handles any exception classes derived from that class"

- id: C3
  claim: concurrent.futures is a high-level callable interface.
  source: S2
  evidence: "high-level interface for asynchronously executing callables."

- id: C4
  claim: Execution can use threads.
  source: S2
  evidence: "The asynchronous execution can be performed with threads"

## Sources

- [S1] Built-in Exceptions. Python Software Foundation. https://docs.python.org/3/library/exceptions.html . License: PSF License Version 2. Accessed 2026-11-06.

- [S2] concurrent.futures — Launching parallel tasks. Python Software Foundation. https://docs.python.org/3/library/concurrent.futures.html . License: PSF License Version 2. Accessed 2026-11-06.


License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted). Source: Python Software Foundation (Python 3 docs). PSF License Version 2 (https://docs.python.org/3/license.html).