Exception Chaining in Python 3

How can I raise an exception in Python and attach another exception as extra info.
That’s called exception chaining. (This article is AI generated. I’ll rewrite it on occasion.)

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1. Using raise ... from ...

try:
    int("abc")
except ValueError as e:
    raise RuntimeError("Parsing failed") from e

Output:

RuntimeError: Parsing failed

The above exception was the direct cause of the following exception:
ValueError: invalid literal for int() with base 10: 'abc'

→ from e attaches the original exception as __cause__.

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2. Using raise ... inside an except block (implicit chaining)

try:
    int("abc")
except ValueError:
    raise RuntimeError("General error")

Python automatically sets __context__ here, so the original exception is shown in the traceback, even without from.

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3. Storing it as an attribute

If you don’t want chaining but just want to keep the original exception as information, you can save it in your own exception class:

class MyError(Exception):
    def __init__(self, message, original=None):
        super().__init__(message)
        self.original = original

try:
    1/0
except ZeroDivisionError as e:
    raise MyError("Custom error", original=e)

Then you can access .original when catching the exception.

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