Re-raise exception with a different type and message, preserving existing information

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Python 3 introduced exception chaining (as described in PEP 3134). This allows, when raising an exception, to cite an existing exception as the “cause”:

try:
    frobnicate()
except KeyError as exc:
    raise ValueError("Bad grape") from exc

The caught exception (exc, a KeyError) thereby becomes part of (is the “cause of”) the new exception, a ValueError. The “cause” is available to whatever code catches the new exception.

By using this feature, the __cause__ attribute is set. The built-in exception handler also knows how to report the exception’s “cause” and “context” along with the traceback.

In Python 2, it appears this use case has no good answer (as described by Ian Bicking and Ned Batchelder). Bummer.

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