Exception Chains in Python

TIL, that you can hide the exception context in Python.

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Saw somthing like this in some code today.

raise RuntimeError("Cannot divide by zero") from None

Had to do a bit of a search to understand it. Turns out it hides the exception context.

What does this mean?

Example 1/3: Simple Exception

try:
    1/0
except ZeroDivisionError as e:
    raise RuntimeError("Cannot divide by zero")

This gives us:

Traceback (most recent call last):
  File "divzero.py", line 2, in <module>
    1/0
    ~^~
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "divzero.py", line 4, in <module>
    raise RuntimeError("Cannot divide by zero")
RuntimeError: Cannot divide by zero

We get the raised exception and then the exception that caused the raised exception.

Example 2/3: Exception with Chain

try:
    1/0
except ZeroDivisionError as e:
    raise RuntimeError("Cannot divide by zero") from e

which gives:

Traceback (most recent call last):
  File "divzero.py", line 2, in <module>
    1/0
    ~^~
ZeroDivisionError: division by zero

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "divzero.py", line 4, in <module>
    raise RuntimeError("Cannot divide by zero") from e
RuntimeError: Cannot divide by zero

We get the raised exception that caused the re-raised exception.

Finally…

Example 3/3: Exception without a Chain

try:
    1/0
except ZeroDivisionError as e:
    raise RuntimeError("Cannot divide by zero") from None

Gives a much simpler traceback:

  File "divzero.py", line 4, in <module>
    raise RuntimeError("Cannot divide by zero") from None
RuntimeError: Cannot divide by zero

It’s just the raised exception - now we just see a ‘Runtime Error’, not a ‘ZeroDivisionError’.

This’ll be useful for raising exceptions that might be user-facing or in a library: nobody needs to know the details - just the resultant exception.

Is there a PEP for this?

Of course there’s a PEP for this!

PEP 0409 - It’s been available since 2012!