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© 2019 Bloomberg Finance L.P. All rights reserved. Exceptional Exceptions: How to properly raise, handle and create them EuroPython 2019 July 11, 2019 Mario Corchero @mariocj89

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Let’s just raise an exception, right?

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Let’s just raise an exception, right?

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Let’s go step by step

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. How to capture an exception

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. What can go in the except?

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. What can go in the except?

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Using finally

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Using else

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Order of execution?

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. What about returning in the middle?

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Logging exceptions Use exc_info to include exception information

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Logging exceptions

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Logging exceptions Errors should never pass silently. Unless explicitly silenced.

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Logging exceptions

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Logging exceptions But... When I have...

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Logging exceptions But... What if I know...

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Logging exceptions

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Logging exceptions

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Logging exceptions

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Logging exceptions

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Scoping of the except See PEP 3110. In short: exception -> traceback -> stack frame -> exception

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Raise it!

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. The tricky, raise within except

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© 2018 Bloomberg Finance L.P. All rights reserved. The tricky, raise within except

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Reraise

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© 2018 Bloomberg Finance L.P. All rights reserved. Reraise

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Reraise

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© 2018 Bloomberg Finance L.P. All rights reserved. Reraise

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Chaining exceptions

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© 2018 Bloomberg Finance L.P. All rights reserved. Chaining exceptions

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Eliding previous exception

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Eliding previous exception

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. False friends: NotImplemented Passes without printing. 37,762 hits in GitHub as of July 2019.

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Raise on a thread?

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© 2018 Bloomberg Finance L.P. All rights reserved. Raise on a thread?

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. The exception hierarchy Check https://docs.python.org/3/library/exceptions.html

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Exception attributes

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Exception attributes

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Exception attributes

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Creating new classes

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Creating new classes

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Exception hierarchy

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Multiple inheritance • Decimal: • Requests:

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Can I use Error codes?

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Conclusions

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Extra content!

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. New magic in 3.8

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. New magic in 3.8

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© 2019 Bloomberg Finance L.P. All rights reserved. Cycles when reraising

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© 2019 Bloomberg Finance L.P. All rights reserved. Cycles when reraising

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Returns in finally

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Returns in finally

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© 2018 Bloomberg Finance L.P. All rights reserved. © 2019 Bloomberg Finance L.P. All rights reserved. Returns in finally

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© 2019 Bloomberg Finance L.P. All rights reserved. Thank you! Questions?? Also, ask me about travelling to Extremadura!