# Python api handle exceptions with @fdb.transactional

**URL:** <https://forums.foundationdb.org/t/python-api-handle-exceptions-with-fdb-transactional/2919>\
**Category:** Using FoundationDB\
**Created:** [September 30, 2021, 3:46pm UTC](https://forums.foundationdb.org/t/python-api-handle-exceptions-with-fdb-transactional/2919 "2021-09-30T15:46:28Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![laita](https://avatars.discourse-cdn.com/v4/letter/l/77aa72/32.png) [@laita](https://forums.foundationdb.org/u/laita)\
**Post date:** [September 30, 2021, 3:46pm UTC](https://forums.foundationdb.org/t/python-api-handle-exceptions-with-fdb-transactional/2919/1 "2021-09-30T15:46:28Z")

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Hello,  
I am a new user of foundationdb and using it now in our projects. I wonder about using fdb.transactional decorator and handling errors and exceptions.

I wonder if I am missing something is the following example of code (not the final code):

```auto
def bank_account(self, params):
      self._fdb_create_accoun(self.db, param)

@fdb.transactional
def _fdb_create_account(tr, param)
     tr['account_id'] = params['name']
     tr['date'] = params['date']

```

I know that decorator includes a try/except but what happen if transaction failed and what error to handle in caller method bank\_account ?

Thank you for advices

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**Author:** ![ajbeamon](https://sea1.discourse-cdn.com/foundationdb/user_avatar/forums.foundationdb.org/ajbeamon/32/13_2.png) [@ajbeamon](https://forums.foundationdb.org/u/ajbeamon)\
**Post date:** [September 30, 2021, 7:30pm UTC](https://forums.foundationdb.org/t/python-api-handle-exceptions-with-fdb-transactional/2919/2 "2021-09-30T19:30:06Z")

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The transaction decorator will cause the transaction to automatically retry certain FDB errors. If your function throws a non-FDB error, it would bubble out to `bank_account`. Additionally, any non-retryable FDB errors will be thrown, as well as any retryable error if you exceed your configured retry limit (based on a transaction option). FDB errors will be of type `fdb.FDBError`.

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<div class="post-metadata">

**Author:** ![amirouche](https://sea1.discourse-cdn.com/foundationdb/user_avatar/forums.foundationdb.org/amirouche/32/2096_2.png) [@amirouche](https://forums.foundationdb.org/u/amirouche)\
**Post date:** [October 10, 2021, 7:30am UTC](https://forums.foundationdb.org/t/python-api-handle-exceptions-with-fdb-transactional/2919/3 "2021-10-10T07:30:47Z")

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I second what ajbeamon wrote. Any user exception will not be catched and raise as usual. See

> <https://github.com/apple/foundationdb/blob/0cce7743255f539f7811aa0776e9cf625c7771e4/bindings/python/fdb/impl.py#L272-L281>

Line 280, the `except FDBError` will only catch FoundationDB errors.

`tr.on_error(...)` might raise a FDB error in case the transaction was retried too many times or that is a non-retryable error (e.g. transaction too large ?), and which case the `while not commited` will stop. Just reading the Python code it seems that the transaction might retry indefinitly, but it is never the case.

By the way, given the fact that a transaction can be retried, you must make sure that the decorated function is **idempotent** e.g. you can’t send an email inside the transaction function and expect that it will only be sent only-once.

(On a somewhat related note, I am wondering whether the retry logic, should not be the responsability of the bindings instead of the C driver, in some cases you do not want to retry too much time if the transaction happens during an end-user HTTP request-response. One way to workaround it would be to have an “effort” or “timeout” in milliseconds provided by the user, and fallback to another approach in case the transaction can not succeed because of competition. Now that I wrote that, I figured it can be implemented in custom bindings or alternative `@transactional` decorator)