CODEWITHSUNDEEP
pythondatabaseexceptiontornado
bazzilic
bazzilic

Reputation: 828

Correct way to handle exceptions when working with database

When I am executing a simple query (I am using tornado.database module) I handle exceptions like this:

try:
    self.application.db.execute('DELETE FROM table_one WHERE a = 1')
except Exception, e:
    logging.warning('DB exception: %s' % e)
    self.set_status(500)
    return

If I want to do a transaction though, it would be smart to rollback it in case of exception:

try:
    self.application.db.execute('START TRANSACTION')
    self.application.db.execute('DELETE FROM table_one WHERE a = 1')
    self.application.db.execute('DELETE FROM table_two WHERE b = 2')
    self.application.db.execute('COMMIT')
except Exception, e:
    logging.warning('DB exception: %s' % e)
    self.set_status(500)
    self.application.db.execute('ROLLBACK')
    return

But what if rollback will cause an exception too (e.g. if connection fails)? Do I need to put a nested try-except block inside of the except block?

Upvotes: 1

Views: 12219

Answers (2)

Muhammad Soliman
Muhammad Soliman

Reputation: 23876

better to check recommendations from here

according to PEP, main points I would like to highlight here:

  • don't catch on general Exception
  • don't raise general exceptions

Upvotes: 0

Matei Florescu
Matei Florescu

Reputation: 1195

Placing a nested try ... except block in except block is a solution. But I would go for using finally:

try:
  ...
except ... :

finally:
   # cleanup (close the connection, etc...)

I mean if the rollback failed, there is pretty much nothing else to do then log the exception and cleanup, right?

Upvotes: 2

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