Python test if object exists

Question

I consider it bad style to use try: except: for flow control, but I can't figure out how to write the following code to test if a DB field in Django exists. This is the "dirty" code which works:

@receiver(pre_save, sender=UserProfile)
def create_user_profile(sender, instance=None, **kwargs):

    try:
        print str(instance.bank_account)
    except:
        print 'No account'

I would rather wanna do something like this, but I get an exception when the if statement is run and the object does not exist:

@receiver(pre_save, sender=UserProfile)
def create_user_profile(sender, instance=None, **kwargs):

    if instance.bank_account is None:
        print 'No account'
    else:
        print str(instance.bank_account)

Out of curiosity: are you following a project guideline or is it a personal preference? I'd consider your unqualified except bad style, but not if you used except AttributeError. I'd much prefer your first example to your second then. — Eduardo Ivanec
– Eduardo Ivanec, Commented Mar 13, 2012 at 10:55
Python is a language that encourages the 'ask for forgiveness, not permission' mantra. Unless you think it's more likely that the account won't exist most of the time, just catch a (specific) exception. docs.python.org/glossary.html#term-eafp — Gareth Latty
– Gareth Latty, Commented Mar 13, 2012 at 11:16

roam · Accepted Answer · 2012-03-13 11:06:44Z

3

I'm guessing you encounter a BankAccount.DoesNotExist error? If that's the case, you could reverse the relationship between UserProfile and BankAccount. Have a look at Django ticket #10227 to find out what's going on here.

That being said, if I needed to work with your code I'd rather see an explicit try...except instead of a workaround, unless that workaround actually makes sense.

answered Mar 13, 2012 at 11:06

roam

1,3459 silver badges8 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

j7nn7k Over a year ago

That ticket is describing my exact problem. Thx. I will use try: except DoesNotExist:

Eduardo Ivanec · Accepted Answer · 2012-03-13 10:57:50Z

2

I much prefer your first example to your second, but perhaps you could use hasattr(instance,'bank_account') as a safeguard?

answered Mar 13, 2012 at 10:57

Eduardo Ivanec

11.9k2 gold badges41 silver badges42 bronze badges

Comments

Marcin · Accepted Answer · 2012-03-13 11:02:26Z

2

First of all, your first example is the preferred way of doing this (although, you might like to narrow down the except to just the exceptions you expect for this operation).

I imagine that your if statement is blowing up when instance is None; the solution is to add a test to your if statement, to test instance first:

if instance and instance.bank_account:
    print str(instance.bank_account)
else: print 'No account'

If you don't know if the field exists, you could replace instance.bank_account with getattr(instance, 'bank_account', None), which both retrieves the value, and deals with the possibility of the attribute not existing.

answered Mar 13, 2012 at 11:02

Marcin

50.1k18 gold badges137 silver badges207 bronze badges

Collectives™ on Stack Overflow

Python test if object exists

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related