CODEWITHSUNDEEP
pythondjango
Chase Roberts
Chase Roberts

Reputation: 9376

Python django how to properly test if a queryset returned a result

I haven't had a very thorough training in python and sometimes don't know the correct way of doing things. One of these things is testing if my resultQuery returned a result or not. I find myself doing this a lot:

    try:
        user = User.objects.get(email=email)
    except DoesNotExist:
        user = User()

I don't know about python but try catches in other languages are supposed to be for exceptions and not for normal program flow. How would you do this with an if else? I guess I want something similar to:

if request.GET.get('email','') is not None:
    email = request.GET['email'])

Upvotes: 5

Views: 2401

Answers (4)

Lorenz Lo Sauer
Lorenz Lo Sauer

Reputation: 24730

Django defines a class EmptyQuerySet that can be used for special implementations when strict isinstance(...) testing is necessary.

A straightforward way is as follows:

try:
    user = User.objects.get(email=email)
    if  user:
        # ...
    else:
        # ...

except DoesNotExist:
    user = User()

Django provides a get method to directly retrieve one single Model instance or will raise an Exception. In this case the normal way is to check for Exceptions such as DoesNotExist

To retrieve entire rows from a database query as you normally would, use the user.objects.filter(...) method. The returned QuerySet instance provides you with .count(), .exists() methods.

Upvotes: 0

Peter DeGlopper
Peter DeGlopper

Reputation: 37319

I agree that catching the exception is the normal pattern here. If you do need to check for whether or not a queryset returns results, the exists method is one way to do so that I don't think has been mentioned yet. The docs run through the various performance implications, most notably that if you know the queryset will be evaluated it's better to just check the boolean interpretation of the queryset.

Upvotes: 0

jro
jro

Reputation: 9474

Your exception example is most commonly the preferred way to do things. Wikipedia has quite a nice description about this:

Python style calls for the use of exceptions whenever an error condition might arise. Rather than testing for access to a file or resource before actually using it, it is conventional in Python to just go ahead and try to use it, catching the exception if access is rejected.

Exceptions are often used as an alternative to the if-block (...). A commonly-invoked motto is EAFP, or "It is Easier to Ask for Forgiveness than Permission."

Exceptions aren't necessarily costly in Python. Again, quoting the Wikipedia example:

if hasattr(spam, 'eggs'):
    ham = spam.eggs
else:
    handle_error()

... versus:

try:
    ham = spam.eggs
except AttributeError:
    handle_error()

These two code samples have the same effect, although there will be performance differences. When spam has the attribute eggs, the EAFP sample will run faster. When spam does not have the attribute eggs (the "exceptional" case), the EAFP sample will run slower.

Specifically for Django, I would use the exception example. It is the standard way to do things in that framework, and following standards is never a bad thing :).

Upvotes: 4

Rohan
Rohan

Reputation: 53326

You can use .filter() instead of .get(), and also make use of django's optimized queryset evaluation.

You can do

qs = User.objects.filter(email=email)
if qs :
    user = qs[0]
...

Upvotes: 1

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