Python lazy multiple conditions check

Question

I have a lot of conditions to check, but condition evaluation is heavy (e.g. condition requires database access), so I have to check them lazily.

Normally, such check could be written in if clause:

if type in FOOD_PRIZES and Prize.objects.filter(type=type).exists():
    pass

If the number of conditions are increasing then if clause becomes ugly.

I can make list of condition lambdas and use all method, but it looks ugly too:

conditions = [
  lambda: type in FOOD_PRIZES,
  lambda: Prize.objects.filter(type=type).exists()
] 

if all(condition() for condition in conditions):
   pass

Is there a better way to make code shorter? Is there another ways to make conditions lazy?

This doesn't seem to be a python question since you state that the issue is due to query time. In that case, the python code makes no difference; it's a micro optimisation. — roganjosh, Feb 03 '18 at 16:32
@vaultah I used `a == b` as an example. Condition could be something heavy like Django `exists` method. — potykion, Feb 03 '18 at 16:32
Given that your objection is that your solution *"looks ugly"* and you ask for one that *"looks better"*, which is wholly based on your opinion, how do you even expect this to get answered? — jonrsharpe, Feb 03 '18 at 16:33
@jonrsharpe OK, is there a way to make code shorter that list of lambdas with `all` call? — potykion, Feb 03 '18 at 16:36
@jonrsharpe I like to write less code, but since shorter code does not mean better code, then my question is not about code style. Sorry for the confusion. — potykion, Feb 03 '18 at 17:03

score 0 · Accepted Answer · edited Feb 03 '18 at 17:34

Your best bet is to continue what you're doing — but put your fastest conditions to check first.

all() will short-circuit, meaning that as soon as a condition evaluates to False, it will stop processing conditions, saving you the time you would expend by running the other queries.

As for what looks best — that's up to you. But it's far better to have effective, readable code than "attractive" code! And short code isn't always better. Verbosity often makes code more readable to others.

Just be careful. For example, if subsequent conditions are dependent on the first, using all can break. For example, given x='6.5', if isinstance(x, float) and x>5.5 would work but all((isinstance(x, float), x>5.5)) would error.

Python lazy multiple conditions check

1 Answers1