Write Pythonic Code Like a Seasoned Developer Transcripts
Chapter: Foundational Concepts
Lecture: Testing for None
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0:01
Next let's look at testing for a special case. Imagine that we are going to call a function and that might return a list
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and that list might have items in it or it might not, maybe we are trying to do some kind of search and there is just no results,
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and yet, if it was an unable to perform a search, we'd like to indicate that differently than if there is just no results.
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In that case, we can actually change our algorithm just a little bit and test for something different. Let's look at PyCharm.
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So here I have this function called find_accounts and you give it some search text;
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it checks to see if the database was available and if it's not available, it returns None, which we know is False.
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Otherwise, it's going to actually do a search against our database and return a list of account ids. However, there might not be any matching results
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in which case that list could be empty, so you might be tempted to say "if not accounts", then maybe I want to just like print something out like this,
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"else", let's actually try to say we are going to print the accounts. This part is going to work fine if there are results, we would list them.
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However, it could be that our search just returns no results, in which case this list would evaluate the False,
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so we are going to change this, we are going to test actually if accounts is None, when you are testing against singletons, and None is a singleton,
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there is only one instance of it per process, we'll use "is" and that actually compares at the pointers, not just the inherit truthiness of the objects
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or some kind of overloaded comparison operator. So now our code is going to work as expected,
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if we literally get nothing back because the DB is unavailable, then we can say oh, DB is not available, otherwise, we might print them out.
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And of course, that set might be empty, we might want to put in additional testing here to say
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"well, if it's empty" like your search works but there is no results, whatever. But the Pythonic thing here is to notice that we are using "is"
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to test against singleton and a special case of singletons of course is the None keyword, which is one of the most common.
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So here we have this in a graphic, so we are calling find_accounts, giving it some search text and we are going to get some results back
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and we want to make sure that not only is it truthy, but it's actually worth specifically testing that there is something we got back
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rather than a None pointer, so "if accounts is not None", and notice, when you negate "is", you say "is not",
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rather "than not account is", which would also work but is less Pythonic.