CODEWITHSUNDEEP
python
xrisk
xrisk

Reputation: 3898

How to make a function return nothing?

I have a function called crawl which will return a link to a website. Then I do something like:

found.append(crawl()) (found is a list)

This works fine as long as crawl returns a valid link, but sometimes it does not return anything. So a value of None gets added to the list.

So my question is that, is it possible to return something from crawl that will not not add anything to the list?

Upvotes: 2

Views: 2051

Answers (4)

Eric Appelt
Eric Appelt

Reputation: 2973

If a function returns it has to return an object, even if that object is None. However, there is another answer being overlooked, and that is to raise an exception rather than returning None.

As other people point out, one approach is to check if the returned object is None before appending it to the list:

link = crawl()
if link is not None:
    found.append(link)

The other approach is to define some exception, perhaps WebsiteNotFoundError, and have crawl execute raise WebsiteNotFoundError instead of return None. Then you can write:

try:
    found.append(crawl())
except WebsiteNotFoundError:
    pass  # or take appropriate action

The advantage of the exception handling approach is that it is generally faster than checking for None if returning None is a relatively rare occurrence compared to returning a valid link. Depending on the use, it may be more readable in the sense that the code naturally explains what is going wrong with the function.

Upvotes: 1

Steve Barnes
Steve Barnes

Reputation: 28370

What you could do is to pass the list in to the crawl function and if there is anything to add append it otherwise not. Something like:

def crawl(found):
   """ Add results to found list """
   # do your stuff saving to result
   if result is not None:
       found.append(result)

# Call this as 
   crawl(found)

Upvotes: 2

Jay Bosamiya
Jay Bosamiya

Reputation: 3209

This is not possible directly.

You can test for a None being returned though, using the following code.

returned_link = crawl()
if returned_link is not None:
    found.append(returned_link)

Upvotes: 1

Daniel
Daniel

Reputation: 42758

In Python nothing is something: None. You have to use if-condition:

link = crawl()
if link is not None:
    found.append(link)

or let crawl return a list, which can contain one or zero elements:

found.extend(crawl())

Upvotes: 10

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