Thursday, March 22, 2007

Python: Returning Multiple Things of Different Types

If you are a strong believer in statically typed languages, you won't want to read this post!

Sometimes it makes sense for a function to return multiple things. In such cases, it's common to just return them in a tuple: "return (count, new_obj)".

Sometimes you might want to return objects of different types based on whether an operation succeeded or not. For instance, if the operation was successful, you might return "(True, obj)". If it failed, you might return "(False, reason)". Often, you can use exceptions to handle this situation.

Sometimes you might want to return objects of different types based on arguments to the function. For instance, did the caller ask for the data in this format or that format?

Sometimes you'll want all of these variations at the same time. In such cases, I have found that using a dict for your return value is a good solution. For instance, here is a piece of a docstring that I wrote yesterday:
    Return a dict (which I often call "response") with one or more of the
following as appropriate:

This is a bool to indicate success.

If unsuccessful, this is a Pylons response object that your
controller can return. It will contain an appropriate error
message for the user.

Upon success, this is the file handle returned by
``urllib2.urlopen``. Per urllib2, it has an ``info()`` method
containing things like headers, etc.

Upon success, if you set ``parse_xml_document=True``, this will
contain the parsed xml_document. I'll take care of parsing errors
with an appropriate pylons_response for you.
The calling code then looks like this:
        server_response = talk_to_server(parse_xml_document=True)
if not server_response['successful']:
return server_response['pylons_response']
etag = server_response['file_handle'].info().getheader('ETag')
xml_document = server_response['xml_document']
If you read it out loud, the code "reads" easily, and yet it has the flexibility to contain all the different things I need to return. If I ask for something that isn't there, I get an exception, which is life as usual for a Python programmer.


Drew Perttula said...

I've seen this in a few places, but with the slight change that the return is a new type, and the keys are attributes. The benefits are:

1. slightly shorter syntax to access to the results:
server_response = talk(p)
xml_document = server_response.xml_document
# except if you give server_response a short name, then you might not even bother reassigning the xml_document value to its own variable name

2. clearer output when people print or inspect the result:
>>> print talk(p)
<server_response with some interesting summary of the pieces of the result>
>>> help(talk(p).xml_document) # might work if you use properties

Also, your (False, reason) example doesn't look very pythonic to me. I can't think of why you wouldn't use an exception, which itself might be annotated with some other attributes.

Pythonic Avocado said...

I like it.

Jason said...

And in Python 2.6, you'll also be able to use named tuples via collections.NamedTuple.

Anonymous said...

Why do you say strongly typed advocates should not ready your post?
I return proper values or error values all the time. In a completely strongly typed a and checked manner. And with simpler syntax. Just pick your language.
(Mine is Haskell.)

Simon Wittber said...

Actually, the text reads:

If you are a strong believer in statically typed languages, you won't want to read this post!

Strong != Static

Anonymous said...

What anonymous above is referring to is Haskell's Maybe datatype. See also the Either datatype just below that.

James Graves

Shannon -jj Behrens said...

I know about Maybe and Either. Thanks for the reminder, though.

When I said "If you are a strong believer in statically typed languages, you won't want to read this post!" I was referring to the fact that many Java programmers might get upset about the fact that I was using a dict in order to get around having to declare exactly what I was going to return.

Naturally, Maybe in Haskell is strongly typed, so it's a different ball of wax. Furthermore, in Haskell, it's quite natural to return a big list of stuff and do pattern matching on the return value.