Skip to main content

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.


Anonymous 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.
John Speno said…
I like it.
EY 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
jjinux 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.

Popular posts from this blog

Drawing Sierpinski's Triangle in Minecraft Using Python

In his keynote at PyCon, Eben Upton, the Executive Director of the Rasberry Pi Foundation, mentioned that not only has Minecraft been ported to the Rasberry Pi, but you can even control it with Python. Since four of my kids are avid Minecraft fans, I figured this might be a good time to teach them to program using Python. So I started yesterday with the goal of programming something cool for Minecraft and then showing it off at the San Francisco Python Meetup in the evening.

The first problem that I faced was that I didn't have a Rasberry Pi. You can't hack Minecraft by just installing the Minecraft client. Speaking of which, I didn't have the Minecraft client installed either ;) My kids always play it on their Nexus 7s. I found an open source Minecraft server called Bukkit that "provides the means to extend the popular Minecraft multiplayer server." Then I found a plugin called RaspberryJuice that implements a subset of the Minecraft Pi modding API for Bukkit s…

Apple: iPad and Emacs

Someone asked my boss's buddy Art Medlar if he was going to buy an iPad. He said, "I figure as soon as it runs Emacs, that will be the sign to buy." I think he was just trying to be funny, but his statement is actually fairly profound.

It's well known that submitting iPhone and iPad applications for sale on Apple's store is a huge pain--even if they're free and open source. Apple is acting as a gatekeeper for what is and isn't allowed on your device. I heard that Apple would never allow a scripting language to be installed on your iPad because it would allow end users to run code that they hadn't verified. (I don't have a reference for this, but if you do, please post it below.) Emacs is mostly written in Emacs Lisp. Per Apple's policy, I don't think it'll ever be possible to run Emacs on the iPad.

Emacs was written by Richard Stallman, and it practically defines the Free Software movement (in a manner of speaking at least). Stal…

JavaScript: Porting from react-css-modules to babel-plugin-react-css-modules (with Less)

I recently found a bug in react-css-modules that prevented me from upgrading react-mobx which prevented us from upgrading to React 16. Then, I found out that react-css-modules is "no longer actively maintained". Hence, whether I wanted to or not, I was kind of forced into moving from react-css-modules to babel-plugin-react-css-modules. Doing the port is mostly straightforward. Once I switched libraries, the rest of the port was basically:
Get ESLint to pass now that react-css-modules is no longer available.Get babel-plugin-react-css-modules working with Less.Get my Karma tests to at least build.Get the Karma tests to pass.Test things thoroughly.Fight off merge conflicts from the rest of engineering every 10 minutes ;) There were a few things that resulted in difficult code changes. That's what the rest of this blog post is about. I don't think you can fix all of these things ahead of time. Just read through them and keep them in mind as you follow the approach above.…