StringList – Is It A String? Is It A List? Depends On How You Use It!

At my job, I had an API problem: I had a object property that could contain one or more strings. I assumed it would be a list, some of my users felt that ['value',] was harder to write than 'value'. I found myself making the same mistake. So, I solved the problem, then I took the rough class I wrote and polished it. It’s up on my github, at

So what is it?

It’s a class that tries to make a slick API when there can be one, or many values for a property. The class can be instantiated with either a single value or many. If you use many, it will act like a list. If you use a single value (a string), it will act like a string. If it’s a string, and you use the .append() method, it becomes a list. Bam.

How would I use it?

Like so (see the README for a more detailed example, and the tests for comprehensive usage):

class Thing(object):
    Arbitrary class representing a single thing with one or more roles.

    role = StringList()

    def __init__(self, role=None):
        self.role = role

# initialize with a single string value        
obj = Thing('single role')

# initialize with multiple string values
obj = Thing(('one', 'two', 'three'))

# set to a single string value
obj.roles = 'new role'

# set to many string values
obj.roles = 'primary', 'secondary'

# when it's a string, it works like a string
obj = Thing('role1')

obj.roles = obj.upper()

# convert to a list using .append()

# now its a list
obj.roles[1] == 'role2'


  • The module sports 100% test coverage.
  • There is a buildout in the repo. Just run python; bin/buildout and then you can run the tests with bin/nosetests
  • It was originally developed to avoid ['h', 'e', 'l', 'l', 'o'] mistakes when a single string was used instead of a list.
  • This is the first time I’ve used descriptors in Python. Very cool.
  • It would not have been possible without this marvelous post about Python’s magic methods (big thanks to Rafe Kettler).
  • Some things were not as straight-forward as they could have been, given Python’s string implementation. Of special interest is the implementation of the __delitem__ and __iter__ methods. In both cases, the base str class doesn’t have either method, so instead of doing the sane thing and proxying the call, I had to fall back to re-doing the operation in the method.
This entry was posted in Uncategorized and tagged , , , , , , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s