Should I use a class or dictionary?

A class in python is a dict underneath. You do get some overhead with the class behavior, but you won't be able to notice it without a profiler. In this case, I believe you benefit from the class because:

• All your logic lives in a single function
• It is easy to update and stays encapsulated
• If you change anything later, you can easily keep the interface the same

Why would you make this a dictionary? What's the advantage? What happens if you later want to add some code? Where would your __init__ code go?

Classes are for bundling related data (and usually code).

Dictionaries are for storing key-value relationships, where usually the keys are all of the same type, and all the values are also of one type. Occasionally they can be useful for bundling data when the key/attribute names are not all known up front, but often this a sign that something's wrong with your design.

Keep this a class.

Use a dictionary unless you need the extra mechanism of a class. You could also use a namedtuple for a hybrid approach:

>>> from collections import namedtuple
>>> request = namedtuple("Request", "environ request_method url_scheme")
>>> request
<class '__main__.Request'>
>>> request.environ = "foo"
>>> request.environ
'foo'

Performance differences here will be minimal, although I would be surprised if the dictionary wasn't faster.