REST API helpers

There are a number of helpers that make building out the REST API easier.

Etags

HTTP etags are a way for clients to decide whether their copy of a resource has changed or not. Mailman’s REST API calculates this in a cheap and dirty way. Pass in the dictionary representing the resource and that dictionary gets modified to contain the etag under the http_etag key.

>>> from mailman.rest.helpers import etag
>>> resource = dict(geddy='bass', alex='guitar', neil='drums')
>>> json_data = etag(resource)
>>> print(resource['http_etag'])
"6929ecfbda2282980a4818fb75f82e812077f77a"

For convenience, the etag function also returns the JSON representation of the dictionary after tagging, since that’s almost always what you want.

>>> import json
>>> data = json.loads(json_data)

# This is pretty close to what we want, so it's convenient to use.
>>> dump_msgdata(data)
alex     : guitar
geddy    : bass
http_etag: "6929ecfbda2282980a4818fb75f82e812077f77a"
neil     : drums

POST and PUT unpacking

Another helper unpacks POST and PUT request variables, validating and converting their values.

>>> from mailman.rest.validator import Validator
>>> validator = Validator(one=int, two=str, three=bool)

>>> class FakeRequest:
...     params = None
...     content_type = 'application/x-www-form-urlencoded'
>>> FakeRequest.params = dict(one='1', two='two', three='yes')

On valid input, the validator can be used as a **keyword argument.

>>> def print_request(one, two, three):
...     print(repr(one), repr(two), repr(three))
>>> print_request(**validator(FakeRequest))
1 'two' True

On invalid input, an exception is raised.

>>> FakeRequest.params['one'] = 'hello'
>>> print_request(**validator(FakeRequest))
Traceback (most recent call last):
...
ValueError: Invalid Parameter "one": invalid literal for int() with base 10: 'hello'.

On missing input, an exception is raised.

>>> del FakeRequest.params['one']
>>> print_request(**validator(FakeRequest))
Traceback (most recent call last):
...
ValueError: Missing Parameter: one

If more than one key is missing, it will be reflected in the error message.

>>> del FakeRequest.params['two']
>>> print_request(**validator(FakeRequest))
Traceback (most recent call last):
...
ValueError: Missing Parameter: one, two

Extra keys are also not allowed.

>>> FakeRequest.params = dict(one='1', two='two', three='yes',
...                           four='', five='')
>>> print_request(**validator(FakeRequest))
Traceback (most recent call last):
...
ValueError: Unexpected parameters: five, four

However, if optional keys are missing, it’s okay.

>>> validator = Validator(one=int, two=str, three=bool,
...                       four=int, five=int,
...                       _optional=('four', 'five'))

>>> FakeRequest.params = dict(one='1', two='two', three='yes',
...                           four='4', five='5')
>>> def print_request(one, two, three, four=None, five=None):
...     print(repr(one), repr(two), repr(three), repr(four), repr(five))
>>> print_request(**validator(FakeRequest))
1 'two' True 4 5

>>> del FakeRequest.params['four']
>>> print_request(**validator(FakeRequest))
1 'two' True None 5

>>> del FakeRequest.params['five']
>>> print_request(**validator(FakeRequest))
1 'two' True None None

But if the optional values are present, they must of course also be valid.

>>> FakeRequest.params = dict(one='1', two='two', three='yes',
...                           four='no', five='maybe')
>>> print_request(**validator(FakeRequest))
Traceback (most recent call last):
...
ValueError: Invalid Parameter "five": invalid literal for int() with base 10: 'maybe'. Invalid Parameter "four": invalid literal for int() with base 10: 'no'.

Arrays

Some POST forms include more than one value for a particular key. This is how lists and arrays are modeled. The validator does the right thing with such form data. Specifically, when a key shows up multiple times in the form data, a list is given to the validator.

# We can't use a normal dictionary because we'll have multiple keys, but
# the validator only wants to call .items() on the object.
>>> class MultiDict:
...     def __init__(self, *params): self.values = list(params)
...     def items(self): return iter(self.values)
>>> form_data = MultiDict(
...     ('one', '1'),
...     ('many', '3'),
...     ('many', '4'),
...     ('many', '5'),
...     )

This is a validation function that ensures the value is a list.

>>> def must_be_list(value):
...     if not isinstance(value, list):
...         raise ValueError('not a list')
...     return [int(item) for item in value]

This is a validation function that ensure the value is not a list.

>>> def must_be_scalar(value):
...     if isinstance(value, list):
...         raise ValueError('is a list')
...     return int(value)

And a validator to pull it all together.

>>> validator = Validator(one=must_be_scalar, many=must_be_list)
>>> FakeRequest.params = form_data
>>> values = validator(FakeRequest)
>>> print(values['one'])
1
>>> print(values['many'])
[3, 4, 5]

The list values are guaranteed to be in the same order they show up in the form data.

>>> FakeRequest.params = MultiDict(
...     ('one', '1'),
...     ('many', '3'),
...     ('many', '5'),
...     ('many', '4'),
...     )
>>> values = validator(FakeRequest)
>>> print(values['one'])
1
>>> print(values['many'])
[3, 5, 4]

PATCH Unpacking

PATCH requests are different from PUT and POST because with the latter, you’re changing the entire resource, so all expected attributes must exist. With the former, you’re only changing a subset of the attributes, so you only validate the ones that exist in the request.

>>> from mailman.rest.validator import PatchValidator
>>> from mailman.rest.helpers import GetterSetter
>>> values = dict(one=GetterSetter(int),
...               two=GetterSetter(str),
...               three=GetterSetter(bool))
>>> FakeRequest.params = dict(one=1)
>>> validator = PatchValidator(FakeRequest, values)

PatchValidator can be used to update the attributes of an object directly:

>>> class FakeObject:
...     one = 2
>>> fakeobj = FakeObject()
>>> validator.update(fakeobj, FakeRequest)
>>> print(fakeobj.one)
1

JSON Unpacking

Request can optionally consist of JSON body as parameters. If the Content-Type header is set to application/json, request’s body is parsed to set request.media as a dict object

>>> validator = Validator(one=int, two=str, three=bool)

>>> class FakeRequest:
...     params = None
...     content_type = 'application/json'
...     media = None
>>> FakeRequest.media = dict(one='1', two='two', three='yes')

On valid input, the validator can be used as a **keyword argument.

>>> def print_request(one, two, three):
...     print(repr(one), repr(two), repr(three))
>>> print_request(**validator(FakeRequest))
1 'two' True