papi 0.1.0

Build RESTful APIs with minimal boilerplate

# papi

## Low-Boilerplate RESTful APIs

## Introduction

papi is a library that allows you to build powerful
[RESTful](https://en.wikipedia.org/wiki/Restful) web services on top of plain
WSGI by writing backends as simple and semantic classes, and then feeding them
to its equally simple WSGI wrapper function.

## Features

- Proper RESTful semantics over HTTP(S): GET, PUT, POST, DELETE map to
retrieve / list resources, create, update, delete
- Automatic routing
- Automatic [HATEOAS](https://en.wikipedia.org/wiki/HATEOAS) decoration (adds
links to parent, self, and children, on every JSON response)
- Semi-automatic content type negotiation: JSON is handled transparently, other
content types are easy to support in your backend code
- Automatic translations of failures to HTTP error responses; uses the 4xx
range of status codes correctly
- Runs on any compliant WSGI host, making it suitable for deployment under a
wide range of web servers and protocols
- Method override: fake unsupported HTTP methods through GET parameters or
headers

## Installing

Installing with pip:

```bash
pip install papi
```

You probably want to do this in a virtualenv.

## Conceptual Model

Papi's concept of a RESTful API is that of a tree-shaped data structure,
consisting of "leaf" nodes called "documents" and "branch" nodes called
"collections". Both are modelled as *resources*, and a resource can act as a
document, as a collection, or both. Documents have a body (potentially
available in multiple flavors, matching different MIME content types);
collections have child resources, and the library code maps this resource tree
onto a URL path structure. As RESTfulness has it, HTTP methods indicate the
kind of operation on this tree, and the HATEOAS philosophy is applied by
tagging documents and collections with metadata when possible.

## Usage

### Defining A Resource

To implement a working RESTful API, you need to define a root resource.
Resources can act as documents (having a body), collections (having child
resources), or hybrids (having both a body and child resources). For the root
resource, you almost certainly want a collection-style resource, otherwise your
API will only ever contain one document.

Note that `Resource` is not a base class, it's just an implicit interface. Papi
resolves method calls through duck typing, there is no need to inherit or
formally implement anything, just add the methods you need, and that's it.
Adding other methods is of course no problem at all.

The relevant methods for a resource are:

```python
def get_structured_body(self, digest=False)
def get_typed_body(self, mime_pattern)
```

Get the payload data for the resource itself; implementing these methods makes
the resource a document.

`get_typed_body` is always tried first; it should return a pair of `(mime_type,
body)` to indicate that a body is available that matches the `mime_pattern`, or
`None` to tell Papi that this MIME type cannot be satisfied.

For some "special" MIME types (currently only `text/json` and
`application/json`), the `get_structured_body` method is tried when
`get_typed_body` fails; this method is supposed to return a native Python data
structure. Currently, the only requirement is that the returned data must be
JSON-encodable, but in the future, other types may be supported (e.g. XML,
plain text, HTML, ...), so it's best to stick with "vanilla" data structures
that directly correspond to JSON types: `dict`, `list`, `tuple`, `int`,
`float`, `bool`, `str` and `None` are all safe to use, others might not. The
`digest` argument indicates whether the full body should be returned, or a
"digest" version that contains only the essential properties. `digest` will be
`True` when called on a child resource in a collection listing context, `False`
when the resource is requested directly.

The crux with these two methods is that **documents derived from
`get_typed_body` are never parsed, and no metadata is ever added**. This means
that if you want to have Papi add HATEOAS links and a list of child resources
to the response, you must implement `get_structured_body`, and if you also
implement `get_typed_body`, it must return `None` for at least the JSON content
types (and, in the future, any content type you want to have tagged with
metadata).

```python
def get_children(self, *args, **kwargs)
def get_child(self, name)
```

These methods need to be implemented for resources that act as collections.
`get_children` returns a list of `(name, resource)` pairs, and can take the
following arguments to alter its behavior:

- `offset`: the number of items to skip from the beginning of the list. Works
like Python's `x[offset:]` construct, or the `OFFSET` part in an SQL `LIMIT`
clause.
- `count`: the number of items to return, starting at the `offset` if provided.
Works like Python's `x[:count]` construct, or the `COUNT` part in an SQL
`LIMIT` clause.
- `page`: when `count` is specified, you can provide a page number instead of
an `offset`. Page numbers are 1-based, and each page contains `count`
entries, so `page=2, count=10` retrieves items 10 through 19.

`get_child` gets a single child resource; the `name` parameter, throughout
Papi's Python API, refers to a resource's primary key. We call it "name",
because ideally, it should be a somewhat descriptive, meaningful natural
identifier for the object it represents, which, when possible, is more in line
with the RESTful philosophy, and makes for naturally beautiful URIs.
`http://example.org/api/fruit/apples/granny_smith` is a much nicer URI than
`http://example.org/api/5d75e3/35b0bd/d68c481bb1f4`.

```python
def create(self, input, content_type=None)
def store(self, input, name, content_type=None)
def delete(self, name)
```

These methods can optionally be implemented to turn a readonly resource into a
writeable collection. Note that *all* write operations are defined on the
parent resource, even though at the HTTP level, some are exposed on the
resource itself - for example, `POST /root/child1` maps to the resource named
`"child1"` under the parent resource `"root"`, but the method that gets called
is the `store` method of the `root` resource. This is for two reasons: one, the
child resource to store may not exist yet (this is the case for `PUT`
requests), and two, the resource itself does not know its own name, nor does it
need to.

Some notes on these methods:

- The `input` argument will contain a file-like object, which means you can use
the usual `read()` etc. methods on it to extract the body. Parsing is your
own responsibility, Papi does not do this for you. Particularly, there is no
write equivalent to the `get_structured_body` method; however, processing
JSON documents is usually a simple matter of calling `json.loads`.
- The difference between `create` and `store` is that `create` must generate a
name for the received document, and return a `name, body` tuple (where `body`
is a digest that describes the document that has been created, in a
JSON-encodable data structure according to the same rules as
`get_structured_body`); multiple calls to `create` should create multiple
distinct documents, and return distinct names. Conceptually, `create`
*always* creates a new document. By contrast, `store` takes a document name
as an argument, so it does not generate one itself, and multiple calls with
the same name will overwrite one another. While `store` may also create new
documents (if the `name` does not exist yet), it should overwrite (update)
documents when the name already exists.

### Serving A Resource

Serving a resource is simple; the `serve_resource` function can be used to turn
a valid resource into a WSGI application, like this:

```python
def application(env, start_response):
return serve_resource(root_resource, env, start_response)
```

And from there, it's a matter of feeding that function to a WSGI server (see
the [WSGI documentation](https://wsgi.readthedocs.io/en/latest/) for details).

### Give It A Spin

The included example application (`example/app.py`) implements a simple
in-memory database that supports plain-text payloads for documents; all the
resources in it are read/write document/collection hybrids, which means that
data can be added at any point in the tree. Assuming that this application runs
in a WSGI server on localhost:5000, we can try a few requests (we'll use cURL
for these examples):

```bash
> curl 'http://localhost:5000/' # Fetch the root resource

{"_parent": {"href": "/"}, "_self": {"href": "/"}, "_items": [{"_parent":
{"href": "/"}, "_self": {"href": "/things"}, "_name": "things"}]}
```

That's not very readable, but we can use the `pretty` parameter to pretty-print
JSON output:

```bash
> curl 'http://localhost:5000/?pretty=1'
{
"_parent": {
"href": "/"
},
"_self": {
"href": "/"
},
"_items": [
{
"_parent": {
"href": "/"
},
"_self": {
"href": "/things"
},
"_name": "things"
}
]
}
```

This tells us a few things:

- The URI for this resource (`_self`) is `/`
- The URI for this resource's parent (`_parent`) is also '/' (this is actually
a misfeature currently; the root node should not actually report a parent)
- The resource contains child resources (`_items`)
- To be specific, it contains *one* child resource, named `things`, with a URI
of `/things`.

As you can see, this HATEOAS metadata makes the API fully discoverable; the
resource tells us its own location within the API, as well as those of its
parent and children.

Let's look at the child resource "things":

```bash
> curl 'http://localhost:5000/things/?pretty=1'
{
"_parent": {
"href": "/"
},
"_self": {
"href": "/things"
},
"_items": [
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/apple"
},
"_value": "I am an apple. Eat me.",
"_name": "apple"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/banana"
},
"_value": "I'll bend either way for you.",
"_name": "banana"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/nut"
},
"_value": "I'm nuts!",
"_name": "nut"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/onion"
},
"_value": "Hurt me, and I will make you cry.",
"_name": "onion"
}
],
"_name": "things"
}
```

Oh joy! What a bunch of things! And they're still fully HATEOAS-discoverable,
so let's see what happens when we try to fetch an onion:

```bash
> curl 'http://localhost:5000/things/onion/?pretty=1'
Hurt me, and I will make you cry.
```
That's weird. No JSON. Why is that? Right, content negotiation. Our example
resource supports `text/plain` as well as JSON; curl, by default, specifies
that it accepts `*/*`, that is, *anything*, and because Papi prefers "typed"
bodies over "structured" bodies, the first type that matches (which happens to
be `text/plain`) is what we get. If we were serving, say, images through our
API, this would be *exactly* the desired behavior. We can still request JSON
though, we just have to override the `Accept` header:

```bash
> curl 'http://localhost:5000/things/onion/?pretty=1' -H 'Accept: text/json'
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/onion"
},
"_value": "Hurt me, and I will make you cry.",
"_name": "onion"
}
```
All is well!

So far, we have only requested things that existed. Of course requesting
something that doesn't exist yields a 404 error; we'll use cURL's `-i` option
to show HTTP headers:

```bash
> curl 'http://localhost:5000/things/nope/?pretty=1' -i
HTTP/1.1 404 Not Found
Content-type: text/plain;charset=utf8

Not Found
```
That makes sense.

What happens if we request a content type that the resource doesn't support?

```bash
> curl 'http://localhost:5000/things/onion/?pretty=1' -i -H 'Accept: img/png'
HTTP/1.1 406 Not Acceptable
Content-type: text/plain;charset=utf8

Not Acceptable
```
It does the right thing.

So far we've only been *reading* from the API; let's try *writing* things.
According to standard RESTful procedures, we can create new documents by
using the HTTP `PUT` method:

```bash
> curl 'http://localhost:5000/things/potato' -XPUT -i -H 'Content-Type: text/plain'
HTTP/1.1 200 OK
Content-type: application/json

{"_parent": {"href": "/things"}, "_self": {"href": "/things/potato"}, "_value": "Slice me, dice me, fry me"}
```

The status code `200` indicates that the document was indeed created, and
fetching the `_self` URI confirms this:

```bash
> curl 'http://localhost:5000/things/potato/?pretty=1'
Slice me, dice me, fry me
```

And of course, this new document supports JSON as well:
```bash
> curl 'http://localhost:5000/things/potato/?pretty=1' -H 'Accept: text/json'
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/potato"
},
"_value": "Slice me, dice me, fry me",
"_name": "potato"
}
```

Note that if you want to access the API from a web browser, it will almost
certainly not support any HTTP methods other than `GET` and `POST` (plus a few
that we don't care much about here, such as `HEAD` and `OPTIONS`); `PUT` and
`DELETE`, in particular, will not work. Because of this, Papi has a method
override feature: if you add a `_method` parameter to the query string, or a
`X-Method-Override` header to the request, the value of that will override the
actual request method. So the following curl requests would all produce the
same behavior:

```bash
> curl 'http://localhost:5000/things/potato' -XPUT -i -H 'Content-Type: text/plain'
> curl 'http://localhost:5000/things/potato?_method=PUT' -XPOST -i -H 'Content-Type: text/plain'
> curl 'http://localhost:5000/things/potato' -XPOST -i -H 'X-Method-Override: PUT' -H 'Content-Type: text/plain'
```

An alternative way of creating new documents is using the HTTP method `POST` on
the *parent* resource, leaving the responsibility of generating a suitable
unique name for the new document to the parent resource. This is what that
looks like:

```bash
> curl 'http://localhost:5000/things?pretty=1' -XPOST -i -H 'Content-Type: text/plain' -d'Carrot on a stick'
HTTP/1.1 200 OK
Content-type: application/json

{"_parent": {"href": "/things"}, "_self": {"href": "/things/carrot"}, "_value": "Carrot on a stick"}
```

Our example resource is configured to generate names based on the first word of
the input, so that's what we get: `"carrot"`.

Other than the `PUT` method, however, `POST` will always create a new document,
rather than overwrite an existing one, so if we `POST` the same thing again,
the API is required to either deny the request with a `Conflict` response, or
create a new document with a different unique name. Our example application
opts for the second solution:

```bash
> curl 'http://localhost:5000/things?pretty=1' -XPOST -i -H 'Content-Type: text/plain' -d'Carrot on a stick'
HTTP/1.1 200 OK
Content-type: application/json

{"_parent": {"href": "/things"}, "_self": {"href": "/things/BL6yCijd8x4Mwzcf-carrot"}, "_value": "Carrot on a stick"}
```

As you can see, the name is disambiguated by prepending a random token. Listing
the `/things` resource shows that two documents have actually been created:

```bash
> curl 'http://localhost:5000/things?pretty=1' -H 'Accept: text/json'
{
"_parent": {
"href": "/"
},
"_self": {
"href": "/things"
},
"_items": [
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/BL6yCijd8x4Mwzcf-carrot"
},
"_value": "Carrot on a stick",
"_name": "BL6yCijd8x4Mwzcf-carrot"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/apple"
},
"_value": "I am an apple. Eat me.",
"_name": "apple"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/banana"
},
"_value": "I'll bend either way for you.",
"_name": "banana"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/carrot"
},
"_value": "Carrot on a stick",
"_name": "carrot"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/nut"
},
"_value": "I'm nuts!",
"_name": "nut"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/onion"
},
"_value": "Hurt me, and I will make you cry.",
"_name": "onion"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/potato"
},
"_value": "Slice me, dice me, fry me",
"_name": "potato"
}
],
"_name": "things"
}
```

And of course our example application also supports deleting items, using the
`DELETE` method:

```bash
> curl 'http://localhost:5000/things/potato/?pretty=1' -i -XDELETE
HTTP/1.1 204 No Content
Content-type: text/plain

```

Note the use of the `204 No Content` status line; since we've deleted a
resource, there is no meaningful content to return, all we get is an empty
success response. And to confirm that the potato has indeed been deleted:

```bash
> curl 'http://localhost:5000/things?pretty=1' -H 'Accept: text/json'
{
"_parent": {
"href": "/"
},
"_self": {
"href": "/things"
},
"_items": [
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/BL6yCijd8x4Mwzcf-carrot"
},
"_value": "Carrot on a stick",
"_name": "BL6yCijd8x4Mwzcf-carrot"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/apple"
},
"_value": "I am an apple. Eat me.",
"_name": "apple"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/banana"
},
"_value": "I'll bend either way for you.",
"_name": "banana"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/carrot"
},
"_value": "Carrot on a stick",
"_name": "carrot"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/nut"
},
"_value": "I'm nuts!",
"_name": "nut"
},
{
"_parent": {
"href": "/things"
},
"_self": {
"href": "/things/onion"
},
"_value": "Hurt me, and I will make you cry.",
"_name": "onion"
}
],
"_name": "things"
}
```