Versioning

Versioning will adhere to PEP 629 format (Major.Minor), which has defined the existing HTML responses to be 1.0. Since this PEP does not introduce new features into the API, rather it describes a different serialization format for the existing features, this PEP does not change the existing 1.0 version, and instead just describes how to serialize that into JSON.

Similar to PEP 629, the major version number MUST be incremented if any changes to the new format would result in no longer being able to expect existing clients to meaningfully understand the format.

Likewise, the minor version MUST be incremented if features are added or removed from the format, but existing clients would be expected to continue to meaningfully understand the format.

Changes that would not result in existing clients being unable to meaningfully understand the format and which do not represent features being added or removed may occur without changing the version number.

This is intentionally vague, as this PEP believes it is best left up to future PEPs that make any changes to the API to investigate and decide whether or not that change should increment the major or minor version.

Future versions of the API may add things that can only be represented in a subset of the available serializations of that version. All serializations version numbers, within a major version, SHOULD be kept in sync, but the specifics of how a feature serializes into each format may differ, including whether or not that feature is present at all.

It is the intent of this PEP that the API should be thought of as URL endpoints that return data, whose interpretation is defined by the version of that data, and then serialized into the target serialization format.

JSON Serialization

The URL structure from PEP 503 still applies, as this PEP only adds an additional serialization format for the already existing API.

The following constraints apply to all JSON serialized responses described in this PEP:

• All JSON responses will always be a JSON object rather than an array or other type.
• While JSON doesn’t natively support an URL type, any value that represents an URL in this API may be either absolute or relative as long as they point to the correct location. If relative, they are relative to the current URL as if it were HTML.
• Additional keys may be added to any dictionary objects in the API responses and clients MUST ignore keys that they don’t understand.
• All JSON responses will have a meta key, which contains information related to the response itself, rather than the content of the response.
• All JSON responses will have a meta.api-version key, which will be a string that contains the PEP 629 Major.Minor version number, with the same fail/warn semantics as defined in PEP 629.
• All requirements of PEP 503 that are not HTML specific still apply.

{
  "meta": {
    "api-version": "1.0"
  },
  "projects": [
    {"name": "Frob"},
    {"name": "spamspamspam"}
  ]
}

{
  "meta": {
    "api-version": "1.0"
  },
  "name": "holygrail",
  "files": [
    {
      "filename": "holygrail-1.0.tar.gz",
      "url": "https://example.com/files/holygrail-1.0.tar.gz",
      "hashes": {"sha256": "...", "blake2b": "..."},
      "requires-python": ">=3.7",
      "yanked": "Had a vulnerability"
    },
    {
      "filename": "holygrail-1.0-py3-none-any.whl",
      "url": "https://example.com/files/holygrail-1.0-py3-none-any.whl",
      "hashes": {"sha256": "...", "blake2b": "..."},
      "requires-python": ">=3.7",
      "dist-info-metadata": true
    }
  ]
}

Content-Types

This PEP proposes that all responses from the Simple API will have a standard content type that describes what the response is (a Simple API response), what version of the API it represents, and what serialization format has been used.

The structure of this content type will be:

application/vnd.pypi.simple.$version+format

Since only major versions should be disruptive to clients attempting to understand one of these API responses, only the major version will be included in the content type, and will be prefixed with a v to clarify that it is a version number.

Which means that for the existing 1.0 API, the content types would be:

• JSON: application/vnd.pypi.simple.v1+json
• HTML: application/vnd.pypi.simple.v1+html

In addition to the above, a special “meta” version is supported named latest, whose purpose is to allow clients to request the absolute latest version, without having to know ahead of time what that version is. It is recommended however, that clients be explicit about what versions they support.

To support existing clients which expect the existing PEP 503 API responses to use the text/html content type, this PEP further defines text/html as an alias for the application/vnd.pypi.simple.v1+html content type.

Version + Format Selection

Now that there is multiple possible serializations, we need a mechanism to allow clients to indicate what serialization formats they’re able to understand. In addition, it would be beneficial if any possible new major version to the API can be added without disrupting existing clients expecting the previous API version.

To enable this, this PEP standardizes on the use of HTTP’s Server-Driven Content Negotiation.

While this PEP won’t fully describe the entirety of server-driven content negotiation, the flow is roughly:

1. The client makes an HTTP request containing an Accept header listing all of the version+format content types that they are able to understand.
2. The server inspects that header, selects one of the listed content types, then returns a response using that content type (treating the absence of an Accept header as Accept: */*).
3. If the server does not support any of the content types in the Accept header then they are able to choose between 3 different options for how to respond:
   1. Select a default content type other than what the client has requested and return a response with that.
   2. Return a HTTP 406 Not Acceptable response to indicate that none of the requested content types were available, and the server was unable or unwilling to select a default content type to respond with.
   3. Return a HTTP 300 Multiple Choices response that contains a list of all of the possible responses that could have been chosen.
4. The client interprets the response, handling the different types of responses that the server may have responded with.

This PEP does not specify which choices the server makes in regards to handling a content type that it isn’t able to return, and clients SHOULD be prepared to handle all of the possible responses in whatever way makes the most sense for that client.

However, as there is no standard format for how a 300 Multiple Choices response can be interpreted, this PEP highly discourages servers from utilizing that option, as clients will have no way to understand and select a different content-type to request. In addition, it’s unlikely that the client could understand a different content type anyways, so at best this response would likely just be treated the same as a 406 Not Acceptable error.

This PEP does require that if the meta version latest is being used, the server MUST respond with the content type for the actual version that is contained in the response (i.e. A Accept: application/vnd.pypi.simple.latest+json request that returns a v1.x response should have a Content-Type of application/vnd.pypi.simple.v1+json).

The Accept header is a comma separated list of content types that the client understands and is able to process. It supports three different formats for each content type that is being requested:

• $type/$subtype
• $type/*
• */*

For the use of selecting a version+format, the most useful of these is $type/$subtype, as that is the only way to actually specify the version and format you want.

The order of the content types listed in the Accept header does not have any specific meaning, and the server SHOULD consider all of them to be equally valid to respond with. If a client wishes to specify that they prefer a specific content type over another, they may use the Accept header’s quality value syntax.

This allows a client to specify a priority for a specific entry in their Accept header, by appending a ;q= followed by a value between 0 and 1 inclusive, with up to 3 decimal digits. When interpreting this value, an entry with a higher quality has priority over an entry with a lower quality, and any entry without a quality present will default to a quality of 1.

However, clients should keep in mind that a server is free to select any of the content types they’ve asked for, regardless of their requested priority, and it may even return a content type that they did not ask for.

To aid clients in determining the content type of the response that they have received from an API request, this PEP requires that servers always include a Content-Type header indicating the content type of the response. This is technically a backwards incompatible change, however in practice pip has been enforcing this requirement so the risks for actual breakages is low.

An example of how a client can operate would look like:

import email.message
import requests

def parse_content_type(header: str) -> str:
    m = email.message.Message()
    m["content-type"] = header
    return m.get_content_type()

# Construct our list of acceptable content types, we want to prefer
# that we get a v1 response serialized using JSON, however we also
# can support a v1 response serialized using HTML. For compatibility
# we also request text/html, but we prefer it least of all since we
# don't know if it's actually a Simple API response, or just some
# random HTML page that we've gotten due to a misconfiguration.
CONTENT_TYPES = [
    "application/vnd.pypi.simple.v1+json",
    "application/vnd.pypi.simple.v1+html;q=0.2",
    "text/html;q=0.01",  # For legacy compatibility
]
ACCEPT = ", ".join(CONTENT_TYPES)


# Actually make our request to the API, requesting all of the content
# types that we find acceptable, and letting the server select one of
# them out of the list.
resp = requests.get("https://pypi.org/simple/", headers={"Accept": ACCEPT})

# If the server does not support any of the content types you requested,
# AND it has chosen to return a HTTP 406 error instead of a default
# response then this will raise an exception for the 406 error.
resp.raise_for_status()


# Determine what kind of response we've gotten to ensure that it is one
# that we can support, and if it is, dispatch to a function that will
# understand how to interpret that particular version+serialization. If
# we don't understand the content type we've gotten, then we'll raise
# an exception.
content_type = parse_content_type(resp.headers.get("content-type", ""))
match content_type:
    case "application/vnd.pypi.simple.v1+json":
        handle_v1_json(resp)
    case "application/vnd.pypi.simple.v1+html" | "text/html":
        handle_v1_html(resp)
    case _:
        raise Exception(f"Unknown content type: {content_type}")

If a client wishes to only support HTML or only support JSON, then they would just remove the content types that they do not want from the Accept header, and turn receiving them into an error.

TUF Support - PEP 458

PEP 458 requires that all API responses are hashable and that they can be uniquely identified by a path relative to the repository root. For a Simple API repository, the target path is the Root of our API (e.g. /simple/ on PyPI). This creates challenges when accessing the API using a TUF client instead of directly using a standard HTTP client, as the TUF client cannot handle the fact that a target could have multiple different representations that all hash differently.

PEP 458 does not specify what the target path should be for the Simple API, but TUF requires that the target paths be “file-like”, in other words, a path like simple/PROJECT/ is not acceptable, because it technically points to a directory.

The saving grace is that the target path does not have to actually match the URL being fetched from the Simple API, and it can just be a sigil that the fetching code knows how to transform into the actual URL that needs to be fetched. This same thing can hold true for other aspects of the actual HTTP request, such as the Accept header.

Ultimately figuring out how to map a directory to a filename is out of scope for this PEP (but it would be in scope for PEP 458), and this PEP defers making a decision about how exactly to represent this inside of PEP 458 metadata.

However, it appears that the current WIP branch against pip that attempts to implement PEP 458 is using a target path like simple/PROJECT/index.html. This could be modified to include the API version and serialization format using something like simple/PROJECT/vnd.pypi.simple.vN.FORMAT. So the v1 HTML format would be simple/PROJECT/vnd.pypi.simple.v1.html and the v1 JSON format would be simple/PROJECT/vnd.pypi.simple.v1.json.

In this case, since text/html is an alias to application/vnd.pypi.simple.v1+html when interacting through TUF, it likely will make the most sense to normalize to the more explicit name.

Likewise the latest metaversion should not be included in the targets, only explicitly declared versions should be supported.

Does this mean PyPI is planning to drop support for HTML/PEP 503?

No, PyPI has no plans at this time to drop support for PEP 503 or HTML responses.

While this PEP does give repositories the flexibility to do that, that largely exists to ensure that things like using the Endpoint Configuration mechanism is able to work, and to ensure that clients do not make any assumptions that would prevent, at some point in the future, gracefully dropping support for HTML.

The existing HTML responses incur almost no maintenance burden on PyPI and there is no pressing need to remove them. The only real benefit to dropping them would be to reduce the number of items cached in our CDN.

If in the future PyPI does wish to drop support for them, doing so would almost certainly be the topic of a PEP, or at a minimum a public, open, discussion and would be informed by metrics showing any impact to end users.

Why JSON instead of X format?

JSON parsers are widely available in most, if not every, language. A JSON parser is also available in the Python standard library. It’s not the perfect format, but it’s good enough.

Why not add X feature?

The general goal of this PEP is to change or add very little. We will instead focus largely on translating the existing information contained within our HTML responses into a sensible JSON representation. This will include PEP 658 metadata required for packaging tooling.

The only real new capability that is added in this PEP is the ability to have multiple hashes for a single file. That was done because the current mechanism being limited to a single hash has made it painful in the past to migrate hashes (md5 to sha256) and the cost of making the hashes a dictionary and allowing multiple is pretty low.

The API was generally designed to allow further extension through adding new keys, so if there’s some new piece of data that an installer might need, future PEPs can easily make that available.

Why include the filename when the URL has it already?

We could reduce the size of our responses by removing the filename key and expecting clients to pull that information out of the URL.

Currently this PEP chooses not to do that, largely because PEP 503 explicitly required that the filename be available via the anchor tag of the links, though that was largely because something had to be there. It’s not clear if repositories in the wild always have a filename as the last part of the URL or if they’re relying on the filename in the anchor tag.

It also makes the responses slightly nicer to read for a human, as you get a nice short unique identifier.

If we got reasonable confidence that mandating the filename is in the URL, then we could drop this data and reduce the size of the JSON response.

Why not break out other pieces of information from the filename?

Currently clients are expected to parse a number of pieces of information from the filename such as project name, version, ABI tags, etc. We could break these out and add them as keys to the file object.

This PEP has chosen not to do that because doing so would increase the size of the API responses, and most clients are going to require the ability to parse that information out of file names anyways regardless of what the API does. Thus it makes sense to keep that functionality inside of the clients.

Why Content Negotiation instead of multiple URLs?

Another reasonable way to implement this would be to duplicate the API routes and include some marker in the URL itself for JSON. Such as making the URLs be something like /simple/foo.json, /simple/_index.json, etc.

This makes some things simpler like TUF integration and fully static serving of a repository (since .json files can just be written out).

However, this is two pretty major issues:

• Our current URL structure relies on the fact that there is an URL that represents the “root”, / to serve the list of projects. If we want to have separate URLs for JSON and HTML, we would need to come up with some way to have two root URLs.

  Something like / being HTML and /_index.json being JSON, since _index isn’t a valid project name could work. But / being HTML doesn’t work great if a repository wants to remove support for HTML.

  Another option could be moving all of the existing HTML URLs under a namespace while making a new namespace for JSON. Since /<project>/ was defined, we would have to make these namespaces not valid project names, so something like /_html/ and /_json/ could work, then just redirect the non namespaced URLs to whatever the “default” for that repository is (likely HTML, unless they’ve disabled HTML then JSON).

• With separate URLs, there’s no good way to support zero configuration discovery that a repository supports the JSON URLs without making additional HTTP requests to determine if the JSON URL exists or not.

  The most naive implementation of this would be to request the JSON URL and fall back to the HTML URL for every single request, but that would be horribly performant and violate the goal of minimal additional HTTP requests.

  The most likely implementation of this would be to make some sort of repository level configuration file that somehow indicates what is supported. We would have the same namespace problem as above, with the same solution, something like /_config.json or so could hold that data, and a client could first make an HTTP request to that, and if it exists pull it down and parse it to learn about the capabilities of this particular repository.

• The use of Accept also allows us to add versioning into this field

All being said, it is the opinion of this PEP that those three issues combined make using separate API routes a less desirable solution than relying on content negotiation to select the most ideal representation of the data.

Does this mean that static servers are no longer supported?

In short, no, static servers are still (almost) fully supported by this PEP.

The specifics of how they are supported will depend on the static server in question. For example:

• S3: S3 fully supports custom content types, however it does not support any form of content negotiation. In order to have a server hosted on S3, you would have to use the “Endpoint configuration” style of negotiation, and users would have to configure their clients explicitly.
• GitHub Pages: GitHub pages does not support custom content types, so the S3 solution is not currently workable, which means that only text/html repositories would function.
• Apache: Apache fully supports server-driven content negotiation, and would just need to be configured to map the custom content types to specific extension.

Why not add an application/json alias like text/html?

This PEP believes that it is best for both clients and servers to be explicit about the types of the API responses that are being used, and a content type like application/json is the exact opposite of explicit.

The existence of the text/html alias exists as a compromise primarily to ensure that existing consumers of the API continue to function as they already do. There is no such expectation of existing clients using the Simple API with a application/json content type.

In addition, application/json has no versioning in it, which means that if there is ever a 2.x version of the Simple API, we will be forced to make a decision. Should application/json preserve backwards compatibility and continue to be an alias for application/vnd.pypi.simple.v1+json, or should it be updated to be an alias for application/vnd.pypi.simple.v2+json?

This problem doesn’t exist for text/html, because the assumption is that HTML will remain a legacy format, and will likely not gain any new features, much less features that require breaking compatibility. So having it be an alias for application/vnd.pypi.simple.v1+html is effectively the same as having it be an alias for application/vnd.pypi.simple.latest+html, since 1.x will likely be the only HTML version to exist.

The largest benefit to adding the application/json content type is that there do things that do not allow you to have custom content types, and require you to select one of their preset content types. The main example of this being GitHub Pages, which the lack of application/json support in this PEP means that static repositories will no longer be able to be hosted on GitHub Pages unless GitHub adds the application/vnd.pypi.simple.v1+json content type.

This PEP believes that the benefits are not large enough to add that content type alias at this time, and that its inclusion would likely be a footgun waiting for unsuspecting people to accidentally pick it up. Especially given that we can always add it in the future, but removing things is a lot harder to do.

Why add a application/vnd.pypi.simple.v1+html?

The PEP expects the HTML version of the API to become legacy, so one option it could take is not add the application/vnd.pypi.simple.v1+html content type, and just use text/html for that.

This PEP has decided that adding the new content type is better overall, since it makes even the legacy format more self describing and makes them both more consistent with each other. Overall I think it’s more confusing if the +html version doesn’t exist.

Why v1.0 and not v1.1 or v2.0?

This PEP is still wholly backwards compatible with clients that could read the existing v1.0 API, can still continue to read the API after these changes have been made. In PEP 629, the qualification for a major version bump is:

Incrementing the major version is used to signal a backwards incompatible change such that existing clients would no longer be expected to be able to meaningfully use the API.

The changes in this PEP do not meet that bar, nothing has changed in a way that existing clients would no longer be expected to be able to meaningfully use the API.

That means we should still be within the v1.x version line.

The question of whether we should be v1.1 or v1.0 is a more interesting one, and there are a few ways of looking at it:

• We’ve exposed new features to the API (the project name on the project page, multiple hashes), which is a sign that we should increment the minor version.
• The new features exist wholly within the JSON serialization, which means that no client that currently is requesting the HTML 1.0 page, would ever see any of the new features anyways, so for them it is effectively still v1.0.
• No major client has implemented support for PEP 629 yet, which means that the minor version numbering is largely academic at this point anyways, since it exists to let clients provide feedback to end users.

The second and third points above end up making the first point kind of meaningless, and with that, it makes more sense to just call everything v1.0 and be stricter about updating versions into the future.