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PEP 680 -- tomllib: Support for Parsing TOML in the Standard Library

PEP:680
Title:tomllib: Support for Parsing TOML in the Standard Library
Author:Taneli Hukkinen, Shantanu Jain <hauntsaninja at gmail.com>
Sponsor:Petr Viktorin <encukou at gmail.com>
Discussions-To:https://discuss.python.org/t/13040
Status:Draft
Type:Standards Track
Created:01-Jan-2022
Python-Version:3.11
Post-History:11-Jan-2022

Abstract

This PEP proposes adding the tomllib module to the standard library for parsing TOML (Tom's Obvious Minimal Language, https://toml.io).

Motivation

TOML is the format of choice for Python packaging, as evidenced by PEP 517, PEP 518 and PEP 621. This creates a bootstrapping problem for Python build tools, forcing them to vendor a TOML parsing package or employ other undesirable workarounds, and causes serious issues for repackagers and other downstream consumers. Including TOML support in the standard library would neatly solve all of these issues.

Further, many Python tools are now configurable via TOML, such as black, mypy, pytest, tox, pylint and isort. Many that are not, such as flake8, cite the lack of standard library support as a main reason why. Given the special place TOML already has in the Python ecosystem, it makes sense for it to be an included battery.

Finally, TOML as a format is increasingly popular (for the reasons outlined in PEP 518), with various Python TOML libraries having about 2000 reverse dependencies on PyPI (for comparison, requests has about 28000 reverse dependencies). Hence, this is likely to be a generally useful addition, even looking beyond the needs of Python packaging and related tools.

Rationale

This PEP proposes basing the standard library support for reading TOML on the third-party library tomli (github.com/hukkin/tomli).

Many projects have recently switched to using tomli, such as pip, build, pytest, mypy, black, flit, coverage, setuptools-scm and cibuildwheel.

tomli is actively maintained and well-tested. It is about 800 lines of code with 100% test coverage, and passes all tests in the proposed official TOML compliance test suite, as well as the more established BurntSushi/toml-test suite.

Specification

A new module tomllib will be added to the Python standard library, exposing the following public functions:

def load(
    fp: SupportsRead[bytes],
    /,
    *,
    parse_float: Callable[[str], Any] = ...,
 ) -> dict[str, Any]: ...

def loads(
    s: str,
    /,
    *,
    parse_float: Callable[[str], Any] = ...,
) -> dict[str, Any]: ...

tomllib.load deserializes a binary file-like object containing a TOML document to a Python dict. The fp argument must have a read() method with the same API as io.RawIOBase.read().

tomllib.loads deserializes a str instance containing a TOML document to a Python dict.

The parse_float argument is a callable object that takes as input the original string representation of a TOML float, and returns a corresponding Python object (similar to parse_float in json.load). For example, the user may pass a function returning a decimal.Decimal, for use cases where exact precision is important. By default, TOML floats are parsed as instances of the Python float type.

The returned object contains only basic Python objects (str, int, bool, float, datetime.{datetime,date,time}, list, dict with string keys), and the results of parse_float.

tomllib.TOMLDecodeError is raised in the case of invalid TOML.

Note that this PEP does not propose tomllib.dump or tomllib.dumps functions; see Including an API for writing TOML for details.

Maintenance Implications

Stability of TOML

The release of TOML 1.0.0 in January 2021 indicates the TOML format should now be officially considered stable. Empirically, TOML has proven to be a stable format even prior to the release of TOML 1.0.0. From the changelog, we can see that TOML has had no major changes since April 2020, and has had two releases in the past five years (2017-2021).

In the event of changes to the TOML specification, we can treat minor revisions as bug fixes and update the implementation in place. In the event of major breaking changes, we should preserve support for TOML 1.x.

Maintainability of proposed implementation

The proposed implementation (tomli) is pure Python, well tested and weighs in at under 1000 lines of code. It is minimalist, offering a smaller API surface area than other TOML implementations.

The author of tomli is willing to help integrate tomli into the standard library and help maintain it, as per this post. Furthermore, Python core developer Petr Viktorin has indicated a willingness to maintain a read API, as per this post.

Rewriting the parser in C is not deemed necessary at this time. It is rare for TOML parsing to be a bottleneck in applications, and users with higher performance needs can use a third-party library (as is already often the case with JSON, despite Python offering a standard library C-extension module).

TOML support a slippery slope for other things

As discussed in the Motivation section, TOML holds a special place in the Python ecosystem, for reading PEP 518 pyproject.toml packaging and tool configuration files. This chief reason to include TOML in the standard library does not apply to other formats, such as YAML or MessagePack.

In addition, the simplicity of TOML distinguishes it from other formats like YAML, which are highly complicated to construct and parse.

An API for writing TOML may, however, be added in a future PEP.

Backwards Compatibility

This proposal has no backwards compatibility issues within the standard library, as it describes a new module. Any existing third-party module named tomllib will break, as import tomllib will import the standard library module. However, tomllib is not registered on PyPI, so it is unlikely that any module with this name is widely used.

Note that we avoid using the more straightforward name toml to avoid backwards compatibility implications for users who have pinned versions of the current toml PyPI package. For more details, see the Alternative names for the module section.

Security Implications

Errors in the implementation could cause potential security issues. However, the parser's output is limited to simple data types; inability to load arbitrary classes avoids security issues common in more "powerful" formats like pickle and YAML. Also, the implementation will be in pure Python, which reduces security issues endemic to C, such as buffer overflows.

How to Teach This

The API of tomllib mimics that of other well-established file format libraries, such as json and pickle. The lack of a dump function will be explained in the documentation, with a link to relevant third-party libraries (e.g. tomlkit, tomli-w, pytomlpp).

Reference Implementation

The proposed implementation can be found at https://github.com/hukkin/tomli

Rejected Ideas

Basing on another TOML implementation

Several potential alternative implementations exist:

  • tomlkit is well established, actively maintained and supports TOML 1.0.0. An important difference is that tomlkit supports style roundtripping. As a result, it has a more complex API and implementation (about 5x as much code as tomli). Its author does not believe that tomlkit is a good choice for the standard library.
  • toml is a very widely used library. However, it is not actively maintained, does not support TOML 1.0.0 and has a number of known bugs. Its API is more complex than that of tomli. It allows customising output style through a complicated encoder API, and some very limited and mostly unused functionality to preserve input style through an undocumented decoder API. For more details on its API differences from this PEP, refer to Appendix A.
  • pytomlpp is a Python wrapper for the C++ project toml++. Pure Python libraries are easier to maintain than extension modules.
  • rtoml is a Python wrapper for the Rust project toml-rs and hence has similar shortcomings to pytomlpp. In addition, it does not support TOML 1.0.0.
  • Writing an implementation from scratch. It's unclear what we would get from this; tomli meets our needs and the author is willing to help with its inclusion in the standard library.

Including an API for writing TOML

There are several reasons to not include an API for writing TOML.

The ability to write TOML is not needed for the use cases that motivate this PEP: core Python packaging tools, and projects that need to read TOML configuration files.

Use cases that involve editing an existing TOML file (as opposed to writing a brand new one) are better served by a style preserving library. TOML is intended as a human-readable and -editable configuration format, so it's important to preserve comments, formatting and other markup. This requires a parser whose output includes style-related metadata, making it impractical to output plain Python types like str and dict. Furthermore, it substantially complicates the design of the API.

Even without considering style preservation, there are too many degrees of freedom in how to design a write API. For example, what default style (indentation, vertical and horizontal spacing, quotes, etc) should the library use for the output, and how much control should users be given over it? How should the library handle input and output validation? Should it support serialization of custom types, and if so, how? While there are reasonable options for resolving these issues, the nature of the standard library is such that we only get "one chance to get it right".

Currently, no CPython core developers have expressed willingness to maintain a write API, or sponsor a PEP that includes one. Since it is hard to change or remove something in the standard library, it is safer to err on the side of exclusion for now, and potentially revisit this later.

Therefore, writing TOML is left to third-party libraries. If a good API and relevant use cases for it are found later, write support can be added in a future PEP.

Assorted API details

Types accepted as the first argument of tomllib.load

The toml library on PyPI allows passing paths (and lists of path-like objects, ignoring missing files and merging the documents into a single object) to its load function. However, allowing this here would be inconsistent with the behavior of json.load, pickle.load and other standard library functions. If we agree that consistency here is desirable, allowing paths is out of scope for this PEP. This can easily and explicitly be worked around in user code, or by using a third-party library.

The proposed API takes a binary file, while toml.load takes a text file and json.load takes either. Using a binary file allows us to ensure UTF-8 is the encoding used (ensuring correct parsing on platforms with other default encodings, such as Windows), and avoid incorrectly parsing files containing single carriage returns as valid TOML due to universal newlines in text mode.

Type accepted as the first argument of tomllib.loads

While tomllib.load takes a binary file, tomllib.loads takes a text string. This may seem inconsistent at first.

Quoting the TOML v1.0.0 specification:

A TOML file must be a valid UTF-8 encoded Unicode document.

tomllib.loads does not intend to load a TOML file, but rather the document that the file stores. The most natural representation of a Unicode document in Python is str, not bytes.

It is possible to add bytes support in the future if needed, but we are not aware of any use cases for it.

Controlling the type of mappings returned by tomllib.load[s]

The toml library on PyPI accepts a _dict argument in its load[s] functions, which works similarly to the object_hook argument in json.load[s]. There are several uses of _dict found on https://grep.app; however, almost all of them are passing _dict=OrderedDict, which should be unnecessary as of Python 3.7. We found two instances of relevant use: in one case, a custom class was passed for friendlier KeyErrors; in the other, the custom class had several additional lookup and mutation methods (e.g. to help resolve dotted keys).

Such a parameter is not necessary for the core use cases outlined in the Motivation section. The absence of this can be pretty easily worked around using a wrapper class, transformer function, or a third-party library. Finally, support could be added later in a backward-compatible way.

Removing support for parse_float in tomllib.load[s]

This option is not strictly necessary, since TOML floats should be implemented as "IEEE 754 binary64 values", which is equivalent to a Python float on most architectures.

The TOML specification uses the word "SHOULD", however, implying a recommendation that can be ignored for valid reasons. Parsing floats differently, such as to decimal.Decimal, allows users extra precision beyond that promised by the TOML format. In the author of tomli's experience, this is particularly useful in scientific and financial applications. This is also useful for other cases that need greater precision, or where end-users include non-developers who may not be aware of the limits of binary64 floats.

There are also niche architectures where the Python float is not a IEEE 754 binary64 value. The parse_float argument allows users to achieve correct TOML semantics even on such architectures.

Alternative names for the module

Ideally, we would be able to use the toml module name.

However, the toml package on PyPI is widely used, so there are backward compatibility concerns. Since the standard library takes precedence over third party packages, libraries and applications who current depend on the toml package would likely break when upgrading Python versions due to the many API incompatibilities listed in Appendix A, even if they pin their dependency versions.

To further clarify, applications with pinned dependencies are of greatest concern here. Even if we were able to obtain control of the toml PyPI package name and repurpose it for a backport of the proposed new module, we would still break users on new Python versions that included it in the standard library, regardless of whether they have pinned an older version of the existing toml package. This is unfortunate, since pinning would likely be a common response to breaking changes introduced by repurposing the toml package as a backport (that is incompatible with today's toml).

Finally, the toml package on PyPI is not actively maintained, but as of yet, efforts to request that the author add other maintainers have been unsuccessful, so action here would likely have to be taken without the author's consent.

Instead, this PEP proposes the name tomllib. This mirrors plistlib and xdrlib, two other file format modules in the standard library, as well as other modules, such as pathlib, contextlib and graphlib.

Other names considered but rejected include:

  • tomlparser. This mirrors configparser, but is perhaps somewhat less appropriate if we include a write API in the future.
  • tomli. This assumes we use tomli as the basis for implementation.
  • toml under some namespace, such as parser.toml. However, this is awkward, especially so since existing parsing libraries like json, pickle, xml, html etc. would not be included in the namespace.

Appendix A: Differences between proposed API and toml

This appendix covers the differences between the API proposed in this PEP and that of the third-party package toml. These differences are relevant to understanding the amount of breakage we could expect if we used the toml name for the standard library module, as well as to better understand the design space. Note that this list might not be exhaustive.

  1. No proposed inclusion of a write API (no toml.dump[s])

    This PEP currently proposes not including a write API; that is, there will be no equivalent of toml.dump or toml.dumps, as discussed at Including an API for writing TOML.

    If we included a write API, it would be relatively straightforward to convert most code that uses toml to the new standard library module (acknowledging that this is very different from a compatible API, as it would still require code changes).

    A significant fraction of toml users rely on this, based on comparing occurrences of "toml.load" to occurrences of "toml.dump".

  2. Different first argument of toml.load

    toml.load has the following signature:

    def load(
        f: Union[SupportsRead[str], str, bytes, list[PathLike | str | bytes]],
        _dict: Type[MutableMapping[str, Any]] = ...,
        decoder: TomlDecoder = ...,
    ) -> MutableMapping[str, Any]: ...
    

    This is quite different from the first argument proposed in this PEP: SupportsRead[bytes].

    Recapping the reasons for this, previously mentioned at Types accepted as the first argument of tomllib.load:

    • Allowing paths (and even lists of paths) as arguments is inconsistent with other similar functions in the standard library.
    • Using SupportsRead[bytes] allows us to ensure UTF-8 is the encoding used, and avoid incorrectly parsing single carriage returns as valid TOML.

    A significant fraction of toml users rely on this, based on manual inspection of occurrences of "toml.load".

  3. Errors

    toml raises TomlDecodeError, vs. the proposed PEP 8-compliant TOMLDecodeError.

    A significant fraction of toml users rely on this, based on occurrences of "TomlDecodeError".

  4. toml.load[s] accepts a _dict argument

    Discussed at Controlling the type of mappings returned by tomllib.load[s].

    As mentioned there, almost all usage consists of _dict=OrderedDict, which is not necessary in Python 3.7 and later.

  5. toml.load[s] support an undocumented decoder argument

    It seems the intended use case is for an implementation of comment preservation. The information recorded is not sufficient to roundtrip the TOML document preserving style, the implementation has known bugs, the feature is undocumented and we could only find one instance of its use on https://grep.app.

    The toml.TomlDecoder interface exposed is far from simple, containing nine methods.

    Users are likely better served by a more complete implementation of style-preserving parsing and writing.

  6. toml.dump[s] support an encoder argument

    Note that we currently propose to not include a write API; however, if that were to change, these differences would likely become relevant.

    The encoder argument enables two use cases:

    • control over how custom types should be serialized, and
    • control over how output should be formatted.

    The first is reasonable; however, we could only find two instances of this on https://grep.app. One of these two used this ability to add support for dumping decimal.Decimal, which a potential standard library implementation would support out of the box. If needed for other types, this use case could be well served by the equivalent of the default argument in json.dump.

    The second use case is enabled by allowing users to specify subclasses of toml.TomlEncoder and overriding methods to specify parts of the TOML writing process. The API consists of five methods and exposes substantial implementation detail.

    There is some usage of the encoder API on https://grep.app; however, it appears to account for a tiny fraction of the overall usage of toml.

  7. Timezones

    toml uses and exposes custom toml.tz.TomlTz timezone objects. The proposed implementation uses datetime.timezone objects from the standard library.

Source: https://github.com/python/peps/blob/master/pep-0680.rst