Tomli

A lil' TOML parser

Table of Contents generated with mdformat-toc

Intro
Installation
Usage
FAQ
Performance

Intro

Tomli is a Python library for parsing TOML. It is fully compatible with TOML v1.0.0.

A version of Tomli, the tomllib module, was added to the standard library in Python 3.11 via PEP 680. Tomli continues to provide a backport on PyPI for Python versions where the standard library module is not available and that have not yet reached their end-of-life.

Installation

pip install tomli

Usage

Parse a TOML string

import tomli

toml_str = """
[[players]]
name = "Lehtinen"
number = 26

[[players]]
name = "Numminen"
number = 27
"""

toml_dict = tomli.loads(toml_str)
assert toml_dict == {
    "players": [{"name": "Lehtinen", "number": 26}, {"name": "Numminen", "number": 27}]
}

Parse a TOML file

import tomli

with open("path_to_file/conf.toml", "rb") as f:
    toml_dict = tomli.load(f)

The file must be opened in binary mode (with the "rb" flag). Binary mode will enforce decoding the file as UTF-8 with universal newlines disabled, both of which are required to correctly parse TOML.

Handle invalid TOML

import tomli

try:
    toml_dict = tomli.loads("]] this is invalid TOML [[")
except tomli.TOMLDecodeError:
    print("Yep, definitely not valid.")

Note that error messages are considered informational only. They should not be assumed to stay constant across Tomli versions.

Construct `decimal.Decimal`s from TOML floats

from decimal import Decimal
import tomli

toml_dict = tomli.loads("precision-matters = 0.982492", parse_float=Decimal)
assert isinstance(toml_dict["precision-matters"], Decimal)
assert toml_dict["precision-matters"] == Decimal("0.982492")

Note that decimal.Decimal can be replaced with another callable that converts a TOML float from string to a Python type. The decimal.Decimal is, however, a practical choice for use cases where float inaccuracies can not be tolerated.

Illegal types are dict and list, and their subtypes. A ValueError will be raised if parse_float produces illegal types.

Building a `tomli`/`tomllib` compatibility layer

Python versions 3.11+ ship with a version of Tomli: the tomllib standard library module. To build code that uses the standard library if available, but still works seamlessly with Python 3.6+, do the following.

Instead of a hard Tomli dependency, use the following dependency specifier to only require Tomli when the standard library module is not available:

tomli >= 1.1.0 ; python_version < "3.11"

Then, in your code, import a TOML parser using the following fallback mechanism:

import sys

if sys.version_info >= (3, 11):
    import tomllib
else:
    import tomli as tomllib

tomllib.loads("['This parses fine with Python 3.6+']")

FAQ

Why this parser?

it's lil'
pure Python with zero dependencies
the fastest pure Python parser *: 16x as fast as tomlkit, 2.3x as fast as toml
outputs basic data types only
100% spec compliant: passes all tests in BurntSushi/toml-test test suite
thoroughly tested: 100% branch coverage

Is comment preserving round-trip parsing supported?

No.

The tomli.loads function returns a plain dict that is populated with builtin types and types from the standard library only. Preserving comments requires a custom type to be returned so will not be supported, at least not by the tomli.loads and tomli.load functions.

Look into TOML Kit if preservation of style is what you need.

Is there a `dumps`, `write` or `encode` function?

Tomli-W is the write-only counterpart of Tomli, providing dump and dumps functions.

The core library does not include write capability, as most TOML use cases are read-only, and Tomli intends to be minimal.

How do TOML types map into Python types?

TOML type	Python type	Details
Document Root	`dict`
Key	`str`
String	`str`
Integer	`int`
Float	`float`
Boolean	`bool`
Offset Date-Time	`datetime.datetime`	`tzinfo` attribute set to an instance of `datetime.timezone`
Local Date-Time	`datetime.datetime`	`tzinfo` attribute set to `None`
Local Date	`datetime.date`
Local Time	`datetime.time`
Array	`list`
Table	`dict`
Inline Table	`dict`

Performance

The benchmark/ folder in this repository contains a performance benchmark for comparing the various Python TOML parsers. The benchmark can be run with tox -e benchmark-pypi. Running the benchmark on my personal computer output the following:

foo@bar:~/dev/tomli$ tox -e benchmark-pypi
benchmark-pypi installed: attrs==21.4.0,click==8.0.3,pytomlpp==1.0.10,qtoml==0.3.1,rtoml==0.7.1,toml==0.10.2,tomli==2.0.1,tomlkit==0.9.2
benchmark-pypi run-test-pre: PYTHONHASHSEED='3088452573'
benchmark-pypi run-test: commands[0] | python -c 'import datetime; print(datetime.date.today())'
2022-02-09
benchmark-pypi run-test: commands[1] | python --version
Python 3.8.10
benchmark-pypi run-test: commands[2] | python benchmark/run.py
Parsing data.toml 5000 times:
------------------------------------------------------
    parser |  exec time | performance (more is better)
-----------+------------+-----------------------------
     rtoml |    0.891 s | baseline (100%)
  pytomlpp |    0.969 s | 91.90%
     tomli |        4 s | 22.25%
      toml |     9.01 s | 9.88%
     qtoml |     11.1 s | 8.05%
   tomlkit |       63 s | 1.41%

The parsers are ordered from fastest to slowest, using the fastest parser as baseline. Tomli performed the best out of all pure Python TOML parsers, losing only to pytomlpp (wraps C++) and rtoml (wraps Rust).

hauntsaninja/tomli