hukkin / tomli-w

A lil' TOML writer (counterpart to https://github.com/hukkin/tomli)

Tomli-W

A lil' TOML writer

Table of Contents generated with mdformat-toc

• Intro
• Installation
• Usage
  • Write to string
  • Write to file
• FAQ
  • Does Tomli-W sort the document?
  • Does Tomli-W support writing documents with comments or custom whitespace?
  • Why does Tomli-W not write a multi-line string if the string value contains newlines?
  • Is Tomli-W output guaranteed to be valid TOML?

Intro

Tomli-W is a Python library for writing TOML. It is a write-only counterpart to Tomli, which is a read-only TOML parser. Tomli-W is fully compatible with TOML v1.0.0.

Installation

pip install tomli-w

Usage

Write to string

import tomli_w

doc = {"table": {"nested": {}, "val3": 3}, "val2": 2, "val1": 1}
expected_toml = """\
val2 = 2
val1 = 1

[table]
val3 = 3

[table.nested]
"""
assert tomli_w.dumps(doc) == expected_toml

Write to file

import tomli_w

doc = {"one": 1, "two": 2, "pi": 3}
with open("path_to_file/conf.toml", "wb") as f:
    tomli_w.dump(doc, f)

FAQ

Does Tomli-W sort the document?

No, but it respects sort order of the input data, so one could sort the content of the dict (recursively) before calling tomli_w.dumps.

Does Tomli-W support writing documents with comments or custom whitespace?

No.

Why does Tomli-W not write a multi-line string if the string value contains newlines?

This default was chosen to achieve lossless parse/write round-trips.

TOML strings can contain newlines where exact bytes matter, e.g.

s = "here's a newline\r\n"

TOML strings also can contain newlines where exact byte representation is not relevant, e.g.

s = """here's a newline
"""

A parse/write round-trip that converts the former example to the latter does not preserve the original newline byte sequence. This is why Tomli-W avoids writing multi-line strings.

A keyword argument is provided for users who do not need newline bytes to be preserved:

import tomli_w

doc = {"s": "here's a newline\r\n"}
expected_toml = '''\
s = """
here's a newline
"""
'''
assert tomli_w.dumps(doc, multiline_strings=True) == expected_toml

Is Tomli-W output guaranteed to be valid TOML?

No. If there's a chance that your input data is bad and you need output validation, parse the output string once with tomli.loads. If the parse is successful (does not raise tomli.TOMLDecodeError) then the string is valid TOML.