What is TOML?

TOML is a configuration format with explicit types and a table syntax, designed to be unambiguous where YAML is clever.

Who uses it

Rust and Python projects, and anything with a pyproject.toml or Cargo.toml.

Writing TOML

Each row becomes an entry in an array of tables. Keys are sanitised to bare keys where possible and quoted where not.

What it can hold

TOML is a data interchange format. TOML has no null, so a missing value has to become an empty string or an absent key. A machine can read the table back out of it without guessing, which is what makes it safe to convert into and out of.

It keeps

Types. A number stays a number and a boolean stays a boolean, so whatever reads the file next does not have to guess from how a value looks.
A line break inside a cell, which survives rather than ending the row.

It cannot express

A null. An absent value and an empty one look the same, and nothing in the file says which was meant.
Any formatting. Bold, colour, column widths and merged cells have nowhere to go.
A partial read. The document has to be complete to mean anything, so a truncated file yields nothing and a large one has to be held in memory whole.
A header row. Column names are repeated on every record instead, which is why a file in this format is larger than the table it holds.

What it looks like

Example TOML

[[staff]]
id = "007"
name = "Halima Yusuf"
role = "Analyst, data"
started = "2024-03-15"
hours = "38.5"

[[staff]]
id = "012"
name = "Jonah \"Jo\" Pryce"
role = "Engineer"
started = "2025-11-02"
hours = ""

[[staff]]
id = "104"
name = "Wei Chen"
role = "Manager"
started = "2023-06-30"
hours = "40"

Questions

How do I open TOML files?

TOML is written by Tablizer rather than read by it. Open the file in whichever tool the format belongs to.

Is TOML a good format for sharing data?

It depends who is receiving it. Rust and Python projects, and anything with a pyproject.toml or Cargo.toml. If the recipient is outside that group, a plain CSV travels further than anything else.