02.4YAML tabs

YAML tab fixer

YAML does not allow tabs for indentation. Paste a file that fails with “found character that cannot start any token” to see every tab, rewrite it with spaces at a consistent depth and check that the result parses.

YAML

services:
	web:
		image: nginx:1.27
		ports:
			- "8080:80"
		environment:
			- name: MODE
			  value: production
		# health check every 30 seconds
		healthcheck:
			test:	["CMD", "curl", "-f", "http://localhost"]
			interval: 30s
	worker:
		build: .
		command: |
			python worker.py \
			  --queue default

or drop a file on this box

Parser check

Original: parse error at line 2, column 1

Tabs are not allowed as indentation.

2→web:^

Fixed: valid YAML

One document parsed without errors.

1services:↵

2Δ··web:↵

3Δ····image:·nginx:1.27↵

4Δ····ports:↵

5Δ······-·"8080:80"↵

6Δ····environment:↵

7Δ······-·name:·MODE↵

8Δ········value:·production↵

9Δ····#·health·check·every·30·seconds↵

10Δ····healthcheck:↵

11Δ······test:·["CMD",·"curl",·"-f",·"http://localhost"]↵

12Δ······interval:·30s↵

13Δ··worker:↵

14Δ····build:·.↵

15Δ····command:·|↵

16Δ······python·worker.py·\↵

17Δ········--queue·default↵

Tabs on 16 lines: 14 in indentation, 1 after key: or -, 2 inside block scalar indentation. 16 lines rewritten with spaces.

The errors a tab causes

found character '\t' that cannot start any token: PyYAML, Ruby and Go (and so Kubernetes, Ansible and Docker Compose) hitting a tab at the start of a line, or right after key: or -.
tab characters must not be used in indentation (js-yaml) and Tabs are not allowed as indentation (the yaml package).
mapping values are not allowed here or bad indentation of a mapping entry: often a tab that an editor showed at one width and the parser counted differently.

The fixer expands each tab to the column it reached in your editor (set Tabs were to match; for files that mix tabs and spaces it is guessed), then rebuilds the indentation level by level, so children stay inside their parents. Keys under a - list item stay lined up with the first key. Block scalars (| and >) keep their content as it is, tabs inside it included. Both versions are then parsed with the yaml package, so you can see where the original fails and that the fixed one does not.

Questions

Are tabs ever allowed in YAML?

Inside quoted strings and block scalar content, yes, and YAML 1.2 allows them as separators. libyaml-based parsers still reject a tab after a colon or dash, which is why that option is on.

Which tab width should I pick?

The width your editor showed tabs at, usually 2, 4 or 8. For a file that mixes tabs and spaces, the guess is the width at which the nesting comes out consistent; check the Original view to confirm.

Is my file uploaded?

No. Fixing and parsing happen in your browser.

YAML tab fixer

YAML

Options

Parser check

The errors a tab causes

Questions