TopolearnKnowing how YAML guesses types prevents values from silently becoming the wrong type in your app.
A scalar is a single value. Written unquoted (a plain scalar), its type is inferred by the parser's schema. With the YAML 1.2 core schema:
- integers:
42,-7,0o17(octal),0x1F(hex); - floats:
3.14,1e3,.inf,-.inf,.nan; - booleans:
true,false(alsoTrue,TRUE); - null:
null,~, or nothing at all after the colon; - everything else is a string:
hello world,eu-west-1,v1.2.3.
Quotes force a string: "42" and 'true' are strings. Strings rarely need quotes, but some values do (next module): those that look like another type, or that start with a character that has meaning in YAML such as *, &, !, {, [, @, % or a backtick, or that contain : or #.
Dates like 2026-10-17 are a special case: YAML 1.1 parsers (including PyYAML) turn them into date objects, while the 1.2 core schema keeps them as strings. Quote them if the type matters.
A key with nothing after the colon has the value null, not an empty string. Write "" for an empty string.
Strings in plain style can't span multiple lines in a readable way; multi-line text gets its own syntax.
Example
count: 42 # int
ratio: 0.75 # float
big: 1e6 # float 1000000.0 (a string in YAML 1.1 parsers)
enabled: true # bool
nothing: ~ # null
empty: # null as well, not ""
blank: "" # empty string
version: "3.10" # string; unquoted 3.10 would be the float 3.1
zip: "01234" # string; keeps the leading zero
region: eu-west-1 # string
mode: 0o644 # int 420 in YAML 1.2 (YAML 1.1 parsers want 0644)Remember
- Plain scalars are typed by appearance: int, float, bool, null or string
- Quote a value to force a string, e.g. version: "3.10"
- An empty value is null, not an empty string
Watch out for
- Writing python_version: 3.10, which parses as the float 3.1.