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StrictYAML is a type-safe YAML parser that parses and validates a restricted subset of the YAML specification.


  • Beautiful API
  • Refusing to parse the ugly, hard to read and insecure features of YAML like the Norway problem.
  • Strict validation of markup and straightforward type casting.
  • Clear, readable exceptions with code snippets and line numbers.
  • Acting as a near-drop in replacement for pyyaml, ruamel.yaml or poyo.
  • Ability to read in YAML, make changes and write it out again with comments preserved.
  • Not speed, currently.

Simple example:

# All about the character
name: Ford Prefect
age: 42
- Towel
from strictyaml import load, Map, Str, Int, Seq, YAMLError

Default parse result:

>>> load(yaml_snippet)
YAML({'name': 'Ford Prefect', 'age': '42', 'possessions': ['Towel']})

All data is string, list or OrderedDict:

>>> load(yaml_snippet).data
{'name': 'Ford Prefect', 'age': '42', 'possessions': ['Towel']}

Quickstart with schema:

from strictyaml import load, Map, Str, Int, Seq, YAMLError

schema = Map({"name": Str(), "age": Int(), "possessions": Seq(Str())})

42 is now parsed as an integer:

>>> person = load(yaml_snippet, schema)
{'name': 'Ford Prefect', 'age': 42, 'possessions': ['Towel']}

A YAMLError will be raised if there are syntactic problems, violations of your schema or use of disallowed YAML features:

# All about the character
name: Ford Prefect
age: 42

For example, a schema violation:

    person = load(yaml_snippet, schema)
except YAMLError as error:
while parsing a mapping
  in "<unicode string>", line 1, column 1:
    # All about the character
     ^ (line: 1)
required key(s) 'possessions' not found
  in "<unicode string>", line 3, column 1:
    age: '42'
    ^ (line: 3)

If parsed correctly:

from strictyaml import load, Map, Str, Int, Seq, YAMLError, as_document

schema = Map({"name": Str(), "age": Int(), "possessions": Seq(Str())})

You can modify values and write out the YAML with comments preserved:

person = load(yaml_snippet, schema)
person['age'] = 43
# All about the character
name: Ford Prefect
age: 43
- Towel

As well as look up line numbers:

>>> person = load(yaml_snippet, schema)
>>> person['possessions'][0].start_line

And construct YAML documents from dicts or lists:

print(as_document({"x": 1}).as_yaml())
x: 1


$ pip install strictyaml

Why StrictYAML?

There are a number of formats and approaches that can achieve more or less the same purpose as StrictYAML. I've tried to make it the best one. Below is a series of documented justifications:

Using StrictYAML

How to:

Compound validators:

Scalar validators:


Design justifications

There are some design decisions in StrictYAML which are controversial and/or not obvious. Those are documented here:

Star Contributors

  • @wwoods
  • @chrisburr
  • @jnichols0

Other Contributors

  • @eulores
  • @WaltWoods
  • @ChristopherGS
  • @gvx
  • @AlexandreDecan
  • @lots0logs
  • @tobbez
  • @jaredsampson
  • @BoboTIG

StrictYAML also includes code from ruamel.yaml, Copyright Anthon van der Neut.


  • Before writing any code, please read the tutorial on contributing to hitchdev libraries.
  • Before writing any code, if you're proposing a new feature, please raise it on github. If it's an existing feature / bug, please comment and briefly describe how you're going to implement it.
  • All code needs to come accompanied with a story that exercises it or a modification to an existing story. This is used both to test the code and build the documentation.