StrictYAML

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

Priorities:

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
possessions:
- 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)
>>> person.data
{'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:

try:
    person = load(yaml_snippet, schema)
except YAMLError as error:
    print(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
print(person.as_yaml())

# All about the character
name: Ford Prefect
age: 43
possessions:
- Towel

As well as look up line numbers:

>>> person = load(yaml_snippet, schema)
>>> person['possessions'][0].start_line
5

And construct YAML documents from dicts or lists:

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

x: 1

Install

$ 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:

Restrictions:

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.

Contributing

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.