Type-safe YAML parser and validator.
Clone or download

README.md

StrictYAML

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.
  • 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(OrderedDict([('name', 'Ford Prefect'), ('age', '42'), ('possessions', ['Towel'])]))

All data is string, list or OrderedDict:

>>> load(yaml_snippet).data
OrderedDict([('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
OrderedDict([('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

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

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:

Contributors

  • @gvx
  • @AlexandreDecan
  • @lots0logs
  • @tobbez

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 (more typically), a modification to an existing story. This is used both to test the code and build the documentation.