PyYAML Documentation

work in progress

RPG-ish descriptions are stolen from  the Angband rogue-like game. Names of the heroes are generated with  MudNames.

Basic usage

Start with importing the yaml package.

>>> import yaml

load()

Warning: It is not safe to call yaml.load with any data received from an untrusted source! yaml.load is as powerful as pickle.load and so may call any Python function. Check the yaml.safe_load function though.

The function yaml.load converts a YAML document to a Python object.

>>> yaml.load("""
... - Hesperiidae
... - Papilionidae
... - Apatelodidae
... - Epiplemidae
... """)

['Hesperiidae', 'Papilionidae', 'Apatelodidae', 'Epiplemidae']

yaml.load accepts a string, a Unicode string, an open file object, or an open Unicode file object. A string or a file must be encoded with utf-8, utf-16-be or utf-16-le encoding. yaml.load detects the encoding by checking the BOM (byte order mark) sequence at the beginning of the string/file. If no BOM is present, the utf-8 encoding is assumed.

yaml.load returns a Python object.

>>> yaml.load(u"""
... hello: Привет!
... """)

{'hello': u'\u041f\u0440\u0438\u0432\u0435\u0442!'}

>>> stream = file('document.yaml', 'r')    # 'document.yaml' contains a single YAML document.
>>> yaml.load(stream)
[...]    # A Python object corresponding to the document.

if a string or a file contains several documents, you may load them all with the yaml.load_all function.

>>> documents = """
... ---
... name: The Set of Gauntlets 'Pauraegen'
... description: >
...     A set of handgear with sparks that crackle
...     across its knuckleguards.
... ---
... name: The Set of Gauntlets 'Paurnen'
... description: >
...   A set of gauntlets that gives off a foul,
...   acrid odour yet remains untarnished.
... ---
... name: The Set of Gauntlets 'Paurnimmen'
... description: >
...   A set of handgear, freezing with unnatural cold.
... """

>>> for data in yaml.load_all(documents):
...     print data

{'description': 'A set of handgear with sparks that crackle across its knuckleguards.\n',
'name': "The Set of Gauntlets 'Pauraegen'"}
{'description': 'A set of gauntlets that gives off a foul, acrid odour yet remains untarnished.\n',
'name': "The Set of Gauntlets 'Paurnen'"}
{'description': 'A set of handgear, freezing with unnatural cold.\n',
'name': "The Set of Gauntlets 'Paurnimmen'"}

PyYAML allows you to construct a Python object of any type.

>>> yaml.load("""
... none: [~, null]
... bool: [true, false, on, off]
... int: 42
... float: 3.14159
... list: [LITE, RES_ACID, SUS_DEXT]
... dict: {hp: 13, sp: 5}
... """)

{'none': [None, None], 'int': 42, 'float': 3.1415899999999999,
'list': ['LITE', 'RES_ACID', 'SUS_DEXT'], 'dict': {'hp': 13, 'sp': 5},
'bool': [True, False, True, False]}

Even instances of Python classes can be constructed using the !!python/object tag.

>>> class Hero:
...     def __init__(self, name, hp, sp):
...         self.name = name
...         self.hp = hp
...         self.sp = sp
...     def __repr__(self):
...         return "%s(name=%r, hp=%r, sp=%r)" % (
...             self.__class__.__name__, self.name, self.hp, self.sp)

>>> yaml.load("""
... !!python/object:__main__.Hero
... name: Welthyr Syxgon
... hp: 1200
... sp: 0
... """)

Hero(name='Welthyr Syxgon', hp=1200, sp=0)

Note that the ability to construct an arbitrary Python object may be dangerous if you receive a YAML document from an untrusted source such as Internet. The function yaml.safe_load limits this ability to simple Python objects like integers or lists.

dump()

The yaml.dump function accepts a Python object and produces a YAML document.

>>> print yaml.dump({'name': 'Silenthand Olleander', 'race': 'Human',
... 'traits': ['ONE_HAND', 'ONE_EYE']})

name: Silenthand Olleander
race: Human
traits: [ONE_HAND, ONE_EYE]

yaml.dump accepts the second optional argument, which must be an open file. In this case, yaml.dump will write the produced YAML document into the file. Otherwise, yaml.dump returns the produced document.

>>> stream = file('document.yaml', 'w')
>>> yaml.dump(data, stream)    # Write a YAML representation of data to 'document.yaml'.
>>> print yaml.dump(data)      # Output the document to the screen.

If you need to dump several YAML documents to a single stream, use the function yaml.dump_all. yaml.dump_all accepts a list or a generator producing Python objects to be serialized into a YAML document. The second optional argument is an open file.

>>> print yaml.dump([1,2,3], explicit_start=True)
--- [1, 2, 3]

>>> print yaml.dump_all([1,2,3], explicit_start=True)
--- 1
--- 2
--- 3

You may even dump instances of Python classes.

>>> class Hero:
...     def __init__(self, name, hp, sp):
...         self.name = name
...         self.hp = hp
...         self.sp = sp
...     def __repr__(self):
...         return "%s(name=%r, hp=%r, sp=%r)" % (
...             self.__class__.__name__, self.name, self.hp, self.sp)

>>> print yaml.dump(("Galain Ysseleg", hp=-3, sp=2))

!!python/object:__main__.Hero {hp: -3, name: Galain Ysseleg, sp: 2}

yaml.dump supports a number of keyword arguments that specify formatting details for the emitter. For instance, you may set the preferred intendation and width or use the canonical YAML format.

>>> print yaml.dump(range(50))

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,
  23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42,
  43, 44, 45, 46, 47, 48, 49]

>>> print yaml.dump(range(50), width=50, indent=4)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
    16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27,
    28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39,
    40, 41, 42, 43, 44, 45, 46, 47, 48, 49]

>>> print yaml.dump(range(5), canonical=True)

---
!!seq [
  !!int "0",
  !!int "1",
  !!int "2",
  !!int "3",
  !!int "4",
]

YAML syntax

A good introduction to the YAML syntax is  Chapter 2 of the YAML specification.

You may also check  the YAML cookbook. Note that it is focused on a Ruby implementation and uses the old YAML 1.0 syntax.

Here we present most common YAML constructs together with the corresponding Python objects.

Documents

YAML stream is a collection of zero or more documents. An empty stream contains no documents. Documents are separated with ---. Documents may optionally end with .... A single document may or may not be marked with ---.

Example of an implicit document:

- Multimedia
- Internet
- Education

Example of an explicit document:

---
- Afterstep
- CTWM
- Oroborus
...

Example of several documents in the same stream:

---
- Ada
- APL
- ASP
- Assembly
- Awk
---
- Basic
---
- C
- C#    # Note that comments are denoted with ' #' (space and #).
- C++
- Cold Fusion

Block sequences

In the block context, sequence entries are denoted by - (dash and space):

# YAML
- The Dagger 'Narthanc'
- The Dagger 'Nimthanc'
- The Dagger 'Dethanc'

# Python
["The Dagger 'Narthanc'", "The Dagger 'Nimthanc'", "The Dagger 'Dethanc'"]

Block sequences can be nested:

# YAML
-
  - HTML
  - LaTeX
  - SGML
  - VRML
  - XML
  - YAML
-
  - BSD
  - GNU Hurd
  - Linux

# Python
[['HTML', 'LaTeX', 'SGML', 'VRML', 'XML', 'YAML'], ['BSD', 'GNU Hurd', 'Linux']]

It's not necessary to start a nested sequence with a new line:

# YAML
- 1.1
- - 2.1
  - 2.2
- - - 3.1
    - 3.2
    - 3.3

# Python
[1.1, [2.1, 2.2], [[3.1, 3.2, 3.3]]]

A block sequence may be nested to a block mapping. Note that in this case it is not necessary to indent the sequence.

# YAML
left hand:
- Ring of Teleportation
- Ring of Speed
right hand:
- Ring of Resist Fire
- Ring of Resist Cold
- Ring of Resist Poison

# Python
{'right hand': ['Ring of Resist Fire', 'Ring of Resist Cold', 'Ring of Resist Poison'],
'left hand': ['Ring of Teleportation', 'Ring of Speed']}

Block mappings

In the block context, keys and values of mappings are separated by : (colon and space):

# YAML
base armor class: 0
base damage: [4,4]
plus to-hit: 12
plus to-dam: 16
plus to-ac: 0

# Python
{'plus to-hit': 12, 'base damage': [4, 4], 'base armor class': 0, 'plus to-ac': 0, 'plus to-dam': 16}

Complex keys are denoted with ? (question mark and space):

# YAML
? !!python/tuple [0,0]
: The Hero
? !!python/tuple [0,1]
: Treasure
? !!python/tuple [1,0]
: Treasure
? !!python/tuple [1,1]
: The Dragon

# Python
{(0, 1): 'Treasure', (1, 0): 'Treasure', (0, 0): 'The Hero', (1, 1): 'The Dragon'}

Block mapping can be nested:

# YAML
hero:
  hp: 34
  sp: 8
  level: 4
orc:
  hp: 12
  sp: 0
  level: 2

# Python
{'hero': {'hp': 34, 'sp': 8, 'level': 4}, 'orc': {'hp': 12, 'sp': 0, 'level': 2}}

A block mapping may be nested in a block sequence:

# YAML
- name: PyYAML
  status: 4
  license: MIT
  language: Python
- name: PySyck
  status: 5
  license: BSD
  language: Python

# Python
[{'status': 4, 'language': 'Python', 'name': 'PyYAML', 'license': 'MIT'},
{'status': 5, 'license': 'BSD', 'name': 'PySyck', 'language': 'Python'}]

Flow collections

The syntax of flow collections in YAML is very close to the syntax of list and dictionary constructors in Python:

# YAML
{ str: [15, 17], con: [16, 16], dex: [17, 18], wis: [16, 16], int: [10, 13], chr: [5, 8] }

# Python
{'dex': [17, 18], 'int': [10, 13], 'chr': [5, 8], 'wis': [16, 16], 'str': [15, 17], 'con': [16, 16]}

Scalars

There are 5 styles of scalars in YAML: plain, single-quoted, double-quoted, literal, and folded:

# YAML
plain: Scroll of Remove Curse
single-quoted: 'EASY_KNOW'
double-quoted: "?"
literal: |    # Borrowed from http://www.kersbergen.com/flump/religion.html
  by hjw              ___
     __              /.-.\
    /  )_____________\\  Y
   /_ /=== == === === =\ _\_
  ( /)=== == === === == Y   \
   `-------------------(  o  )
                        \___/
folded: >
  It removes all ordinary curses from all equipped items.
  Heavy or permanent curses are unaffected.

# Python
{'folded': 'It removes all ordinary curses from all equipped items. \
Heavy or permanent curses are unaffected.\n',
'literal': 'by hjw              ___\n   __              /.-.    /  )_____________\\  Y\n\
 /_ /=== == === === =\\ _\\_\n( /)=== == === === == Y      `-------------------(  o  )\n\
                      \\___/\n',
'single-quoted': 'EASY_KNOW', 'double-quoted': '?', 'plain': 'Scroll of Remove Curse'}

Aliases

Tags

YAML tags and Python types

Reference

Warning: API stability is not guaranteed'''

YAMLError

Tokens

Events

Nodes

Loader

Scanner interface

Parser interface

Composer interface

Constructor interface

Resolver interface

Dumper

Emitter interface

Serializer interface

Representer interface

Resolver interface

YAMLObject

The yaml package

Deviations from the specification

Download and installing

Check it out from the SVN repository  http://svn.pyyaml.org/pyyaml/trunk.

Install it by running

$ python setup.py install

High-level API

Warning: API is not stable and may change in the future

Basic examples

Start with importing the package:

>>> import yaml

Define the input data:

>>> data = """
... - YAML
... - is
... - fun!
... """

The parser accepts string objects, unicode objects, open file objects, and unicode file objects.

Now convert it to a native Python object:

>>> yaml.load(data)
['YAML', 'is', 'fun!']

Conversely, you may convert a Python object into a YAML document:

>>> print yaml.dump(['YAML', 'is', 'fun!'])
- YAML
- is
- fun!

PyYAML 3000 supports many of the types defined in the YAML tags repository:

>>> data = """
... - ~
... - true
... - 3_141_592.653e-6
... - 3000
... - PyYAML3000 birthday: 2006-02-11
... - primes (sort of): !!set { 2, 3, 5, 7, 11, 13 }
... - pairs: !!pairs [1: 2, 3: 4, 5: 6]
... """
>>> for x in yaml.load(data): print x
None
True
3.141592653
3000
{'PyYAML3000 birthday': datetime.datetime(2006, 2, 11, 0, 0)}
{'primes (sort of)': set([2, 3, 5, 7, 11, 13])}
{'pairs': [(1, 2), (3, 4), (5, 6)]}
>>> print yaml.dump([None, True, False, 123, 123.456, 'a string',
... {'a': 'dictionary'}, ['a', 'list']])
- null
- true
- false
- 123
- 123.456
- a string
- a: dictionary
- - a
  - list

The following tags are supported: !!map, !!omap, !!pairs, !!set, !!seq, !!binary, !!bool, !!float, !!int, !!merge, !!null, !!str, !!timestamp, !!value.

Defining custom tags

You may define constructors for your own application-specific tags. You may use either the function yaml.add_constructor or subclass from yaml.YAMLObject.

Instances of yaml.YAMLObject are automatically serialized to YAML and vice versa. You only need to define the YAML tag with the yaml_tag variable.

class Person(yaml.YAMLObject):
    yaml_tag = '!Person'
    def __init__(self, first_name=None, last_name=None, email=None, birthday=None):
        self.first_name = first_name
        self.last_name = last_name
        self.email = email
        self.birthday = birthday
    def __repr__(self):
        return "%s(first_name=%r, last_name=%r, email=%r, birthday=%r)"  \
                % (self.__class__.__name__, self.first_name, self.last_name,
                        self.email, self.birthday)

>>> p = yaml.load("""
... !Person
... first_name: Kirill
... last_name: Simonov
... email: xi(at)resolvent.net
... birthday: null
... """)
>>> print p
Person(first_name='Kirill', last_name='Simonov', email='xi(at)resolvent.net', birthday=None)
>>> print yaml.dump(p)
!Person
last_name: Simonov
first_name: Kirill
email: xi(at)resolvent.net
birthday: null

If you don't want to use metaclass magic, you may define the constructor and representer as functions and register them:

def construct_person(constructor, node):
    # ...
def represent_person(representer, person):
    # ...
yaml.add_constructor('!Person', construct_person)
yaml.add_representer(Person, represent_person)

Parsing and emitting multiple documents in a stream

If an input stream contains several documents, you may load all of them using the yaml.load_all function.

>>> data = """
... This is the first document
... --- # This is an empty document
... ---
... - this
... - is: the
...   last: document
... """
>>> for document in yaml.load_all(data): print document
This is the first document
None
['this', {'is': 'the', 'last': 'document'}]

You may also dump several documents into the same stream using the yaml.dump_all function.

>>> print yaml.dump_all(["The first document", None, ["The", "last", "document"]])
The first document
--- null
---
- The
- last
- document

There are more features, check the source to find out.

Low-level API

PyYAML 3000 provides low-level event-based and easy-to-use parser and emitter API.

Example:

>>> data = """
... --- !tag
... scalar
... ---
... - &anchor item
... - another item
... - *anchor
... ---
... key: value
... ? - complex
...   - key
... : - complex
...   - value
... """
>>> for event in yaml.parse(data): print event
StreamStartEvent()
DocumentStartEvent()
ScalarEvent(anchor=None, tag=u'!tag', implicit=(False, False), value=u'scalar')
DocumentEndEvent()
DocumentStartEvent()
SequenceStartEvent(anchor=None, tag=None, implicit=True)
ScalarEvent(anchor=u'anchor', tag=None, implicit=(True, False), value=u'item')
ScalarEvent(anchor=None, tag=None, implicit=(True, False), value=u'another item')
AliasEvent(anchor=u'anchor')
SequenceEndEvent()
DocumentEndEvent()
DocumentStartEvent()
MappingStartEvent(anchor=None, tag=None, implicit=True)
ScalarEvent(anchor=None, tag=None, implicit=(True, False), value=u'key')
ScalarEvent(anchor=None, tag=None, implicit=(True, False), value=u'value')
SequenceStartEvent(anchor=None, tag=None, implicit=True)
ScalarEvent(anchor=None, tag=None, implicit=(True, False), value=u'complex')
ScalarEvent(anchor=None, tag=None, implicit=(True, False), value=u'key')
SequenceEndEvent()
SequenceStartEvent(anchor=None, tag=None, implicit=True)
ScalarEvent(anchor=None, tag=None, implicit=(True, False), value=u'complex')
ScalarEvent(anchor=None, tag=None, implicit=(True, False), value=u'value')
SequenceEndEvent()
MappingEndEvent()
DocumentEndEvent()
StreamEndEvent()
>>> events = [
... yaml.StreamStartEvent(encoding='utf-8'),
... yaml.DocumentStartEvent(explicit=True),
... yaml.MappingStartEvent(anchor=None, tag=None, implicit=True),
... yaml.ScalarEvent(anchor=None, tag=None, value=u'flow sequence', implicit=(True, True)),
... yaml.SequenceStartEvent(anchor=None, tag=None, flow_style=True, implicit=True),
... yaml.ScalarEvent(anchor=None, tag=None, value=u'123', implicit=(True, False)),
... yaml.ScalarEvent(anchor=None, tag=None, value=u'456', implicit=(True, False)),
... yaml.SequenceEndEvent(),
... yaml.ScalarEvent(anchor=None, tag=None, value=u'block scalar', implicit=(True, True)),
... yaml.ScalarEvent(anchor=None, tag=None, value=u'YAML\nis\nfun!\n', style='|', implicit=(True, True)),
... yaml.MappingEndEvent(),
... yaml.DocumentEndEvent(explicit=True),
... yaml.StreamEndEvent(),
... ]

>>> print yaml.emit(events)
---
flow sequence: [123, 456]
block scalar: |
  YAML
  is
  fun!
...

To Do

Long-term goals:

• fix tabs, indentation for flow collections, indentation for scalars (min=1?), 'y' is !!bool,
• libyaml3000

Deviations from the specification

• rules for tabs in YAML are confusing. We are close, but not there yet. Perhaps both the spec and the parser should be fixed. Anyway, the best rule for tabs in YAML is to not use them at all.
• Byte order mark. The initial BOM is stripped, but BOMs inside the stream are considered as parts of the content. It can be fixed, but it's not really important now.
• Empty plain scalars are not allowed if alias or tag is specified. This is done to prevent anomalities like [ !tag, value], which can be interpreted both as [ !<!tag,> value ] and [ !<!tag> "", "value" ]. The spec should be fixed.
• Indentation of flow collections. The spec requires them to be indented more then their block parent node. Unfortunately this rule many intuitively correct constructs invalid, for instance,

  block: {
  } # this is indentation violation according to the spec.
  

• ':' is not allowed for plain scalars in the flow mode. {1:2} is interpreted as { 1 : 2 }.

Notes

• SpecBugs?