Lightweight declarative YAML and XML data binding for Python
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Lightweight declarative YAML and XML data binding for Python

Guts is written by Sebastian Heimann It is released under the MIT license. See the file LICENSE for more details.


  • PyYAML: Download it from and follow the given installation instructions or install it through your system's package manager (e.g. the python-yaml package on Debian based Linuxes).


A file might look like:

from guts import *

class Song(Object):
    name = String.T()
    album = String.T(default='')
    artist = String.T(default='')
    year = Int.T(optional=True)

class Playlist(Object):
    xmltagname = 'playlist'
    name = String.T(default='Untitled Playlist')
    comment = String.T(optional=True)
    song_list = List.T(Song.T())

These classes come with automatic __init__:

>>> from playlist import *
>>> song1 = Song(name='Metropolis', artist='Kraftwerk')
>>> song2 = Song(name='I Robot', artist='The Alan Parsons Project', album='I Robot')
>>> playlist = Playlist(song_list=[song1,song2])

They serialize to YAML:

>>> print song1.dump()
--- !playlist.Song
name: Metropolis
artist: Kraftwerk

They also serialize to XML:

>>> print playlist.dump_xml()
  <name>Untitled Playlist</name>
    <name>I Robot</name>
    <album>I Robot</album>
    <artist>The Alan Parsons Project</artist>

Objects can validate themselves:

>>> song1.validate()
>>> song2.year = 1977
>>> song2.validate()
>>> song2.year = 'abc'
>>> song2.validate()
Traceback (most recent call last):
guts.ValidationError: year: "abc" is not of type int

Objects can regularize themselves:

>>> song2.year = '1977'
>>> song2.regularize()
>>> song2.year
>>> type(song2.year)
<type 'int'>

They also deserialize from YAML and XML:

>>> playlist2 = load_string(playlist.dump())
>>> playlist3 = load_xml_string(playlist.dump_xml())

Incremental loading of large YAML or XML files is supported with the guts.iload_all() and guts.iload_all_xml() functions, which are buildt to return generators yielding Guts objects.

This module comes with a rudimentary code generator xmlschema-to-guts to turn (some) XML Schema definitions (XSD) into Python modules containing Guts class hierarchies.

Here is an example using the first example in the W3C XML Schema Primer. The Schema shall be defined in po.xsd:

<xsd:schema xmlns:xsd="">

    <xsd:documentation xml:lang="en">
     Purchase order schema for
     Copyright 2000 All rights reserved.

  <xsd:element name="purchaseOrder" type="PurchaseOrderType"/>

  <xsd:element name="comment" type="xsd:string"/>

  <xsd:complexType name="PurchaseOrderType">
      <xsd:element name="shipTo" type="USAddress"/>
      <xsd:element name="billTo" type="USAddress"/>
      <xsd:element ref="comment" minOccurs="0"/>
      <xsd:element name="items"  type="Items"/>
    <xsd:attribute name="orderDate" type="xsd:date"/>

  <xsd:complexType name="USAddress">
      <xsd:element name="name"   type="xsd:string"/>
      <xsd:element name="street" type="xsd:string"/>
      <xsd:element name="city"   type="xsd:string"/>
      <xsd:element name="state"  type="xsd:string"/>
      <xsd:element name="zip"    type="xsd:decimal"/>
    <xsd:attribute name="country" type="xsd:NMTOKEN"

  <xsd:complexType name="Items">
      <xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
            <xsd:element name="productName" type="xsd:string"/>
            <xsd:element name="quantity">
                <xsd:restriction base="xsd:positiveInteger">
                  <xsd:maxExclusive value="100"/>
            <xsd:element name="USPrice"  type="xsd:decimal"/>
            <xsd:element ref="comment"   minOccurs="0"/>
            <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
          <xsd:attribute name="partNum" type="SKU" use="required"/>

  <!-- Stock Keeping Unit, a code for identifying products -->
  <xsd:simpleType name="SKU">
    <xsd:restriction base="xsd:string">
      <xsd:pattern value="\d{3}-[A-Z]{2}"/>


A corresponding XML file po.xml might look like this:

<?xml version="1.0"?>
<purchaseOrder orderDate="1999-10-20">
   <shipTo country="US">
      <name>Alice Smith</name>
      <street>123 Maple Street</street>
      <city>Mill Valley</city>
   <billTo country="US">
      <name>Robert Smith</name>
      <street>8 Oak Avenue</street>
      <city>Old Town</city>
   <comment>Hurry, my lawn is going wild</comment>
      <item partNum="872-AA">
         <comment>Confirm this is electric</comment>
      <item partNum="926-AA">
         <productName>Baby Monitor</productName>

Using the Guts code generator

$ xmlschema-to-guts po.xsd >

will produce a Python module

from guts import *

class SKU(StringPattern):
    pattern = '\\d{3}-[A-Z]{2}'

class Comment(String):
    xmltagname = 'comment'

class Quantity(Int):

class USAddress(Object):
    country = String.T(default='US', optional=True, xmlstyle='attribute')
    name = String.T()
    street = String.T()
    city = String.T()
    state = String.T()
    zip = Float.T()

class Item(Object):
    part_num = SKU.T(xmlstyle='attribute')
    product_name = String.T()
    quantity = Quantity.T()
    us_price = Float.T(xmltagname='USPrice')
    comment = Comment.T(optional=True)
    ship_date = DateTimestamp.T(optional=True)

class Items(Object):
    item_list = List.T(Item.T())

class PurchaseOrderType(Object):
    order_date = DateTimestamp.T(optional=True, xmlstyle='attribute')
    ship_to = USAddress.T()
    bill_to = USAddress.T()
    comment = Comment.T(optional=True)
    items = Items.T()

class PurchaseOrder(PurchaseOrderType):
    xmltagname = 'purchaseOrder'

And we can use it e.g. to parse the example XML file po.xml from above:

>>> from po import *
>>> order = load_xml(filename='po.xml')
>>> print order  # dumps YAML
--- !po.PurchaseOrder
order_date: '1999-10-20'
ship_to: !po.USAddress
  name: Alice Smith
  street: 123 Maple Street
  city: Mill Valley
  state: CA
  zip: 90952.0
bill_to: !po.USAddress
  name: Robert Smith
  street: 8 Oak Avenue
  city: Old Town
  state: PA
  zip: 95819.0
comment: Hurry, my lawn is going wild
items: !po.Items
  - !po.Item
    part_num: 872-AA
    product_name: Lawnmower
    quantity: 1
    us_price: 148.95
    comment: Confirm this is electric
  - !po.Item
    part_num: 926-AA
    product_name: Baby Monitor
    quantity: 1
    us_price: 39.98
    ship_date: '1999-05-21'