1. Simone Marzola
  2. tgext.datahelpers



About Data Helpers

tgext.datahelpers is a collection of utilities to help manage stored data in common web applications.

tgext.datahelpers contains:

  • Validators to fetch objects from database by Id (both SQLA and Ming)
  • Store Attachments by just declaring a Column in your model
  • Store image and thumbnails by just declaring a Column in your model


tgext.datahelpers can be installed both from pypi or from bitbucket:

easy_install tgext.datahelpers

should just work for most of the users


tgext.datahelpers.validators provides the SQLAEntityConverter and MingEntityConverter that convert the given parameter (which is expected to be the primary key of an object) to the actually object itself:

from tgext.datahelpers.validators import SQLAEntityConverter

@validate({'doc':SQLAEntityConverter(Document)}, error_handler=index)
def photo(self, doc):
    return redirect(doc.photo.url)

Validators module provides also the validated_handler utility which makes possible to apply validation also to error_handlers. Normally TurboGears would execute HCC based validation only when the controller method is called through a request, when you use a controller method as the error_handler of another method the error_handler HCC gets skipped and arguments are passed as they are.

Using validated_handler it is possible to change this behavior and apply validation even to error_handlers:

from tgext.datahelpers.validators import SQLAEntityConverter, validated_handler

def next(self, doc):
    return dict(doc=doc+1)

def photo(self, doc):
    return redirect(doc.photo.url)

In the previous example when calling /photo/3 if document 3 is not available it would be retrieved the successive document by calling next. If validated_handler gets removed from photo HCC you would get an error as doc wouldn't be an integer.


tgext.datahelpers.utils provides the slugify function to generate slug urls for entities which are meaningful for users.

The generated slugs includes the entity id so that it can identify unique elements making possible for EntityConverter validators to support retrieving objects from the generated slug. Add slugified=True option to the entity converted to load back an entity by its slug.

Sample usage:

>>> from tgext.datahelpers.utils import slugify

>>> entity = DBSession.query(Entity).get(5)
>>> url = slugify(entity, entity.name)
>>> print url

Utility functions also provide a fail_with object which can be used with turbogears HCC error_handler to report a missing element or forbidden access:

from tgext.datahelpers.validators import SQLAEntityConverter
from tgext.datahelpers.utils import fail_with

@validate({'doc':SQLAEntityConverter(Document, slugified=True)},
def photo(self, doc):
    return redirect(doc.photo.url)

Entities Based Caching

tgext.datahelpers.caching provides the @entitycached decorator which can be used to cache methods (and helpers) based on a parameter which is a Ming or SQLAlchemy entity.

Whenever the entity gets updated the cache is invalidated and the method called again, otherwise calling the method will return the value from the cache.

To determine if the entity has changed it will try to retrieve the cache_key property of the entity, if not available a cache key will be automatically generated using the primary key and updated_at property of the entity.

Sample usage:

from tgext.datahelpers.caching import entitycached

def render_post(post):
    return '<div>%s</div>' % post.html

blog = ''.join(map(render_post, blog_posts))

@entitycached decorator can also be used to cache any function by using the tgext.datahelpers.caching.CacheKey object as a function argument instead of a Ming/SQLAlchemy entity.

If you want to cache an SQLAlchemy query give a look at the sqla_merge option.

@entitycached decorator supports also various options:

  • expire - How long the cached value will be kept around, by default 3 days

  • cache_type - Which type of cache to use, by default memory will be used

  • namespace - The cache namespace, by default this is autogenerated by the cached class and method names

  • sqla_merge - Whenever the cached function return value is a SQLAlchemy query.

    When this option is True you will always get the results instead of the query itself and the resulting objects will be merged back in to the currently existing TurboGears DBSession to avoid DetachedInstanceError exceptions.


tgext.datahelpers.fields provides the Attachment field for SQLAlchemy to provide an easy and convenient way to store attachments.

The Attachment field will permit to assign files to the attribute declared with Attachment type and will store a copy of the file on disk as soon as the object is committed to the database.

The document field will provide a bunch of attributes you can use to access the file:

  • file - A file object pointing to the saved file
  • filename - The name of the saved file
  • url - Url from which the file is fetchable
  • local_path - Local path of the file on disk

Files will be saved in tg.config['attachments_path'] and url will be generated using tg.config['attachments_url']. By default those are set at /public/attachments and /attachments.

The Attachment field accepts a attachment_type parameter which specifies the kind of attachment that it is going to be saved. The default is tgext.datahelpers.fields.AttachedFile which just stores the file itself:

from tgext.datahelpers.fields import Attachment
class Document(DeclarativeBase):
    __tablename__ = 'document'

    uid = Column(Integer, autoincrement=True, primary_key=True)
    file = Column(Attachment)

d = Document(file=open('/myfile.txt'))

d = DBSession.query(Document).first()
print d.file.url


Apart from file objects also instances of cgi.FieldStorage can be assigned to permit to quickly store uploaded files.

Image Attachments with Thumbnail

Using the tgext.datahelpers.fields.AttachedImage as the argument of the Attachment field it is possible to quickly store images with their thumbnail.

The resulting object will provide the same attributes as the generic Attachment one adding two more thumbnail related properties:

  • thumb_local_path - The local path of the image thumbnail
  • thumb_url - The url of the thumbnail

Storing image with thumbnails is as easy as storing the file itself:

from tgext.datahelpers.fields import Attachment, AttachedImage
class Document(DeclarativeBase):
    __tablename__ = 'document'

    uid = Column(Integer, autoincrement=True, primary_key=True)
    image = Column(Attachment(AttachedImage))

d = Document(image=open('/photo.jpg'))

d = DBSession.query(Document).first()
print d.image.url
print d.image.thumb_url

Thumbnail Options

By default thumbnails will be generated with size 128, 128 and in PNG format. This can be changed by sublcassing the AttachedImage class and specifying the thumbnail_size and thumbnail_format attributes:

class BigThumbnailAttachedImage(AttachedImage):
    thumbnail_size = (320, 320)
    thumbnail_format = 'jpg'

class Document(DeclarativeBase):
    __tablename__ = 'document'

    uid = Column(Integer, autoincrement=True, primary_key=True)
    image = Column(Attachment(BigThumbnailAttachedImage))