Files and images

Working with file and image fields, including BLOBs

Plone has dedicated File and Image types, and it is often preferable to use these for managing files and images. However, it is sometimes useful to treat fields on an object as binary data. When working with Dexterity, you can accomplish this by using plone.namedfile and plone.formwidget.namedfile.

The plone.namedfile package includes four field types, all found in the plone.namedfile.field module:

  • NamedFile stores non-BLOB files. This is useful for small files when you don’t want to configure BLOB storage.
  • NamedImage stores non-BLOB images.
  • NamedBlobFile stores BLOB files (see note below). It is otherwise identical to NamedFile.
  • NamedBlobImage stores BLOB images (see note below). It is otherwise identical to NamedImage.

In use, the four field types are all pretty similar. They actually store persistent objects of type plone.namedfile.NamedFile, plone.namedfile.NamedImage, plone.namedfile.NamedBlobFile and plone.namedfile.NamedBlobImage, respectively. Note the different module! These objects have attributes like data, to access the raw binary data, contentType, to get a MIME type, and filename, to get the original filename. The image values also support _height and _width to get image dimensions.

To use the non-BLOB image and file fields, it is sufficient to depend on plone.formwidget.namedfile, since this includes plone.namefile as a dependency. We prefer to be explicit in, however, since we will actually import directly from plone.namedfile:



Again, we do not need separate <include /> lines in configure.zcml for these new dependencies, because we use <includeDependencies />.

For the sake of illustration, we will add an image of the speaker to the Presenter type. In, we add:

from plone.namedfile.field import NamedBlobImage

class IPresenter(model.Schema):

    picture = NamedBlobImage(
        title=_(u"Please upload an image"),

To use this in a view, we can either use a display widget via a DisplayForm, or construct a download URL manually. Since we don’t have a DisplayForm for the Presenter type, we’ll do the latter (of course, we could easily turn the view into a display form as well).

In presenter_templates/, we add this block of TAL:

<div tal:define="picture nocall:context/picture"
    <img tal:attributes="src string:${context/absolute_url}/@@download/picture/${picture/filename};
                         height picture/_height | nothing;
                         width picture/_width | nothing;"

This constructs an image URL using the @@download view from plone.namedfile. This view takes the name of the field containing the file or image on the traversal subpath (/picture), and optionally a filename on a further sub-path. The filename is used mainly so that the URL ends in the correct extension, which can help ensure web browsers display the picture correctly. We also define the height and width of the image based on the values set on the object.

Access to image scales is similar:

<div tal:define="picture nocall:context/picture"
    <img tal:replace="structure context/@@images/picture/scale" />

where scales is large, preview, mini, thumb, tile, icon, or a custom scale. This code generates a full tag, including height and width attributes and alt and title based on the context title. To generate just a URL, use code like:

<img tal:attributes="src string: ${context/absolute_url}/@@images/picture/scale" />

For file fields, you can construct a download URL in a similar way, using an <a /> tag, e.g.:

<a tal:attributes="href string:${context/absolute_url}/@@download/some_field/${context/some_field/filename}" />