Contains image scaling logic for use in Zope environments. It supports Zope 2, Pyramid and other systems build on using the Zope ToolKit (ZTK).
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This package contains image scaling logic for use in Zope environments. It supports Zope 2, grok and other systems build on using the Zope ToolKit (ZTK).

Several design goals were used when writing this package:

  • image scaling to any width, height, width&height should be supported using both up-scaling and down-scaling. Scaling parameters should never be fixed in code. This allows designers to use any image scale they want without having to modify python code.
  • the result of scaling will be an image along with its new size, not a HTML or XHTML tag. We already have excellent tools to generate tags in the form of Zope Pagetemplates, Genshi and other template languages that are much better suited for this purpose.

In addition several implementation goals were defined:

  • image scaling must happen on demand instead of up-front. This reduces initial save time and prevents unnecessary scalings from being generated.
  • image scaling parameters should not be part of the generated URL. Since the number of parameters can change and new parameters may be added in the future this would create overly complex URLs and URL parsing.
  • no HTML rewriting (such as done by repoze.bitblt) should be required.
  • it should be possibly to develop an external storage system which stores scaled images externally and returns a URL which bypasses the application server. This should be configurable via just a filesystem path & base URL.
  • minimum number of external dependencies, allowing this package to be used in many environments.
  • testable without requiring zope.testing. Running test should be sufficient.
  • URLs for scaled images should have an extension which reflects their MIME type. This is facilitates cache (and other front-end services) configuration.


The most common way to use plone.scale is from a HTML template. In TAL syntax a typical usage looks like this:

<img tal:define="scales context/@@image-scaling;
                 thumbnail python:scales.scale('logo', width=64, height=64)"
     tal:attributes="src thumbnail/url;
                     width thumbnail/width;
                     height thumbnail/height" />

This generates a thumbnail of an image field called logo with a maximum size of 64x64 pixels. The dimensions of the resulting image (which might not be exactly 64x64) are set as attributes on the img tag to speed up browser rendering.

If you prefer Genshi syntax and have the IImageScaleStorage interface in scope the syntax looks like this:

<img py:with="thumbnail=IImageScaleStorage(context).scale('logo', width=64, heigh=64)"
     py:attributes="dict(src=thumbnail.url, width=thumbnail.width, height=thumbnail.height" />