The Python expression syntax
TALES python expressions allow the evaluation of arbitrary Python language expressions in your document templates. The syntax for this is:
python:python_expression
The python expression is any valid Python language expression. Python expressions have access to all of the same root contexts and defined variables as other expressions.
Python version
Python expressions make use of the IronPython library to provide the expression backend. At the time of writing, IronPython supports Python version 2.7.
IronPython does have an ongoing effort to add support for 3.x; at the time of writing it is not yet ready for use.
Python expressions are included in an add-on package
Support for python expressions are included in the ZptSharp.PythonExpressions NuGet package.
They are not one of the standard expression types; once the NuGet package is installed to your application they must be activated by using the AddZptPythonExpressions method.
Reserved markup characters must be encoded
As with all usage of expressions inside of markup, any usages of reserved characters must be encoded.
For example, the python expression print("%.2f" % 13.946) makes use of double-quote characters.
The following usage would obviously fail, because the quote characters break the DOM when used inside of an attribute.
<p>The price is <span tal:replace="python:print("%.2f" % 13.946)">0.00</span></p>
The solution is to encode the reserved markup characters, replacing double-quotes with ":
<p>The price is <span tal:replace="python:print(".2f" % 13.946)">0.00</span></p>
Example
Here is a short example of a Python expression in use. It is somewhat contrived but it demonstrates how a Python expression may make use of a defined variable.
<p tal:define="two python:2">
2 plus 2 equals <span tal:replace="python:two + 2">0</span>.
</p>