XHPy 0.83

XHP for Python

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The Apache License 2.0 
Evan Stratford
ROOT \ Programming \ Code Generators
XHPy extends Python syntax so that XML fragments become valid Python expressions.


 * Simplicity: write UI logic in a simple, expressive syntax without the need for external templates or templating languages.
 * Flexibility: use Python expressions freely within XHPy tags, and vice-versa.
 * Security: benefit from automatic escaping of text within XHPy tags.
 * Reusability: build reusable components by subclassing :x:element.

An example

In bar.py:

from xhpy.init import register_xhpy_module
import foo

In foo.py:

from xhpy.pylib import *
class :ui:foo(:x:element):
 attribute list bar
 category %flow
 def render(self):
 a = < ul / >
 for b in self.getAttribute('bar'):
 a.appendChild(< li >{b}< /li >)
 return a
print < div class="baz" >< ui:foo bar={range(3)} / >< /div >

We can now run bar.py as a normal Python script:

$ python bar.py
< div class="baz" >< ul >< li >0< /li >< li >1< /li >< li >2< /li >< /ul >< /div >

Congratulations! You just wrote your first snippet of XHPy.


XHPy adds some new syntax to Python. Line by line replay time!

from xhpy.init import register_xhpy_module

This initializes XHPy and allows you to register modules to be interpreted as XHPy.


Now the foo module in foo.py will be interpreted as XHPy when imported. If foo were a package, all of its submodules would also be registered; this is useful for registering UI libraries.

import foo

To actually use XHPy, however, you will probably want the core library:

from xhpy.pylib import *

Now you have access to all the standard HTML 4.0 elements, the :x:element base class (this is what you build custom components on top of!), and some utilities.

class :ui:foo(:x:element):

Making new components is easy: just subclass :x:element. For your component class to be registered, it must start with : - this clearly distinguishes your components from ordinary Python classes.

attribute list bar

This is an attribute declaration, meaning that :ui:foo allows bar attributes on < ui:foo > tags. Note the

< ui:foo bar={range(3)} / >

later on - like XHP, XHPy uses XML attribute syntax.

category %flow

This is a category declaration - :ui:foo is part of the %flow category. Categories are primarily useful as a way of identifying elements that are similar without using inheritance; for example, the < a > tag in pylib.html has

children (pcdata | %flow)*

indicating that its children must either contain text or be of the %flow category. (So we can put < ui:foo > inside < a >!)

def render(self):

When you print an :x:element (or call str on it), the render() method is invoked; this is where you put your UI logic.

a = < ul / >
for b in self.getAttribute('bar'):
 a.appendChild(< li >{b}< /li >)
return a

Here, < ui:foo > is a thin wrapper around < ul > that allows you to construct an unordered list out of a Python list. Standard HTML elements like < ul > and < li > are automatically rendered - except that, in XHPy, you can use Python expressions within tags, so that


is replaced by the value of b. Note the use of getAttribute() and appendChild():


fetches the value of attribute bar (in this case, range(3)), whereas

a.appendChild(< li >{b}< /li >)

adds < li >{b}< /li > as a child of a = < ul / >.

XHPy is largely based off XHP; for more details on the latter, see the `XHP wiki < https://github.com/facebook/xhp/wiki/`_. The syntax has been adapted for Python; in particular:

 * there are no semicolons;
 * XHPy class names may be used anywhere ordinary Python classes can;
 * XHPy tags ignore internal whitespace, but must externally obey indentation and line continuation rules.

More on the last point:

def foo(href):
 return < a href={href} >< /a >

def bar(href):
 < a href={href} >< /a >

are valid, whereas

def foo(href):
 < a href={href} >
 < /a >

is not, as it introduces an extra dedent after < /a >.

How it works

When you

import xhpy.init

XHPy installs an import hook. This hook traps subsequent import statements, running them through a preprocessor that parses a superset of Python. This preprocessor translates XHPy tags and class names to valid Python, then executes the translated code in module scope.

This is similar to how XHP works, except:

 * with, e.g., pythonenv, you can always use XHPy even without access to system-wide Python package installation directories;
 * by default, Python compiles bytecode .pyc files from your modules, so the preprocessing only needs to be done once when a module is first imported.

Last updated on June 21st, 2012


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