PyconIE 2016 - Kajiki, the fast and validated template engine your were looking for

KAJIKI
THE FAST AND VALIDATED TEMPLATE
ENGINE YOU WERE LOOKING FOR
Alessandro Molina
@__amol__
amol@turbogears.org

Who am I
● Passionate Python Developer
● TurboGears2 core team member
● Beaker caching/session framework
current maintainer
● Author of DukPy js env for Python and
DEPOT file storage framework
● Contributor to Ming, ToscaWidgets2,
Formencode, WebOb, Kajiki, ...

Why?
● There are tens of template engines out
there, but people often only know the
default one of their web framework.
● Different template engines can differ in
important features, not just syntax.
● Working on one has been fun and
interesting.

Template Engines
TYPE NAME URL
Markup + Streamed Genshi http://genshi.edgewall.org/
Text + Compiled Mako http://www.makotemplates.org/
Text + Compiled Jinja http://jinja.pocoo.org/
Markup + Compiled Kajiki http://kajiki.readthedocs.io/

One more Template Engine?
● We loved the fact that Genshi templates
could by opened with any HTML editor
● We loved Genshi syntax, concise and
reflecting the output itself.
● We loved Genshi so much TG2.2 included
it in all projects, to make it available for
pluggable apps and extensions.

Well...
● Genshi was pretty complex. We didn’t
want to maintain such complexity.
● Streaming was powerful but hard to use,
very few people understood genshi
inheritance. And it was really slow.
● A TG team member was experimenting
with the idea of a similar engine.

Kajiki
<html>
<head>
<title py:content="title">This is replaced.</title>
</head>
<body>
These are some fruits:
<ul>
<li py:for="fruit in fruits">
I like ${fruit}s
</li>
</ul>
</body>
</html>

Directives
● py:if & py:else
● py:switch & py:case & py:else
● py:for
● py:def
● py:strip
● py:with
● py:attrs
● py:block & py:extends
● ...

How It Works
● Your template is parsed and converted to
Python code.
● Whenever the template is rendered the
generated python code is executed.
● The python code is yielded by a generator
that resembles the DOM of the document.

Architecture
PARSER
COMPILER
INTERMEDIATE
REPRESENTATION
PYTHON
READ XML AND GENERATE DOM OUT OF IT
WITH SAX
NAVIGATE DOM AND CREATE IR FOR ITS
NODES
ITERATE THE IR NODES TO GENERATE
PYTHON CODE
RUN PYTHON CODE TO RENDER TEMPLATE

Compiled Kajiki
class template:
@kajiki.expose
def __main__():
yield u'<html>n <head>n <title>'
yield self.__kj__.escape(title)
yield u'</title>n </head>n <body>n '
yield local.__kj__.gettext(u'These are some fruits:')
yield u'n <ul>n '
for fruit in fruits:
yield u'<li>'
yield local.__kj__.gettext(u'n I like ')
yield self.__kj__.escape(fruit)
yield local.__kj__.gettext(u'sn ')
yield u'</li>'
yield u'n </ul>n </body>n</html>'

Rendered Kajiki
<html>
<head>
<title>A Kajiki Template</title>
</head>
<body>
These are some fruits:
<ul>
<li>
I like oranges
</li><li>
I like apples
</li>
</ul>
</body>
</html>

That makes it pretty Fast
● Mako
○ Rendered 1M mako templates in 22.217651844
● Kajiki
○ Rendered 1M Kajiki templates in 11.8710489273
%for user in users:
Hello {{ user }}!
%endfor

Hello ${user}!

It’s really just python!
Mad science included!
import kajiki
loader = kajiki.FileLoader('.', force_mode='xml')
tmpl = loader.load('mypage.kajiki')
with open('cython_test.pyx', 'wb') as pyx:
py_text = tmpl.py_text.replace('@kajiki.expose', '@staticmethod')
py_text = py_text.replace('__main__():', '__main__(self, local, users):')
py_text = py_text.replace('template = kajiki.Template(template)', '')
pyx.write(py_text)
import pyximport; pyximport.install()
import cython_test
class template:
@kajiki.expose
def __main__():
return cytmpl.__main__(self, local, users)
template = kajiki.Template(template)
print template(dict(cytmpl=cython_test.template, users=range(100000))).render()

Don’t try this at home
● Huge hack, but theoretically it can work.
With minor tweaks Kajiki itself could
generate cython compatible code.
● Rendering python 11.86215686798
● Rederning cython 9.07893800735

But being Python is easy to debug

Integrates with Python debuggers

Python Syntax Checking
kajiki.template.KajikiSyntaxError: [<string>:9]
invalid syntax
yield local.__kj__.gettext(u'These are some of
my favorite fruits:')
yield u'n <ul>n '
--> for fruit on fuits:
yield u'<li>'

Validated Templates
● As Kajiki understands the document you
are writing (it’s not just text) it can take
steps specific to HTML generation:
○ Error Reporting
○ Escaping
○ Automatic i18n
○ Minification

Malformed HTML Detection
kajiki.xml_template.XMLTemplateParseError:
[./plain.kajiki:10] mismatched tag
I like ${fruit}s
--> 
</ul>
</body>

Escaping handled for us
I like ${fruit}s
</li>
template(dict(
fruits=['<apple>'],
title='A Kajiki Template'
)).render()
<li>
I like <apple>s
</li>

Easy Translations
I like ${fruit}s
</li>
for fruit on fuits:
yield u'<li>'

Minification
tmpl = loader.load('plain.kajiki', strip_text=False)
<ul>
<li>
I like oranges
</li><li>
I like apple
</li><li>
I like kiwis
</li>
</ul>
tmpl = loader.load('plain.kajiki', strip_text=True)
<ul><li>I like oranges</li><li>I like apple</li><li>I like
kiwis</li></ul>

Text engine available too
>>> Template = kajiki.TextTemplate('''
... {%if foo %}
... bar
... {%else%}
... baz
... {%end%}
... ''')
>>> print(Template(dict(foo=True)).render())
bar
>>> print(Template(dict(foo=False)).render())
baz

Feel free to try it!
● Python 2.6, 2.7, 3.2, 3.3, 3.4 and 3.5
● pip install kajiki
● Come and try it!
https://github.com/nandoflorestan/kajiki
● Still young! Feel free to open issues, send
pull requests, suggest features!

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