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pybars3 - Handlebars.js for Python 3 and 2

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Pybars3 provides a template system for Python which is compatible with Handlebars.js. It is a fork of the pybars project that adds Python 3 compatibility and numerous features from Handlebars.js 2.0.

Installation

pip install pybars3

Handlebars.js Compatibility

This is somewhat of a side-project for the current developers, and is maintained for almost purely pragmatic reasons. Being able to share templates between the server and client-side is very useful, and we like having something more powerful than Mustache.

So, with that information, you should realize that the code is probably messy, that there are certainly bugs and not all of Handlebars 2.0, or even 1.1 is currently implemented.

Here is a partial list of features that are supported:

  • @root root data accesor (Handlebars 2.0)
  • @_parent parent scope accesor (Handlebars 2.0)
  • ../ parent scope accessor
  • @index, @key (Handlebars 1.0, 1.2)
  • @first and @last data element in the #each helper (Handlebars 1.1)
  • kwargs passed to partials (Handlebars 2.0)
  • @../index syntax for accessing parent scope data items (Handlebars 2.0)
  • {{[segment literal notation]}} for paths that contain non-word chars (Handlebars 1.1)
  • {{> "quoted partial name"}} for partials that contain non-word chars (Handlebars 1.1)
  • lookup helper for dynamic name access (Handlebars 2.0)
  • Subexpresions (Handlebars 1.3)
  • Lines containing only block statements and whitespace are removed (Handlebars 2.0)
  • pybars.Compiler().precompile() that is equivalent to Handlebars.precompile()
  • {{> (whichPartial) }} dynamic partials (Handlebars 3.0)
  • {{{{raw}}}}{{escaped}}{{{{/raw}}}} raw blocks (Handlebars 2.0)
  • Whitespace control, {{var~}} (Handlebars 1.1)

Feel free to jump in with issues or pull requests.

Usage

For details on the template language see the http://handlebarsjs.com documentation.

Typical usage:

# Get a compiler
from pybars import Compiler
compiler = Compiler()

# Compile the template
source = u"{{>header}}{{#list people}}{{firstName}} {{lastName}}{{/list}}"
template = compiler.compile(source)

# Add any special helpers
def _list(this, options, items):
    result = [u'<ul>']
    for thing in items:
        result.append(u'<li>')
        result.extend(options['fn'](thing))
        result.append(u'</li>')
    result.append(u'</ul>')
    return result
helpers = {'list': _list}

# Add partials
header = compiler.compile(u'<h1>People</h1>')
partials = {'header': header}

# Render the template
output = template({
    'people': [
        {'firstName': "Yehuda", 'lastName': "Katz"},
        {'firstName': "Carl", 'lastName': "Lerche"},
        {'firstName': "Alan", 'lastName': "Johnson"}
    ]}, helpers=helpers, partials=partials)

print(output)

The generated output will be:

<h1>People</h1><ul><li>Yehuda Katz</li><li>Carl Lerche</li><li>Alan Johnson</li></ul>

Handlers

Translating the engine to python required slightly different calling conventions to the JS version:

  • block helpers should accept this, options, *args, **kwargs
  • other helpers should accept this, *args, **kwargs
  • closures in the context should accept this, *args, **kwargs

A template like {{foo bar quux=1}} will pass bar as a positional argument and quux as a keyword argument. Keyword arguments have to be non-reserved words in Python. For instance, print as a keyword argument will fail.

Implementation Notes

Templates with literal boolean arguments like {{foo true}} will have the argument mapped to Python's True or False as appropriate.

For efficiency, rather that passing strings round, pybars passes a subclass of list (strlist) which has a __unicode__ implementation that returns u"".join(self). Template helpers can return any of list, tuple, unicode or strlist instances. strlist exists to avoid quadratic overheads in string processing during template rendering. Helpers that are in inner loops should return list or strlist for the same reason.

NOTE The strlist takes the position of SafeString in the js implementation: when returning a strlist it will not be escaped, even in a regular {{}} expansion.

import pybars

source = u"{{bold name}}"

compiler = pybars.Compiler()
template = compiler.compile(source)

def _bold(this, name):
    return pybars.strlist(['<strong>', name, '</strong>'])
helpers = {'bold': _bold}

output = template({'name': 'Will'}, helpers=helpers)
print(output)

The data facility from the JS implementation has not been ported at this point, if there is demand for it it would be quite easy to add. Similarly the stringParams feature has not been ported - quote anything you wish to force to a string in a helper call.

Dependencies

  • Python 2.6-2.7, 3.3+
  • PyMeta3

Development

Running tests:

python tests.py

To display the AST and generated Python code, execute:

python tests.py --debug

To run a specific test:

python tests.py TestAcceptance.test_subexpression

Or to debug a specific test:

python tests.py --debug TestAcceptance.test_subexpression

Copyright

Copyright (c) 2015 Will Bond, Mjumbe Wawatu Ukweli, 2012 Canonical Ltd

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation, version 3 only.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.
GNU Lesser General Public License version 3 (see the file LICENSE).