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cinje 1.1.0

A Pythonic and ultra fast template engine DSL.

© 2015-2016 Alice Bevan-McGregor and contributors.
https://github.com/marrow/cinje

What is cinje?

Cinje is a modern, elegant template engine constructed as a Python domain specific language (DSL) that integrates into your applications as any other Python code would: by importing them. Your templates are transformed from their source into clean, straightforward, and understandable Python source prior to the Python interpreter compiling it to bytecode.

What kind of name is cinje?!

It’s a word from the constructed language Lojban. A combination of Hindi “śikana”, English “wrinkle”, and Chinese “zhé”. It translates as “is a wrinkle/crease/fold [shape] in”. It’s also a Hungarian noun representing the posessive third-person singular form of “cin”, meaning “tin”. The “c” makes a “sh” sound, the “j” makes a “jy” sound almost like the “is” in “vision”. Correct use does not capitalize the name except at the beginning of sentences.

Rationale and Goals

There is no shortage of template engines available in the Python ecosystem. The following items help differentiate cinje from the competition:

  • There are few to no high-performance template engines which support:
    • Mid-stream flushing for delivery of partial content as it generates. The vast majority of engines buffer the entire template during rendering, returning the result once at the end. This is disadvantageous for any content which involves large amounts of computation, and prevents browsers from eagerly loading external static assets. By comparison, cinje supports a : flush command to yield the buffer generated so far.
    • Direct use as a WSGI iterable body. In cinje, template functions are generators which can be used directly as a WSGI body. With no explicit : flush commands behaviour matches other engines: the buffer will be yielded once, at the end.
  • Virtually all require boilerplate to “load” then “render” the template, such as instantiating a Template class and calling a render method, which is silly and a waste of repeated developer effort. Alternatively, complex framework-specific adapters can be used for boilerplate-free engine use, but this solution is sub-optimal. Since almost all generate Python code in the end, why not treat the templates as Python modules to start with and let the language, which already has all of this machinery, do what it was designed to do? This is what cinje does.
  • Virtually all perform low-level parsing, lexing, and Abstract Syntax Tree (AST) manipulation. These things are difficult for developers new to the language to understand. Additionally, many manually orchestrate Python’s own parsing and compilation phases, and some even manually manage the bytecode cache. This greatly increases the complexity of the engine itself.
  • Only a small minority of engines offer extensible syntax which allows for the creation of new directives.
  • Performance is less important than streaming functionality, but it should be at least “par” with similar engines such as mako or tenjin for complete rendering times. Utilizing streaming functionality should not impose undue overhead. The capability to stream and be reasonably fast should neither obfuscate the template engine code nor obfuscate the generated template code.

Installation

Installing cinje is easy, just execute the following in a terminal:

pip install cinje

Note: We strongly recommend always using a container, virtualization, or sandboxing environment of some kind when developing using Python; installing things system-wide is yucky (for a variety of reasons) nine times out of ten. We prefer light-weight virtualenv, others prefer solutions as robust as Vagrant.

If you add cinje to the install_requires argument of the call to setup() in your application’s setup.py file, cinje will be automatically installed and made available when your own application or library is installed. We recommend using “less than” version numbers to ensure there are no unintentional side-effects when updating. Use cinje<1.2 to get all bugfixes for the current release, and cinje<2.0 to get bugfixes and feature updates while ensuring that large breaking changes are not installed.

While cinje does not have any hard dependencies on any other package, it is strongly recommended that applications using cinje also install the markupsafe package to provide more efficient string escaping and some additional functionality.

Development Version

Development takes place on GitHub in the cinje project. Issue tracking, documentation, and downloads are provided there.

Installing the current development version requires Git, a distributed source code management system. If you have Git you can run the following to download and link the development version into your Python runtime:

git clone https://github.com/marrow/cinje.git
(cd cinje; python setup.py develop)

You can then upgrade to the latest version at any time:

(cd cinje; git pull; python setup.py develop)

If you would like to make changes and contribute them back to the project, fork the GitHub project, make your changes, and submit a pull request. This process is beyond the scope of this documentation; for more information see GitHub’s documentation.

Getting Started

In order for imports of cinje template functions to correctly transform the source you must first import cinje in order to register the file encoding. This may sound like magic, but it’s not: it’s just the Python unicode decoding hook in the cinje.encoding module. Once this has been done you can directly import functions from cinje modules.

Your cinje template files are Python modules like any other: they should have a .py filename extension and begin with the the encoding declaration:

# encoding: cinje

This tells Python to process the file using the cinje codec prior to interpreting the code. Cinje itself assumes the file is actually UTF-8 encoded.

Calling a cinje function is identical to calling a generator function, as all cinje template functions—those containing text—are generators. Normal template functions generate unicode fragments. Wrapper template functions will at some point generate a None value; you can iterate up to that point, and subsequently continue iterating after that point using the cinje.util.interrupt iterator to iterate up to the first None.

Primarily for testing small chunks of template template code in actual unit tests, two helpful functions are provided:

  • cinje.fragment(string, name="anonymous", **context) Transform a template fragment into a callable function.

    Only one function may be declared, either manually, or automatically. If automatic defintition is chosen the resulting function takes no arguments. Additional keyword arguments are passed through as global variables.

  • cinje.flatten(input, file=None, encoding=None, errors='strict') Return a flattened representation of a cinje chunk stream.

    This has several modes of operation. If no file argument is given, output will be returned as a string. The type of string will be determined by the presence of an encoding; if one is given the returned value is a binary string, otherwise the native unicode representation. If a file is present, chunks will be written iteratively through repeated calls to file.write(), and the amount of data (characters or bytes) written returned. The type of string written will be determined by encoding, just as the return value is when not writing to a file-like object. The errors argument is passed through when encoding.

    We can highly recommend using the various streaming IO containers available in the io module, though tempfile classes are also quite useful.

  • cinje.stream(input, encoding=None, errors='strict') Safely iterate a template generator, ignoring None values and optionally stream encoding. Used internally by cinje.flatten, this allows for easy use of a template generator as a WSGI body.

You can always also transform arbitrary template source by passing it through .decode('cinje'), which would return the resulting transformed source code.

Basic Syntax

If you have prior experience using template engines, the syntax should feel quite familiar. Lines prefixed with a colon (:) are “code”. Lines prefixed with a hash mark (#) are comments. All other lines are treated as template text. Template text is not allowed at the module level as it is not valid for a module to yield.

Code lines are processed by each of the different “block” and “inline” processor classes and runs of template text are processed by the cinje.inline.text processor, with replacements processed by the cinje.util.chunk helper function.

Text lines can have a “continuation” marker (\) on the end to denote that no newline should be emitted there.

We use a shell-like argument format for illustrating the syntax.

Variable Replacement

There are several flavours of variable replacement available. Within these use of curly braces is allowed only if the braces are balanced. Any of the helper functions mentioned can be overridden at the module or function level.

All variable replacement is a simple transformation of the source text into a function call wrapped version of the source text.

HTML/XML Escaped Replacement

${<expr>}_escape(<expr>)

The default replacement operator is a Python expression surrounded by ${ and }. In the generated code your expression will be wrapped in a call to _escape() which defaults to the escape function imported from the cinje.helpers module. If markupsafe is installed its escaping function will be used, otherwise the Python- standard html.escape function will be used. Please see the MarkupSafe documentation for a full description of the additional capabilities it offers. The result is appended to the current buffer.

cinje Python Result
${2+2} _escape(2+2) "4"
${"<i>Hi.</i>"} _escape("<i>Hi.</i>") "&lt;i&gt;Hi.&lt;/i&gt;"

Unescaped Replacement

#{<expr>}_bless(<expr>)

The less-safe replacement does not escape HTML entities; you should be careful where this is used. For trusted data, though, this form is somewhat more efficient. In the generated code your expression will be wrapped in a call to _bless() which defaults to the bless function imported from the cinje.helpers module. If markupsafe is installed its Markup class will be used, otherwise the Python str function will be used. The result is appended to the current buffer.

cinje Python Result
#{27*42} _bless(27*42) "1134"
#{"<i>Hi.</i>"} _bless("<i>Hi.</i>") "<i>Hi.</i>"

HTML Attributes Replacement

&{<argspec>}_args(<argspec>)

A frequent pattern in reusable templates is to provide some method to emit key/value pairs, with defaults, as HTML or XML attributes. To eliminate boilerplate cinje provides a replacement which handles this naturally and can help users, especially users new to template engines, avoid certain common but hideous structures to conditionally add attributes.

Attributes which are literally True have no emitted value. Attributes which are literally False or None are omitted. Non-string iterables are treated as a space-separated set of strings, for example, for use as a set of CSS classes. Trailing underscores are removed, to allow for use of Python-reserved words. Single underscores (_) within the key are replaced with hyphens. Double underscores (__) within a key are replaced with colons.

A value can be provided, then defaults provided using the key=value keyword argument style; if the key does not have a value in the initial argument, the default will be used.

cinje Python Result
&{autocomplete=True} _args(autocomplete=True) " autocomplete"
&{autocomplete=False} _args(autocomplete=False) "" (empty)
&{data_key="value"} _args(data_key="value") ' data-key="value"'
&{xmlns__foo="bob"} _args(xmlns__foo="bob") ' xmlns:bob="foo"'
&{name="Bob Dole"} _args(name="Bob Dole") ' name="Bob Dole"'
&{somevar, default=27} _args(somevar, default="hello") (depends on somevar)

A preceeding space will be emitted automatically if any values would be emitted. The following would be correct:

<meta&{name=name, content=content}>

Formatted Replacement

%{<expr> <argspec>}_bless(<expr>).format(<argspec>)

Modern string formatting in Python utilizes the str.format string formatting system. To facilitate replacements using the advanced formatting features available in markupsafe while removing common boilerplate the “formatted replacement” is made available. Your source expression undergoes some mild reformatting, similar to that applied to function declarations, seen later.

cinje Python
%{somevar 42, num=27} _bless(somevar).format(42, num=27)
%{"Lif: {}  {num}" 42, num=27} _bless("Lif: {}  {num}").format(42, num=27)

Any expression can be used for the “format string” expression, however for sanity’s sake it’s generally a good idea to keep it as a short string literal or provide it from a variable.

Note: The format string is blessed, meaning it should not be sourced from user-supplied data, for security reasons. When MarkupSafe is not installed the replacements are passed through to Python-standard string formatting. If, however, MarkupSafe is installed, then the replacements are escaped prior to formatting and additional functionality is available to make your objects HTML-formatting aware. (See the MarkupSafe documentation.)

JSON Object Replacement

@{<expr>}_json(<expr>)

It is sometimes useful to pass data through a template to JavaScript. This will emit the JSON-serialized version of the expression result.

Block Transformations

Block transformations typically denote some form of scope change or flow control, and must be terminated with an “end” instruction. Blocks not terminated by the end of the file will be automatically terminated, allowing trailing terminators to be elided away and omitted from most templates.

Module Scope

This is an automatic transformer triggered by the start of a source file. It automatically adds a few imports to the top of your file to import the required helpers from cinje.

By default the buffer flag is enabled in all modules.

Function Declaration

: def <name-literal>[ <argspec>]def <name-literal>([<argspec>][<scope-binding>]):

Lines beginning with : def are used to declare functions within your template source:

: def somefunction
        Hello world!
: end

The above transforms to, roughly, the following Python source:

def somefunction(*, _escape=_escape, _bless=_bless):
        _buffer = []
        _buffer.append(_bless("\tHello world!\n"))
        yield ''.join(_buffer)

You do not need the extraneous trailing colon to denote the end of the declaration, nor do you need to provide parenthesis around the argument specification. The optimization keyword-only arguments will be added automatically to the argument specification you give on non-Pypy Python 3 versions. It will gracefully handle integration into your arglist even if your arglist already includes the keyword-only marker, or combinations of *args or **kw.

You can specify flags to enable or disable within the context of a specific function using Python 3 function annotations. These annotations will work for setting and unsetting flags across both Python 2 and Python 3 runtimes.

The most common use of per-function flags is to disable buffering, or enable whitespace stripping:

: def anotherfunction -> !buffer strip
        This won't have a trailing newline, and will be immediately yielded.

The result of this would be:

def anotherfucntion(...):
        yield "This won't have a trailing newline, and will be immediately yielded."

Flags declared in this way will have their effect reversed automatically at the close of the function scope.

Flow Control

: <statement><statement>:

Cinje is fairly agnostic towards most Python flow control statements. The cinje.block.generic transformer handles most Python block scope syntax. These include:

  • Conditionals including if, elif, and else.
  • Iterators including while, and for, inlcuding the else block for for loops.
  • Context managers via with.
  • Exception handling including try, except, finally, and else.

In all cases the only real transformation done is moving the colon from the beginning of the declared line to the end.

A helper is provided called iterate which acts similarly to enumerate but can provide additional details. It’s a generator that yields namedtuple values in the form (first, last, index, total, value). If the current loop iteration represents the first iteration, first will be True. Similarly—and even for generators where a total number of values being iterated could not be calculated beforehand—on the final iteration last will be True. The index value is an atomic counter provided by enumerate, and total will be the total number of elements being iterated if the object being iterated supports length determination. You can loop over its results directly:

: for item in iterate(iterable)
        : if item.first
                …
        : end
: end

You can also unpack them:

: for first, last, index, total, value in iterate(iterable)
        …
: end

If you wish to unpack the values being iterated, you can wrap the additional unpacking in a tuple:

: for first, last, i, total, (foo, bar, baz) in iterate(iterable)
        …
: end

Inline Transformations

Inline transformations are code lines that do not “start” a section that subsequently needs an “end”.

Code

Lines prefixed with a colon (:) that aren’t matched by another transformation rule are treated as inline Python code in the generated module. Within these bits of code you do have access to the helpers and buffer, and so can easily customize template rendering at will.

The only lines acceptable at the module scope are code and comments.

Comments

Basic comments are preserved in the final Python source. Any line starting with the Python-standard line comment prefix, a # hash mark or “pound” symbol, that doesn’t match another rule, will be preserved as a comment. If the line is instead prefixed with a double hash mark ## the comment will be stripped and not included in the final Python module.

Flush

The : flush statement triggers cinje to emit the Python code needed to yield the current contents of the template buffer and clear it. The result, in Python, is roughly analogous to:

yield ''.join(_buffer)
_buffer.clear()

A flush is automatically triggered when falling off the bottom of a template function if it is known that there will be un-flushed text in the buffer. (Processing context marked with the “dirty” flag.)

Text

Text covers every other line present in your template source. Cinje efficiently gathers consecutive lines of template text, collapses runs of static text into single strings, and splits the template text up to process replacements.

Template text is not permitted at the module scope as there can be no way to “yield” the buffer from there. To save on method calls, the following:

<meta&{name=name, content=content}>

Is transformed, roughly, into the following single outer call and three nested calls:

_buffer.extend((
        _bless('<meta'),
        _args(name=name, content=content),
        _bless('>')
))

See the Variable Replacement section for details on the replacement options that are available and how they operate.

Pragma

New in Version 1.1

The : pragma <flag>[ <flag>][...] directive allows you to enable or disable one or more processing flags. Usage is straightforward; to add a flag to the current set of flags:

: pragma flag

To subsequently remove a flag:

: pragma !flag

Multiple flags may be whitespace separated and can mix addition and removal:

: pragma flag !other_flag

No flag may contain whitespace. Built-in flags include:

  • init: The module scope has been prpared. Unsetting this is unwise.
  • text: Text fragments have been utilized within the current function, making this a template function.
  • dirty: It is known to the engine that the current buffer contains content which will need to be flushed.
  • buffer: Enabled by default, its presence tells cinje to use a buffer with explicit flushing. When removed, buffering is disabled and every fragment is flushed as it is encountered, and : use and : using behaviour is altered to yield from instead of adding the child template to the buffer. It is potentially very useful to disable this in the context of : use and : using to make child template : flush statements effective.
  • using: Indicates the _using_stack variable is available at this point in the translated code, i.e. to track nested : using stateements.

Inheritance

Due to the streaming and “native Python code” natures of cinje, template inheritance is generally handled through the standard definition of functions, and passing of those first-class objects around. The most common case, where one template “wraps” another, is handled through the : using and : yield directives.

An example “wrapper” template:

: def page **properties
<html>
        <body&{properties}>
                : yield
        </body>
</html>
: end

When called, functions that include a bare yield (and only one is allowed per function) will flush their buffers automatically prior to the yield, then flush automatically at the end of the function, just like any other. This has the effect of extending the wrapped template’s buffer by, at a minimum, two elements (prefix and suffix), though additional : flush statements within the wrapper are allowed.

Note: Because the bare yield will produce a value of None, wrapping functions like these are not safe for use as a WSGI body iterable without wrapping in a generator to throw away None values.

The syntax for the using directive is : using <expr>[ <argspec>], thus to use this wrapper:

: using page
        <p>Hello world!</p>
: end

Execution of this would produce the following HTML:

<html>
        <body>
                <p>Hello world!</p>
        </body>
</html>

Because wrapping templates are just template functions like any other, you can pass arguments to them. In the above example we’re using arbitrary keyword arguments as an “HTML attribute” replacement. The following:

: using page class_="hero"
: end

Would produce the following:

<html>
        <body class="hero">
        </body>
</html>

Lastly, there is a quick shortcut for consuming a template function and injecting its output into the current buffer:

: use <expr>[ <argspec>]

And directly transforms to:

_buffer.extend(<expr>(<argspec>))

Just like with using, the result of the expression must be a callable generator function.

Version History

Version 1.1

  • Enhanced Pypy support. Pypy does not require optimizations which potentially obfuscate the resulting code. So we don’t do them.
  • Fixed incorrect #{} handling when it was the first non-whitepsace on a line. (#22)
  • Fixed buffer iteration edge case if the first template text in a function is deeper than the function scope. (#21)
  • Python 3-style function annotations can now be used to define function-wide “pragma” additions and removals, even on Python 2. (#8)
  • Pragma processing directives. Processing flags can be set and unset during the translation process using : pragma.
  • Unbuffered mode. Cinje can now operate in unbuffered mode. Each contiguous chunk is individually yielded. (#8)
  • Secret feature. Have a cinje template? Want to more easily peek behind the curtain? (Sssh, it’s a completely unsupported feature that even syntax colors if pygments is installed.) python -m cinje source file.py

Version 1.0

  • Initial release.

License

cinje has been released under the MIT Open Source license.

The MIT License

Copyright © 2015-2016 Alice Bevan-McGregor and contributors.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

 
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