Mako Documentation - The Unicode Chapter

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# Mako Documentation

Version: 0.2.5 Last Updated: 09/07/09 15:28:26

### The Unicode Chapter

The Python language in the 2.x series supports two ways of representing string objects. One is the `string` type and the other is the `unicode` type, both of which extend a type called `basestring`. A key issue in Python, which is hopefully to be resolved in Python 3000, is that objects of type `string` (i.e. created from an expression such as `"hello world"`) contain no information regarding what encoding the data is stored in. For this reason they are often referred to as byte strings. The origins of this come from Python's background of being developed before the Unicode standard was even available, back when strings were C-style strings and were just that, a series of bytes. Strings that had only values below 128 just happened to be ascii strings and were printable on the console, whereas strings with values above 128 would produce all kinds of graphical characters and bells.

Contrast the Python `string` type with the Python `unicode` type. Objects of this type are created whenever you say something like `u"hello world"`. In this case, Python represents each character in the string internally using multiple bytes per character (something similar to UTF-16). Whats important is that when using the `unicode` type to store strings, Python knows the data's encoding; its in its own internal format. Whereas when using the `string` type, it does not.

When Python attempts to treat a byte-string as a string, which means its attempting to compare/parse its characters, to coerce it into another encoding, or to decode it to a unicode object, it has to guess what the encoding is. In this case, it will pretty much always guess the encoding as `ascii`...and if the bytestring contains bytes above value 128, you'll get an error.

There is one operation that Python can do with a non-ascii bytestring, and its a great source of confusion: it can dump the bytestring straight out to a stream or a file, with nary a care what the encoding is. To Python, this is pretty much like dumping any other kind of binary data (like an image) to a stream somewhere. So in a lot of cases, programs that embed all kinds of international characters and encodings into plain byte-strings (i.e. using `"hello world"` style literals) can fly right through their run, sending reams of strings out to whereever they are going, and the programmer, seeing the same output as was expressed in the input, is now under the illusion that his or her program is Unicode-compliant. In fact, their program has no unicode awareness whatsoever, and similarly has no ability to interact with libraries that are unicode aware.

The "pass through encoded data" scheme is what template languages like Cheetah and earlier versions of Myghty do by default. Mako as of version 0.2 also supports this mode of operation using the "disable_unicode=True" flag. However, when using Mako in its default mode of unicode-aware, it requires explicitness when dealing with non-ascii encodings. Additionally, if you ever need to handle unicode strings and other kinds of encoding conversions more intelligently, the usage of raw bytestrings quickly becomes a nightmare, since you are sending the Python interpreter collections of bytes for which it can make no intelligent decisions with regards to encoding.

In normal Mako operation, all parsed template constructs and output streams are handled internally as Python `unicode` objects. Its only at the point of `render()` that this unicode stream is rendered into whatever the desired output encoding is. The implication here is that the template developer must ensure that the encoding of all non-ascii templates is explicit, that all non-ascii-encoded expressions are in one way or another converted to unicode, and that the output stream of the template is handled as a unicode stream being encoded to some encoding.

### Specifying the Encoding of a Template File

This is the most basic encoding-related setting, and it is equivalent to Python's "magic encoding comment", as described in pep-0263. Any template that contains non-ascii characters requires that this comment be present so that Mako can decode to unicode (and also make usage of Python's AST parsing services). Mako's lexer will use this encoding in order to convert the template source into a `unicode` object before continuing its parsing:

    ## -*- coding: utf-8 -*-

    Alors vous imaginez ma surprise, au lever du jour, quand une drôle de petit voix m’a réveillé. Elle disait: « S’il vous plaît… dessine-moi un mouton! »

For the picky, the regular expression used is derived from that of the abovementioned pep:

    #.*coding[:=]\s*([-\w.]+).*\n

The lexer will convert to unicode in all cases, so that if any characters exist in the template that are outside of the specified encoding (or the default of `ascii`), the error will be immediate.

As an alternative, the template encoding can be specified programmatically to either `Template` or `TemplateLookup` via the `input_encoding` parameter:

    t = TemplateLookup(directories=['./'], input_encoding='utf-8')

The above will assume all located templates specify `utf-8` encoding, unless the template itself contains its own magic encoding comment, which takes precedence.

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### Handling Expressions

The next area that encoding comes into play is in expression constructs. By default, Mako's treatment of an expression like this:

    ${"hello world"}

looks something like this:

    context.write(unicode("hello world"))

That is, the output of all expressions is run through the `unicode` builtin. This is the default setting, and can be modified to expect various encodings. The `unicode` step serves both the purpose of rendering non-string expressions into strings (such as integers or objects which contain `__str()__` methods), and to ensure that the final output stream is constructed as a unicode object. The main implication of this is that any raw bytestrings that contain an encoding other than ascii must first be decoded to a Python unicode object. It means you can't say this:

    ${"voix m’a réveillé."}  ## error !

You must instead say this:

    ${u"voix m’a réveillé."}  ## OK !

Similarly, if you are reading data from a file, or returning data from some object that is returning a Python bytestring containing a non-ascii encoding, you have to explcitly decode to unicode first, such as:

    ${call_my_object().decode('utf-8')}

If you want a certain encoding applied to all expressions, override the `unicode` builtin with the `decode` builtin at the `Template` or `TemplateLookup` level:

    t = Template(templatetext, default_filters=['decode.utf8'])

Note that the built-in `decode` object is slower than the `unicode` function, since unlike `unicode` its not a Python builtin, and it also checks the type of the incoming data to determine if string conversion is needed first.

The `default_filters` argument can be used to entirely customize the filtering process of expressions. This argument is described in The default_filters Argument.

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### Defining Output Encoding

Now that we have a template which produces a pure unicode output stream, all the hard work is done. We can take the output and do anything with it.

As stated in the "Usage" chapter, both `Template` and `TemplateLookup` accept `output_encoding` and `encoding_errors` parameters which can be used to encode the output in any Python supported codec:

    from mako.template import Template
    from mako.lookup import TemplateLookup

    mylookup = TemplateLookup(directories=['/docs'], output_encoding='utf-8', encoding_errors='replace')

    mytemplate = mylookup.get_template("foo.txt")
    print mytemplate.render()

And `render_unicode()` will return the template output as a Python `unicode` object:

    print mytemplate.render_unicode()

The above method disgards the output encoding keyword argument; you can encode yourself by saying:

    print mytemplate.render_unicode().encode('utf-8', 'replace')

### Buffer Selection

Mako does play some games with the style of buffering used internally, to maximize performance. Since the buffer is by far the most heavily used object in a render operation, its important!

When calling `render()` on a template that does not specify any output encoding (i.e. its `ascii`), Python's `cStringIO` module, which cannot handle encoding of non-ascii `unicode` objects (even though it can send raw bytestrings through), is used for buffering. Otherwise, a custom Mako class called `FastEncodingBuffer` is used, which essentially is a super dumbed-down version of `StringIO` that gathers all strings into a list and uses `u''.join(elements)` to produce the final output - its markedly faster than `StringIO`.

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### Saying to Heck with it: Disabling the usage of Unicode entirely

Some segements of Mako's userbase choose to make no usage of Unicode whatsoever, and instead would prefer the "passthru" approach; all string expressions in their templates return encoded bytestrings, and they would like these strings to pass right through. The generated template module is also in the same encoding as the template and additionally carries Python's "magic encoding comment" at the top. The only advantage to this approach is that templates need not use `u""` for literal strings; there's an arguable speed improvement as well since raw bytestrings generally perform slightly faster than unicode objects in Python. For these users, they will have to get used to using Unicode when Python 3000 becomes the standard, but for now they can hit the `disable_unicode=True` flag, introduced in version 0.2 of Mako, as so:

    # -*- encoding:utf-8 -*-
    from mako.template import Template

    t = Template("drôle de petit voix m’a réveillé.", disable_unicode=True, input_encoding='utf-8')
    print t.code

The generated module source code will contain elements like these:

    # -*- encoding:utf-8 -*-
    #  ...more generated code ...

    def render_body(context,**pageargs):
        context.caller_stack.push_frame()
        try:
            __M_locals = dict(pageargs=pageargs)
            # SOURCE LINE 1
            context.write('dr\xc3\xb4le de petit voix m\xe2\x80\x99a r\xc3\xa9veill\xc3\xa9.')
            return ''
        finally:
            context.caller_stack.pop_frame()

Where above you can see that the `encoding` magic source comment is at the top, and the string literal used within `context.write` is a regular bytestring.

When `disable_unicode=True` is turned on, the `default_filters` argument which normally defaults to `["unicode"]` now defaults to `["str"]` instead. Setting default_filters to the empty list `[]` can remove the overhead of the `str` call. Also, in this mode you cannot safely call `render_unicode()` - you'll get unicode/decode errors.

Rules for using disable_unicode=True

- don't use this mode unless you really, really want to and you absolutely understand what you're doing
- don't use this option just because you don't want to learn to use Unicode properly; we aren't supporting user issues in this mode of operation. We will however offer generous help for the vast majority of users who stick to the Unicode program.
- it's extremely unlikely this mode of operation will be present in the Python 3000 version of Mako since P3K strings are unicode objects by default; bytestrings are relegated to a "bytes" type that is not intended for dealing with text.

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