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1 .. -*- mode: rst; encoding: utf-8 -*-
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2
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3 ============================
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4 Markup XML Template Language
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5 ============================
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6
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7 Markup provides a simple XML-based template language that is heavily inspired
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8 by Kid_, which in turn was inspired by a number of existing template languages,
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9 namely XSLT_, TAL_, and PHP_.
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10
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11 .. _kid: http://kid-templating.org/
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12 .. _python: http://www.python.org/
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13 .. _xslt: http://www.w3.org/TR/xslt
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14 .. _tal: http://www.zope.org/Wikis/DevSite/Projects/ZPT/TAL
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15 .. _php: http://www.php.net/
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16
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17 This document describes the template language and will be most useful as
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18 reference to those developing Markup templates. Templates are XML files of some
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19 kind (such as XHTML) that include processing directives_ (elements or
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20 attributes identified by a separate namespace) that affect how the template is
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21 rendered, and template expressions_ that are dynamically substituted by
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22 variable data.
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23
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24
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25 .. contents:: Contents
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26 :depth: 3
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27 .. sectnum::
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28
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29 ----------
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30 Python API
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31 ----------
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32
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33 The Python code required for templating with Markup is generally based on the
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34 following pattern:
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35
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36 * Attain a ``Template`` object from a string or file object containing the
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37 template XML source. This can either be done directly, or through a
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38 ``TemplateLoader`` instance.
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39 * Call the ``generate()`` method of the template, passing any data that should
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40 be made available to the template as keyword arguments.
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41 * Serialize the resulting stream using its ``render()`` method.
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42
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43 For example::
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44
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45 from markup.template import Template
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46
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47 tmpl = Template('<h1>$title</h1>')
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48 stream = tmpl.generate(title='Hello, world!')
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49 print stream.render('xhtml')
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50
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51 That code would produce the following output::
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52
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53 <h1>Hello, world!</h1>
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54
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55 However, if you want includes_ to work, you should attain the template instance
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56 through a ``TemplateLoader``, and load the template from a file::
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57
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58 from markup.template import TemplateLoader
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59
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60 loader = TemplateLoader([templates_dir])
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61 tmpl = loader.load('test.html')
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62 stream = tmpl.generate(title='Hello, world!')
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63 print stream.render('xhtml')
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64
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65
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66 .. _`expressions`:
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67
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68 --------------------
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69 Template Expressions
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70 --------------------
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71
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72 Python_ expressions can be used in text and attribute values. An expression is
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73 substituted with the result of its evaluation against the template data.
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74 Expressions need to prefixed with a dollar sign (``$``) and usually enclosed in
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75 curly braces (``{…}``).
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76
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77 If the expression starts with a letter and contains only letters and digits,
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78 the curly braces may be omitted. In all other cases, the braces are required so
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79 that the template processors knows where the expression ends::
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80
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81 >>> from markup.template import Context, Template
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82 >>> tmpl = Template('<em>${items[0].capitalize()} item</em>')
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83 >>> print tmpl.generate(Context(items=['first', 'second']))
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84 <em>First item</em>
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85
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86 Expressions support the full power of Python. In addition, it is possible to
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87 access items in a dictionary using “dotted notation” (i.e. as if they were
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88 attributes), and vice-versa (i.e. access attributes as if they were items in a
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89 dictionary)::
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90
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91 >>> from markup.template import Context, Template
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92 >>> tmpl = Template('<em>${dict.foo}</em>')
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93 >>> print tmpl.generate(Context(dict={'foo': 'bar'}))
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94 <em>bar</em>
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95
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96
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97 .. _`directives`:
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98
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99 -------------------
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100 Template Directives
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101 -------------------
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102
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103 Directives are elements and/or attributes in the template that are identified
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104 by the namespace ``http://markup.edgewall.org/``. They can affect how the
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105 template is rendered in a number of ways: Markup provides directives for
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106 conditionals and looping, among others.
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107
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108 To use directives in a template, the namespace should be declared, which is
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109 usually done on the root element::
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110
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111 <html xmlns="http://www.w3.org/1999/xhtml"
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112 xmlns:py="http://markup.edgewall.org/"
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113 lang="en">
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114 ...
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115 </html>
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116
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117 In this example, the default namespace is set to the XHTML namespace, and the
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118 namespace for Markup directives is bound to the prefix “py”.
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119
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120 All directives can be applied as attributes, and some can also be used as
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121 elements. The ``if`` directives for conditionals, for example, can be used in
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122 both ways::
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123
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124 <html xmlns="http://www.w3.org/1999/xhtml"
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125 xmlns:py="http://markup.edgewall.org/"
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126 lang="en">
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127 ...
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128 <div py:if="foo">
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129 <p>Bar</p>
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130 </div>
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131 ...
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132 </html>
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133
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134 This is basically equivalent to the following::
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135
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136 <html xmlns="http://www.w3.org/1999/xhtml"
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137 xmlns:py="http://markup.edgewall.org/"
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138 lang="en">
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139 ...
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140 <py:if test="foo">
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141 <div>
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142 <p>Bar</p>
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143 </div>
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144 </py:if>
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145 ...
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146 </html>
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147
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148 The rationale behind the second form is that directives do not always map
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149 naturally to elements in the template. In such cases, the ``py:strip``
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150 directive can be used to strip off the unwanted element, or the directive can
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151 simply be used as an element.
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152
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153
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154 Available Directives
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155 ====================
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156
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157
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158 .. _`py:attrs`:
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159
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160 ``py:attrs``
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161 ------------
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162
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163 This directive adds, modifies or removes attributes from the element::
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164
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165 <ul>
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166 <li py:attrs="foo">Bar</li>
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167 </ul>
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168
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169 Given ``foo={'class': 'collapse'}`` in the template context, this would
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170 produce::
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171
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172 <ul>
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173 <li class="collapse">Bar</li>
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174 </ul>
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175
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176 Attributes with the value ``None`` are omitted, so given ``foo={'class': None}``
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177 in the context for the same template this would produce::
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178
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179 <ul>
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180 <li>Bar</li>
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181 </ul>
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182
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183 This directive can only be used as an attribute.
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184
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185
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186 .. _`py:choose`:
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187 .. _`py:when`:
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188 .. _`py:otherwise`:
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189
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190 ``py:choose`` / ``py:when`` / ``py:otherwise``
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191 ----------------------------------------------
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192
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193 This set of directives provides advanced contional processing for rendering one
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194 of several alternatives. The first matching ``py:when`` branch is rendered, or,
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195 if no ``py:when`` branch matches, the ``py:otherwise`` branch is be rendered.
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196
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197 If the ``py:choose`` directive is empty the nested ``py:when`` directives will
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198 be tested for truth::
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199
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200 <div py:choose="">
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201 <span py:when="0 == 1">0</span>
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202 <span py:when="1 == 1">1</span>
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203 <span py:otherwise="">2</span>
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204 </div>
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205
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206 This would produce the following output::
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207
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208 <div>
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209 <span>1</span>
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210 </div>
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211
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212 If the ``py:choose`` directive contains an expression the nested ``py:when``
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213 directives will be tested for equality to the parent ``py:choose`` value::
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214
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215 <div py:choose="1">
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216 <span py:when="0">0</span>
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217 <span py:when="1">1</span>
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218 <span py:otherwise="">2</span>
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219 </div>
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220
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221 This would produce the following output::
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222
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223 <div>
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224 <span>1</span>
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225 </div>
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226
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227
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228 .. _`py:content`:
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229
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230 ``py:content``
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231 --------------
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232
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233 This directive replaces any nested content with the result of evaluating the
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234 expression::
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235
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236 <ul>
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237 <li py:content="bar">Hello</li>
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238 </ul>
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239
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240 Given ``bar='Bye'`` in the context data, this would produce::
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241
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242 <ul>
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243 <li>Bye</li>
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244 </ul>
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245
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246 This directive can only be used as an attribute.
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247
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248
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249 .. _`py:def`:
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250 .. _`macros`:
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251
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252 ``py:def``
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253 ----------
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254
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255 The ``py:def`` directive can be used to create macros, i.e. snippets of
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256 template code that have a name and optionally some parameters, and that can be
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257 inserted in other places::
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258
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259 <div>
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260 <p py:def="greeting(name)" class="greeting">
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261 Hello, ${name}!
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262 </p>
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263 ${greeting('world')}
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264 ${greeting('everyone else')}
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265 </div>
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266
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267 The above would be rendered to::
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268
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269 <div>
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270 <p class="greeting">
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271 Hello, world!
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272 </p>
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273 <p class="greeting">
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274 Hello, everyone else!
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275 </p>
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276 </div>
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277
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278 If a macro doesn't require parameters, it can be defined as well as called
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279 without the parenthesis. For example::
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280
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281 <div>
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282 <p py:def="greeting" class="greeting">
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283 Hello, world!
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284 </p>
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285 ${greeting}
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286 </div>
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287
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288 The above would be rendered to::
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289
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290 <div>
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291 <p class="greeting">
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292 Hello, world!
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293 </p>
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294 </div>
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295
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296 This directive can also be used as an element::
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297
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298 <div>
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299 <py:def function="greeting(name)">
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300 <p class="greeting">Hello, ${name}!</p>
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301 </py:def>
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302 </div>
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303
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304
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305 .. _`py:for`:
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306
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307 ``py:for``
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308 ----------
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309
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310 The element is repeated for every item in an iterable::
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311
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312 <ul>
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313 <li py:for="item in items">${item}</li>
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314 </ul>
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315
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316 Given ``items=[1, 2, 3]`` in the context data, this would produce::
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317
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318 <ul>
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319 <li>1</li><li>2</li><li>3</li>
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320 </ul>
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321
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322 This directive can also be used as an element::
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323
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324 <ul>
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325 <py:for each="item in items">
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326 <li>${item}</li>
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327 </py:for>
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328 </ul>
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329
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330
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331 .. _`py:if`:
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332
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333 ``py:if``
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334 ------------
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335
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336 The element is only rendered if the expression evaluates to a truth value::
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337
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338 <div>
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339 <b py:if="foo">${bar}</b>
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340 </div>
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341
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342 Given the data ``foo=True`` and ``bar='Hello'`` in the template context, this
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343 would produce::
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344
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345 <div>
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346 <b>Hello</b>
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347 </div>
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348
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349 This directive can also be used as an element::
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350
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351 <div>
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352 <py:if test="foo">
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353 <b>${bar}</b>
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354 </py:if>
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355 </div>
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356
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357
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358 .. _`py:match`:
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359 .. _Match Templates:
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360
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361 ``py:match``
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362 ------------
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363
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364 This directive defines a *match template*: given an XPath expression, it
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365 replaces any element in the template that matches the expression with its own
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366 content.
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367
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368 For example, the match template defined in the following template matches any
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369 element with the tag name “greeting”::
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370
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371 <div>
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372 <span py:match="greeting">
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373 Hello ${select('@name')}
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374 </span>
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375 <greeting name="Dude" />
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376 </div>
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377
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378 This would result in the following output::
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379
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380 <div>
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381 <span>
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382 Hello Dude
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383 </span>
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384 </div>
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385
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386 Inside the body of a ``py:match`` directive, the ``select(path)`` function is
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387 made available so that parts or all of the original element can be incorporated
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388 in the output of the match template. See [wiki:MarkupStream#UsingXPath] for
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389 more information about this function.
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390
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391 This directive can also be used as an element::
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392
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393 <div>
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394 <py:match path="greeting">
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395 <span>Hello ${select('@name')}</span>
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396 </py:match>
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397 <greeting name="Dude" />
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398 </div>
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399
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400
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401 .. _`py:replace`:
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402
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403 ``py:replace``
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404 --------------
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405
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406 This directive replaces the element itself with the result of evaluating the
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407 expression::
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408
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409 <div>
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410 <span py:replace="bar">Hello</span>
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411 </div>
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412
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413 Given ``bar='Bye'`` in the context data, this would produce::
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414
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415 <div>
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416 Bye
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417 </div>
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418
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419 This directive can only be used as an attribute.
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420
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421
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422 .. _`py:strip`:
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423
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424 ``py:strip``
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425 ------------
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426
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427 This directive conditionally strips the top-level element from the output. When
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428 the value of the ``py:strip`` attribute evaluates to ``True``, the element is
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429 stripped from the output::
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430
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431 <div>
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432 <div py:strip="True"><b>foo</b></div>
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433 </div>
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434
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435 This would be rendered as::
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436
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437 <div>
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438 <b>foo</b>
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439 </div>
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440
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441 As a shorthand, if the value of the ``py:strip`` attribute is empty, that has
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442 the same effect as using a truth value (i.e. the element is stripped).
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443
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444
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445 .. _`with`:
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446
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447 ``py:with``
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448 -----------
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449
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450 The ``py:with`` directive lets you assign expressions to variables, which can
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451 be used to make expressions inside the directive less verbose and more
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452 efficient. For example, if you need use the expression ``author.posts`` more
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453 than once, and that actually results in a database query, assigning the results
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454 to a variable using this directive would probably help.
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455
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456 For example::
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457
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458 <div>
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459 <span py:with="y=7; z=x+10">$x $y $z</span>
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460 </div>
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461
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462 Given ``x=42`` in the context data, this would produce::
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463
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464 <div>
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465 <span>42 7 52</span>
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466 </div>
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467
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468 This directive can also be used as an element::
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469
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470 <div>
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471 <py:with vars="y=7; z=x+10">$x $y $z</py:with>
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472 </div>
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473
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474 Note that if a variable of the same name already existed outside of the scope
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475 of the ``py:with`` directive, it will **not** be overwritten. Instead, it
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476 will have the same value it had prior to the ``py:with`` assignment.
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477 Effectively, this means that variables are immutable in Markup.
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478
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479
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480 .. _order:
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481
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482 Processing Order
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483 ================
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484
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485 It is possible to attach multiple directives to a single element, although not
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486 all combinations make sense. When multiple directives are encountered, they are
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487 processed in the following order:
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488
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489 #. `py:def`_
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490 #. `py:match`_
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491 #. `py:when`_
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492 #. `py:otherwise`_
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493 #. `py:for`_
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494 #. `py:if`_
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495 #. `py:choose`_
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496 #. `py:with`_
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497 #. `py:replace`_
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498 #. `py:content`_
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499 #. `py:attrs`_
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500 #. `py:strip`_
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501
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502
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503 .. _includes:
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504
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505 --------
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506 Includes
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507 --------
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508
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509 To reuse common snippets of template code, you can include other files using
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510 XInclude_.
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511
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512 .. _xinclude: http://www.w3.org/TR/xinclude/
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513
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514 For this, you need to declare the XInclude namespace (commonly bound to the
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515 prefix “xi”) and use the ``<xi:include>`` element where you want the external
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516 file to be pulled in::
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517
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518 <html xmlns="http://www.w3.org/1999/xhtml"
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519 xmlns:py="http://markup.edgewall.org/"
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520 xmlns:xi="http://www.w3.org/2001/XInclude">
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521 <xi:include href="base.html" />
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522 ...
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523 </html>
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524
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525 Include paths are relative to the filename of the template currently being
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526 processed. So if the example above was in the file "``myapp/index.html``"
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527 (relative to the template search path), the XInclude processor would look for
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528 the included file at "``myapp/base.html``". You can also use Unix-style
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529 relative paths, for example "``../base.html``" to look in the parent directory.
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530
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531 Any content included this way is inserted into the generated output instead of
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532 the ``<xi:include>`` element. The included template sees the same context data.
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533 `Match templates`_ and `macros`_ in the included template are also available to
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534 the including template after the point it was included.
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535
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536 By default, an error will be raised if an included file is not found. If that's
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537 not what you want, you can specify fallback content that should be used if the
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538 include fails. For example, to to make the include above fail silently, you'd
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539 write:
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540
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541 <xi:include href="base.html"><xi:fallback /></xi:include>
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542
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543 See the XInclude_ for more about fallback content. Note though that Markup
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544 currently only supports a small subset of XInclude.
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545
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546 Incudes in Markup are fully dynamic: Just like normal attributes, the `href`
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547 attribute accepts expressions_, and directives_ can be used on the
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548 ``<xi:include />`` element just as on any other element, meaning you can do
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549 things like conditional includes::
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550
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551 <xi:include href="${name}.html" py:if="not in_popup"
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552 py:for="name in ('foo', 'bar', 'baz')" />
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553
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554
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555 .. _comments:
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556
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557 --------
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558 Comments
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559 --------
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560
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561 Normal XML/HTML comment syntax can be used in templates::
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562
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563 <!-- this is a comment -->
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564
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565 However, such comments get passed through the processing pipeline and are by
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566 default included in the final output. If that's not desired, prefix the comment
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567 text with an exclamation mark::
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568
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569 <!-- !this is a comment too, but one that will be stripped from the output -->
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570
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571 Note that it does not matter whether there's whitespace before or after the
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572 exclamation mark, so the above could also be written as follows::
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573
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574 <!--! this is a comment too, but one that will be stripped from the output -->
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