YATL helpers
Consider the following code in a view:
[[=DIV('this', 'is', 'a', 'test', _id='123', _class='myclass')]]
it is rendered as:
<div id="123" class="myclass">thisisatest</div>
DIV is a helper class, i.e., something that can be used to build
HTML programmatically. It corresponds to the HTML <div> tag.
Positional arguments are interpreted as objects contained between the open and close tags. Named arguments that start with an underscore are interpreted as HTML tag attributes (without the underscore). Some helpers also have named arguments that do not start with underscore; these arguments are tag-specific.
Instead of a set of unnamed arguments, a helper can also take a single
list or tuple as its set of components using the * notation and it
can take a single dictionary as its set of attributes using the **,
for example:
[[
contents = ['this', 'is', 'a', 'test']
attributes = {'_id':'123', '_class':'myclass'}
=DIV(*contents, **attributes)
]]
(produces the same output as before).
The following set of helpers:
A, BEAUTIFY, BODY, CAT, CODE, DIV, EM,
FORM, H1, H2, H3, H4, H5, H6, HEAD,
HTML, I, IMG, INPUT, LABEL, LI, LINK,
META, METATAG, OL, OPTION, PRE, SELECT,
SPAN, STRONG, TABLE, TAG, TBODY, TD,
TEXTAREA, TH, THEAD, TR, UL, XML, sanitize,
xmlescape
can be used to build complex expressions that can then be serialized to XML. For example:
[[=DIV(B(I("hello ", "<world>")), _class="myclass")]]
is rendered:
<div class="myclass"><b><i>hello <world></i></b></div>
Helpers can also be serialized into strings, equivalently, with the
__str__ and the xml methods:
>>> print str(DIV("hello world"))
<div>hello world</div>
>>> print DIV("hello world").xml()
<div>hello world</div>
The helpers mechanism in py4web is more than a system to generate HTML without concatenating strings. It provides a server-side representation of the Document Object Model (DOM).
Components of helpers can be referenced via their position, and helpers act as lists with respect to their components:
>>> a = DIV(SPAN('a', 'b'), 'c')
>>> print a
<div><span>ab</span>c</div>
>>> del a[1]
>>> a.append(B('x'))
>>> a[0][0] = 'y'
>>> print a
<div><span>yb</span><b>x</b></div>
Attributes of helpers can be referenced by name, and helpers act as dictionaries with respect to their attributes:
>>> a = DIV(SPAN('a', 'b'), 'c')
>>> a['_class'] = 's'
>>> a[0]['_class'] = 't'
>>> print a
<div class="s"><span class="t">ab</span>c</div>
Note, the complete set of components can be accessed via a list called
a.components, and the complete set of attributes can be accessed via
a dictionary called a.attributes. So, a[i] is equivalent to
a.components[i] when i is an integer, and a[s] is equivalent
to a.attributes[s] when s is a string.
Notice that helper attributes are passed as keyword arguments to the helper. In some cases, however, attribute names include special characters that are not allowed in Python identifiers (e.g., hyphens) and therefore cannot be used as keyword argument names. For example:
DIV('text', _data-role='collapsible')
will not work because «_data-role» includes a hyphen, which will produce a Python syntax error.
In such cases you have a couple of options. You can use the data
argument (this time without a leading underscore) to pass a dictionary
of related attributes without their leading hyphen, and the output will
have the desired combinations e.g.
>>> print DIV('text', data={'role': 'collapsible'})
<div data-role="collapsible">text</div>
or you can instead pass the attributes as a dictionary and make use of
Python’s ** function arguments notation, which maps a dictionary of
(key:value) pairs into a set of keyword arguments:
>>> print DIV('text', **{'_data-role': 'collapsible'})
<div data-role="collapsible">text</div>
Note that more elaborate entries will introduce HTML character entities, but they will work nonetheless e.g.
>>> print DIV('text', data={'options':'{"mode":"calbox", "useNewStyle":true}'})
<div data-options="{"mode":"calbox", "useNewStyle":true}">text</div>
You can also dynamically create special TAGs:
>>> print TAG['soap:Body']('whatever', **{'_xmlns:m':'http://www.example.org'})
<soap:Body xmlns:m="http://www.example.org">whatever</soap:Body>
XML
XML is an object used to encapsulate text that should not be
escaped. The text may or may not contain valid XML. For example, it
could contain JavaScript.
The text in this example is escaped:
>>> print DIV("<b>hello</b>")
<div><b>hello</b></div>
by using XML you can prevent escaping:
>>> print DIV(XML("<b>hello</b>"))
<div><b>hello</b></div>
Sometimes you want to render HTML stored in a variable, but the HTML may contain unsafe tags such as scripts:
>>> print XML('<script>alert("unsafe!")</script>')
<script>alert("unsafe!")</script>
Un-escaped executable input such as this (for example, entered in the body of a comment in a blog) is unsafe, because it can be used to generate Cross Site Scripting (XSS) attacks against other visitors to the page.
The py4web XML helper can sanitize our text to prevent injections
and escape all tags except those that you explicitly allow. Here is an
example:
>>> print XML('<script>alert("unsafe!")</script>', sanitize=True)
<script>alert("unsafe!")</script>
The XML constructors, by default, consider the content of some tags
and some of their attributes safe. You can override the defaults using
the optional permitted_tags and allowed_attributes arguments.
Here are the default values of the optional arguments of the XML
helper.
XML(text, sanitize=False,
permitted_tags=['a', 'b', 'blockquote', 'br/', 'i', 'li',
'ol', 'ul', 'p', 'cite', 'code', 'pre', 'img/'],
allowed_attributes={'a':['href', 'title'],
'img':['src', 'alt'], 'blockquote':['type']})
Built-in helpers
A
This helper is used to build links.
>>> print A('<click>', XML('<b>me</b>'),
_href='http://www.py4web.com')
<a href='http://www.py4web.com'><click><b>me</b></a>
BODY
This helper makes the body of a page.
>>> print BODY('<hello>', XML('<b>world</b>'), _bgcolor='red')
<body bgcolor="red"><hello><b>world</b></body>
CAT
This helper concatenates other helpers, same as TAG[“”].
>>> print CAT('Here is a ', A('link', _href=URL()), ', and here is some ', B('bold text'), '.')
Here is a <a href="/app/default/index">link</a>, and here is some <b>bold text</b>.
CODE
This helper performs syntax highlighting for Python, C, C++, HTML and
py4web code, and is preferable to PRE for code listings. CODE
also has the ability to create links to the py4web API documentation.
Here is an example of highlighting sections of Python code.
>>> print CODE('print "hello"', language='python').xml()
<table><tr style="vertical-align:top;">
<td style="min-width:40px; text-align: right;"><pre style="
font-size: 11px;
font-family: Bitstream Vera Sans Mono,monospace;
background-color: transparent;
margin: 0;
padding: 5px;
border: none;
color: #A0A0A0;
">1.</pre></td><td><pre style="
font-size: 11px;
font-family: Bitstream Vera Sans Mono,monospace;
background-color: transparent;
margin: 0;
padding: 5px;
border: none;
overflow: auto;
white-space: pre !important;
"><span style="color:#185369; font-weight: bold">print </span>
<span style="color: #FF9966">"hello"</span></pre></td></tr></table>
Here is a similar example for HTML
>>> print CODE('<html><body>[[=request.env.remote_add]]</body></html>',
... language='html')
<table>...<code>...
<html><body>[[=request.env.remote_add]]</body></html>
...</code>...</table>
These are the default arguments for the CODE helper:
CODE("print 'hello world'", language='python', link=None, counter=1, styles={})
Supported values for the language argument are “python”,
“html_plain”, “c”, “cpp”, “py4web”, and “html”. The “html” language
interprets tags as “py4web” code, while “html_plain” doesn’t.
If a link value is specified, for example “/examples/global/vars/”,
py4web API references in the code are linked to documentation at the
link URL. For example “request” would be linked to
“/examples/global/vars/request”. In the above example, the link URL is
handled by the “vars” action in the “global.py” controller that is
distributed as part of the py4web “examples” application.
The counter argument is used for line numbering. It can be set to
any of three different values. It can be None for no line numbers, a
numerical value specifying the start number, or a string. If the counter
is set to a string, it is interpreted as a prompt, and there are no line
numbers.
The styles argument is a bit tricky. If you look at the generated
HTML above, it contains a table with two columns, and each column has
its own style declared inline using CSS. The styles attributes
allows you to override those two CSS styles. For example:
CODE(..., styles={'CODE':'margin: 0;padding: 5px;border: none;'})
The styles attribute must be a dictionary, and it allows two
possible keys: CODE for the style of the actual code, and
LINENUMBERS for the style of the left column, which contains the
line numbers. Mind that these styles completely replace the default
styles and are not simply added to them.
DIV
All helpers apart from XML are derived from DIV and inherit its
basic methods.
>>> print DIV('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<div id="0" class="test"><hello><b>world</b></div>
EM
Emphasizes its content.
>>> print EM('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<em id="0" class="test"><hello><b>world</b></em>
FORM
This is one of the most important helpers. In its simple form, it just
makes a <form>...</form> tag, but because helpers are objects and
have knowledge of what they contain, they can process submitted forms
(for example, perform validation of the fields). This will be discussed
in detail in Chapter 10.
>>> print FORM(INPUT(_type='submit'), _action='', _method='post')
<form enctype="multipart/form-data" action="" method="post">
<input type="submit" /></form>
The “enctype” is “multipart/form-data” by default.
The constructor of a FORM, and of SQLFORM, can also take a
special argument called hidden. When a dictionary is passed as
hidden, its items are translated into “hidden” INPUT fields. For
example:
>>> print FORM(hidden=dict(a='b'))
<form enctype="multipart/form-data" action="" method="post">
<input value="b" type="hidden" name="a" /></form>
H1, H2, H3, H4, H5, H6
These helpers are for paragraph headings and subheadings:
>>> print H1('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<h1 id="0" class="test"><hello><b>world</b></h1>
HEAD
For tagging the HEAD of an HTML page.
>>> print HEAD(TITLE('<hello>', XML('<b>world</b>')))
<head><title><hello><b>world</b></title></head>
HTML
This helper is a little different. In addition to making the <html>
tags, it prepends the tag with a doctype string.
>>> print HTML(BODY('<hello>', XML('<b>world</b>')))
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html><body><hello><b>world</b></body></html>
The HTML helper also takes some additional optional arguments that have the following default:
HTML(..., lang='en', doctype='transitional')
where doctype can be ‘strict’, ‘transitional’, ‘frameset’, ‘html5’, or a full doctype string.
I
This helper makes its contents italic.
>>> print I('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<i id="0" class="test"><hello><b>world</b></i>
IMG
It can be used to embed images into HTML:
>>> print IMG(_src='http://example.com/image.png', _alt='test')
{ alt="rest" }
Here is a combination of A, IMG, and URL helpers for including a static image with a link:
>>> print A(IMG(_src=URL('static', 'logo.png'), _alt="My Logo"),
... _href=URL('default', 'index'))
...
<a href="/myapp/default/index">
{ alt="My Logo" }
</a>
INPUT
Creates an <input.../> tag. An input tag may not contain other tags,
and is closed by /> instead of >. The input tag has an optional
attribute _type that can be set to “text” (the default), “submit”,
“checkbox”, or “radio”.
>>> print INPUT(_name='test', _value='a')
<input value="a" name="test" />
It also takes an optional special argument called “value”, distinct from «_value“. The latter sets the default value for the input field; the former sets its current value. For an input of type”text», the former overrides the latter:
>>> print INPUT(_name='test', _value='a', value='b')
<input value="b" name="test" />
For radio buttons, INPUT selectively sets the “checked” attribute:
>>> for v in ['a', 'b', 'c']:
... print INPUT(_type='radio', _name='test', _value=v, value='b'), v
...
<input value="a" type="radio" name="test" /> a
<input value="b" type="radio" checked="checked" name="test" /> b
<input value="c" type="radio" name="test" /> c
and similarly for checkboxes:
>>> print INPUT(_type='checkbox', _name='test', _value='a', value=True)
<input value="a" type="checkbox" checked="checked" name="test" />
>>> print INPUT(_type='checkbox', _name='test', _value='a', value=False)
<input value="a" type="checkbox" name="test" />
LABEL
It is used to create a LABEL tag for an INPUT field.
>>> print LABEL('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<label id="0" class="test"><hello><b>world</b></label>
LI
It makes a list item and should be contained in a UL or OL tag.
>>> print LI('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<li id="0" class="test"><hello><b>world</b></li>
OL
It stands for Ordered List. The list should contain LI tags. OL
arguments that are not LI objects are automatically enclosed in
<li>...</li> tags.
>>> print OL('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<ol id="0" class="test"><li><hello></li><li><b>world</b></li></ol>
OPTION
This should only be used as part of a SELECT/OPTION combination.
>>> print OPTION('<hello>', XML('<b>world</b>'), _value='a')
<option value="a"><hello><b>world</b></option>
As in the case of INPUT, py4web make a distinction between «_value»
(the value of the OPTION), and “value” (the current value of the
enclosing select). If they are equal, the option is “selected”.
>>> print SELECT('a', 'b', value='b'):
<select>
<option value="a">a</option>
<option value="b" selected="selected">b</option>
</select>
P
This is for tagging a paragraph.
>>> print P('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<p id="0" class="test"><hello><b>world</b></p>
PRE
Generates a <pre>...</pre> tag for displaying pre-formatted text.
The CODE helper is generally preferable for code listings.
>>> print PRE('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<pre id="0" class="test"><hello><b>world</b></pre>
SCRIPT
This is include or link a script, such as JavaScript. The content between the tags is rendered as an HTML comment, for the benefit of really old browsers.
>>> print SCRIPT('alert("hello world");', _type='text/javascript')
<script type="text/javascript"><!--
alert("hello world");
//--></script>
SELECT
Makes a <select>...</select> tag. This is used with the OPTION
helper. Those SELECT arguments that are not OPTION objects are
automatically converted to options.
>>> print SELECT('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<select id="0" class="test">
<option value="<hello>"><hello></option>
<option value="<b>world</b>"><b>world</b></option>
</select>
SPAN
Similar to DIV but used to tag inline (rather than block) content.
>>> print SPAN('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<span id="0" class="test"><hello><b>world</b></span>
STYLE
Similar to script, but used to either include or link CSS code. Here the CSS is included:
>>> print STYLE(XML('body {color: white}'))
<style><!--
body { color: white }
//--></style>
and here it is linked:
>>> print STYLE(_src='style.css')
<style src="style.css"><!--
//--></style>
TABLE, TR, TD
These tags (along with the optional THEAD and TBODY helpers) are
used to build HTML tables.
>>> print TABLE(TR(TD('a'), TD('b')), TR(TD('c'), TD('d')))
<table><tr><td>a</td><td>b</td></tr><tr><td>c</td><td>d</td></tr></table>
TR expects TD content; arguments that are not TD objects are
converted automatically.
>>> print TABLE(TR('a', 'b'), TR('c', 'd'))
<table><tr><td>a</td><td>b</td></tr><tr><td>c</td><td>d</td></tr></table>
It is easy to convert a Python array into an HTML table using Python’s
* function arguments notation, which maps list elements to
positional function arguments.
Here, we will do it line by line:
>>> table = [['a', 'b'], ['c', 'd']]
>>> print TABLE(TR(*table[0]), TR(*table[1]))
<table><tr><td>a</td><td>b</td></tr><tr><td>c</td><td>d</td></tr></table>
Here we do all lines at once:
>>> table = [['a', 'b'], ['c', 'd']]
>>> print TABLE(*[TR(*rows) for rows in table])
<table><tr><td>a</td><td>b</td></tr><tr><td>c</td><td>d</td></tr></table>
TBODY
This is used to tag rows contained in the table body, as opposed to header or footer rows. It is optional.
>>> print TBODY(TR('<hello>'), _class='test', _id=0)
<tbody id="0" class="test"><tr><td><hello></td></tr></tbody>
TEXTAREA
This helper makes a <textarea>...</textarea> tag.
>>> print TEXTAREA('<hello>', XML('<b>world</b>'), _class='test')
<textarea class="test" cols="40" rows="10"><hello><b>world</b></textarea>
The only caveat is that its optional “value” overrides its content (inner HTML)
>>> print TEXTAREA(value="<hello world>", _class="test")
<textarea class="test" cols="40" rows="10"><hello world></textarea>
TH
This is used instead of TD in table headers.
>>> print TH('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<th id="0" class="test"><hello><b>world</b></th>
THEAD
This is used to tag table header rows.
>>> print THEAD(TR(TH('<hello>')), _class='test', _id=0)
<thead id="0" class="test"><tr><th><hello></th></tr></thead>
TITLE
This is used to tag the title of a page in an HTML header.
>>> print TITLE('<hello>', XML('<b>world</b>'))
<title><hello><b>world</b></title>
TR
Tags a table row. It should be rendered inside a table and contain
<td>...</td> tags. TR arguments that are not TD objects will
be automatically converted.
>>> print TR('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<tr id="0" class="test"><td><hello></td><td><b>world</b></td></tr>
TT
Tags text as typewriter (monospaced) text.
>>> print TT('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<tt id="0" class="test"><hello><b>world</b></tt>
UL
Signifies an Unordered List and should contain LI items. If its
content is not tagged as LI, UL does it automatically.
>>> print UL('<hello>', XML('<b>world</b>'), _class='test', _id=0)
<ul id="0" class="test"><li><hello></li><li><b>world</b></li></ul>
URL
The URL helper is documented in Chapter 4 URL ../04
Custom helpers
TAG
Sometimes you need to generate custom XML tags. py4web provides TAG,
a universal tag generator.
[[=TAG.name('a', 'b', _c='d')]]
generates the following XML
<name c="d">ab</name>
Arguments “a”, “b”, and “d” are automatically escaped; use the XML
helper to suppress this behavior. Using TAG you can generate
HTML/XML tags not already provided by the API. TAGs can be nested, and
are serialized with str(). An equivalent syntax is:
[[=TAG['name']('a', 'b', c='d')]]
If the TAG object is created with an empty name, it can be used to
concatenate multiple strings and HTML helpers together without inserting
them into a surrounding tag, but this use is deprecated. Use the CAT
helper instead.
Self-closing tags can be generated with the TAG helper. The tag name must end with a “/”.
[[=TAG['link/'](_href='http://py4web.com')]]
generates the following XML:
<link ref="http://py4web.com"/>
Notice that TAG is an object, and TAG.name or TAG['name'] is
a function that returns a temporary helper class.
BEAUTIFY
BEAUTIFY is used to build HTML representations of compound objects,
including lists, tuples and dictionaries:
[[=BEAUTIFY({"a": ["hello", XML("world")], "b": (1, 2)})]]
BEAUTIFY returns an XML-like object serializable to XML, with a nice
looking representation of its constructor argument. In this case, the
XML representation of:
{"a": ["hello", XML("world")], "b": (1, 2)}
will render as:
<table>
<tr><td>a</td><td>:</td><td>hello<br />world</td></tr>
<tr><td>b</td><td>:</td><td>1<br />2</td></tr>
</table>
Server-side DOM and parsing
elements
The DIV helper and all derived helpers provide the search methods
element and elements.
element returns the first child element matching a specified
condition (or None if no match).
elements returns a list of all matching children.
element and elements use the same syntax to specify the matching condition, which allows for three possibilities that can be mixed and matched: jQuery-like expressions, match by exact attribute value, match using regular expressions.
Here is a simple example:
>>> a = DIV(DIV(DIV('a', _id='target', _class='abc')))
>>> d = a.elements('div#target')
>>> d[0][0] = 'changed'
>>> print a
<div><div><div id="target" class="abc">changed</div></div></div>
The un-named argument of elements is a string, which may contain:
the name of a tag, the id of a tag preceded by a pound symbol, the class
preceded by a dot, the explicit value of an attribute in square
brackets.
Here are 4 equivalent ways to search the previous tag by id:
d = a.elements('#target')
d = a.elements('div#target')
d = a.elements('div[id=target]')
d = a.elements('div', _id='target')
Here are 4 equivalent ways to search the previous tag by class:
d = a.elements('.abc')
d = a.elements('div.abc')
d = a.elements('div[class=abc]')
d = a.elements('div', _class='abc')
Any attribute can be used to locate an element (not just id and
class), including multiple attributes (the function element can take
multiple named arguments), but only the first matching element will be
returned.
Using the jQuery syntax “div#target” it is possible to specify multiple search criteria separated by a comma:
a = DIV(SPAN('a', _id='t1'), DIV('b', _class='c2'))
d = a.elements('span#t1, div.c2')
or equivalently
a = DIV(SPAN('a', _id='t1'), DIV('b', _class='c2'))
d = a.elements('span#t1', 'div.c2')
If the value of an attribute is specified using a name argument, it can be a string or a regular expression:
a = DIV(SPAN('a', _id='test123'), DIV('b', _class='c2'))
d = a.elements('span', _id=re.compile('test\d{3}')
A special named argument of the DIV (and derived) helpers is find.
It can be used to specify a search value or a search regular expression
in the text content of the tag. For example:
>>> a = DIV(SPAN('abcde'), DIV('fghij'))
>>> d = a.elements(find='bcd')
>>> print d[0]
<span>abcde</span>
or
>>> a = DIV(SPAN('abcde'), DIV('fghij'))
>>> d = a.elements(find=re.compile('fg\w{3}'))
>>> print d[0]
<div>fghij</div>
components
Here’s an example of listing all elements in an html string:
>>> html = TAG('<a>xxx</a><b>yyy</b>')
>>> for item in html.components:
... print item
...
<a>xxx</a>
<b>yyy</b>
parent and siblings
parent returns the parent of the current element.
>>> a = DIV(SPAN('a'), DIV('b'))
>>> s = a.element('span')
>>> d = s.parent
>>> d['_class']='abc'
>>> print a
<div class="abc"><span>a</span><div>b</div></div>
>>> for e in s.siblings(): print e
<div>b</div>
Replacing elements
Elements that are matched can also be replaced or removed by specifying
the replace argument. Notice that a list of the original matching
elements is still returned as usual.
>>> a = DIV(SPAN('x'), DIV(SPAN('y'))
>>> b = a.elements('span', replace=P('z')
>>> print a
<div><p>z</p><div><p>z</p></div>
replace can be a callable. In this case it will be passed the
original element and it is expected to return the replacement element:
>>> a = DIV(SPAN('x'), DIV(SPAN('y'))
>>> b = a.elements('span', replace=lambda t: P(t[0])
>>> print a
<div><p>x</p><div><p>y</p></div>
If replace=None, matching elements will be removed completely.
>>> a = DIV(SPAN('x'), DIV(SPAN('y'))
>>> b = a.elements('span', replace=None)
>>> print a
<div></div>
flatten
The flatten method recursively serializes the content of the children of a given element into regular text (without tags):
>>> a = DIV(SPAN('this', DIV('is', B('a'))), SPAN('test'))
>>> print a.flatten()
thisisatest
Flatten can be passed an optional argument, render, i.e. a function
that renders/flattens the content using a different protocol. Here is an
example to serialize some tags into Markmin wiki syntax:
>>> a = DIV(H1('title'), P('example of a ', A('link', _href='#test')))
>>> from gluon.html import markmin_serializer
>>> print a.flatten(render=markmin_serializer)
# titles
example of *a link *
At the time of writing we provide markmin_serializer and
markdown_serializer.
Parsing
The TAG object is also an XML/HTML parser. It can read text and convert into a tree structure of helpers. This allows manipulation using the API above:
>>> html = '<h1>Title</h1><p>this is a <span>test</span></p>'
>>> parsed_html = TAG(html)
>>> parsed_html.element('span')[0]='TEST'
>>> print parsed_html
<h1>Title</h1><p>this is a <span>TEST</span></p>
Page layout
Views can extend and include other views in a tree-like structure.
For example, we can think of a view “index.html” that extends “layout.html” and includes “body.html”. At the same time, “layout.html” may include “header.html” and “footer.html”.
The root of the tree is what we call a layout view. Just like any other HTML template file, you can edit it using the py4web administrative interface. The file name “layout.html” is just a convention.
Here is a minimalist page that extends the “layout.html” view and includes the “page.html” view:
[[extend 'layout.html']]
<h1>Hello World</h1>
[[include 'page.html']]
The extended layout file must contain an [[include]] directive,
something like:
<html>
<head>
<title>Page Title</title>
</head>
<body>
[[include]]
</body>
</html>
When the view is called, the extended (layout) view is loaded, and the
calling view replaces the [[include]] directive inside the layout.
Processing continues recursively until all extend and include
directives have been processed. The resulting template is then
translated into Python code. Note, when an application is bytecode
compiled, it is this Python code that is compiled, not the original view
files themselves. So, the bytecode compiled version of a given view is a
single .pyc file that includes the Python code not just for the original
view file, but for its entire tree of extended and included views.
extend,include,blockandsuperare special template directives, not Python commands.
Any content or code that precedes the [[extend ...]] directive will
be inserted (and therefore executed) before the beginning of the
extended view’s content/code. Although this is not typically used to
insert actual HTML content before the extended view’s content, it can be
useful as a means to define variables or functions that you want to make
available to the extended view. For example, consider a view
“index.html”:
[[sidebar_enabled=True]]
[[extend 'layout.html']]
<h1>Home Page</h1>
and an excerpt from “layout.html”:
[[if sidebar_enabled:]]
<div id="sidebar">
Sidebar Content
</div>
[[pass]]
Because the sidebar_enabled assignment in “index.html” comes before
the extend, that line gets inserted before the beginning of
“layout.html”, making sidebar_enabled available anywhere within the
“layout.html” code (a somewhat more sophisticated version of this is
used in the welcome app).
It is also worth pointing out that the variables returned by the controller function are available not only in the function’s main view, but in all of its extended and included views as well.
The argument of an extend or include (i.e., the extended or
included view name) can be a Python variable (though not a Python
expression). However, this imposes a limitation – views that use
variables in extend or include statements cannot be bytecode
compiled. As noted above, bytecode-compiled views include the entire
tree of extended and included views, so the specific extended and
included views must be known at compile time, which is not possible if
the view names are variables (whose values are not determined until run
time). Because bytecode compiling views can provide a significant speed
boost, using variables in extend and include should generally be
avoided if possible.
In some cases, an alternative to using a variable in an include is
simply to place regular [[include ...]] directives inside an
if...else block.
[[if some_condition:]]
[[include 'this_view.html']]
[[else:]]
[[include 'that_view.html']]
[[pass]]
The above code does not present any problem for bytecode compilation
because no variables are involved. Note, however, that the bytecode
compiled view will actually include the Python code for both
“this_view.html” and “that_view.html”, though only the code for one of
those views will be executed, depending on the value of
some_condition.
Keep in mind, this only works for include – you cannot place
[[extend ...]] directives inside if...else blocks.
Layouts are used to encapsulate page commonality (headers, footers, menus), and though they are not mandatory, they will make your application easier to write and maintain. In particular, we suggest writing layouts that take advantage of the following variables that can be set in the controller. Using these well known variables will help make your layouts interchangeable:
response.title
response.subtitle
response.meta.author
response.meta.keywords
response.meta.description
response.flash
response.menu
response.files
Except for menu and files, these are all strings and their
meaning should be obvious.
response.menu menu is a list of 3-tuples or 4-tuples. The three
elements are: the link name, a boolean representing whether the link is
active (is the current link), and the URL of the linked page. For
example:
response.menu = [('Google', False, 'http://www.google.com', []),
('Index', True, URL('index'), [])]
The fourth tuple element is an optional sub-menu.
response.files is a list of CSS and JS files that are needed by your
page.
We also recommend that you use:
[[include 'py4web_ajax.html']]
in the HTML head, since this will include the jQuery libraries and
define some backward-compatible JavaScript functions for special effects
and Ajax. “py4web_ajax.html” includes the response.meta tags in the
view, jQuery base, the calendar datepicker, and all required CSS and JS
response.files.
Default page layout
The “views/layout.html” that ships with the py4web scaffolding application welcome (stripped down of some optional parts) is quite complex but it has the following structure:
<!DOCTYPE html>
<head>
<meta charset="utf-8" />
<title>[[=response.title or request.application]]</title>
...
<script src="[[=URL('static', 'js/modernizr.custom.js')]]"></script>
[[
response.files.append(URL('static', 'css/py4web.css'))
response.files.append(URL('static', 'css/bootstrap.min.css'))
response.files.append(URL('static', 'css/bootstrap-responsive.min.css'))
response.files.append(URL('static', 'css/py4web_bootstrap.css'))
]]
[[include 'py4web_ajax.html']]
[[
# using sidebars need to know what sidebar you want to use
left_sidebar_enabled = globals().get('left_sidebar_enabled', False)
right_sidebar_enabled = globals().get('right_sidebar_enabled', False)
middle_columns = {0:'span12', 1:'span9', 2:'span6'}[
(left_sidebar_enabled and 1 or 0)+(right_sidebar_enabled and 1 or 0)]
]]
[[block head]][[end]]
</head>
<body>
<!-- Navbar ================================================== -->
<div class="navbar navbar-inverse navbar-fixed-top">
<div class="flash">[[=response.flash or '']]</div>
<div class="navbar-inner">
<div class="container">
[[=response.logo or '']]
<ul id="navbar" class="nav pull-right">
[[='auth' in globals() and auth.navbar(mode="dropdown") or '']]
</ul>
<div class="nav-collapse">
[[if response.menu:]]
[[=MENU(response.menu)]]
[[pass]]
</div><!--/.nav-collapse -->
</div>
</div>
</div><!--/top navbar -->
<div class="container">
<!-- Masthead ================================================== -->
<header class="mastheader row" id="header">
<div class="span12">
<div class="page-header">
<h1>
[[=response.title or request.application]]
<small>[[=response.subtitle or '']]</small>
</h1>
</div>
</div>
</header>
<section id="main" class="main row">
[[if left_sidebar_enabled:]]
<div class="span3 left-sidebar">
[[block left_sidebar]]
<h3>Left Sidebar</h3>
<p></p>
[[end]]
</div>
[[pass]]
<div class="[[=middle_columns]]">
[[block center]]
[[include]]
[[end]]
</div>
[[if right_sidebar_enabled:]]
<div class="span3">
[[block right_sidebar]]
<h3>Right Sidebar</h3>
<p></p>
[[end]]
</div>
[[pass]]
</section><!--/main-->
<!-- Footer ================================================== -->
<div class="row">
<footer class="footer span12" id="footer">
<div class="footer-content">
[[block footer]] <!-- this is default footer -->
...
[[end]]
</div>
</footer>
</div>
</div> <!-- /container -->
<!-- The javascript =============================================
(Placed at the end of the document so the pages load faster) -->
<script src="[[=URL('static', 'js/bootstrap.min.js')]]"></script>
<script src="[[=URL('static', 'js/py4web_bootstrap.js')]]"></script>
[[if response.google_analytics_id:]]
<script src="[[=URL('static', 'js/analytics.js')]]"></script>
<script type="text/javascript">
analytics.initialize({
'Google Analytics':{trackingId:'[[=response.google_analytics_id]]'}
});</script>
[[pass]]
</body>
</html>
There are a few features of this default layout that make it very easy to use and customize:
It is written in HTML5 and uses the “modernizr” library for backward compatibility. The actual layout includes some extra conditional statements required by IE and they are omitted for brevity.
It displays both
response.titleandresponse.subtitlewhich can be set in a model or a controller. If they are not set, it adopts the application name as title.It includes the
py4web_ajax.htmlfile in the header which generated all the link and script import statements.It uses a modified version of Twitter Bootstrap for flexible layouts which works on mobile devices and re-arranges columns to fit small screens.
It uses “analytics.js” to connect to Google Analytics.
The
[[=auth.navbar(...)]]displays a welcome to the current user and links to the auth functions like login, logout, register, change password, etc. depending on context.auth.navbaris a helper factory and its output can be manipulated as any other helper. It is placed in an expression to check for auth definition, the expression evaluates to ’’ in case auth is undefined.The
[[=MENU(response.menu)]]displays the menu structure as<ul>...</ul>.[[include]]is replaced by the content of the extending view when the page is rendered.By default it uses a conditional three column (the left and right sidebars can be turned off by the extending views)
It uses the following classes: page-header, main, footer.
It contains the following blocks: head, left_sidebar, center, right_sidebar, footer.
In views, you can turn on and customize sidebars as follows:
[[left_sidebar_enabled=True]]
[[extend 'layout.html']]
This text goes in center
[[block left_sidebar]]
This text goes in sidebar
[[end]]
Customizing the default layout
Customizing the default layout without editing is easy because the welcome application is based on Twitter Bootstrap which is well documented and supports themes. In py4web four static files which are relevant to style:
“css/py4web.css” contains py4web specific styles
“css/bootstrap.min.css” contains the Twitter Bootstrap CSS style
“css/py4web_bootstrap.css” which overrides some Bootstrap styles to conform to py4web needs.
“js/bootstrap.min.js” which includes the libraries for menu effects, modals, panels.
To change colors and background images, try append the following code to layout.html header:
<style>
body { background: url('images/background.png') repeat-x #3A3A3A; }
a { color: #349C01; }
.page-header h1 { color: #349C01; }
.page-header h2 { color: white; font-style: italic; font-size: 14px;}
.statusbar { background: #333333; border-bottom: 5px #349C01 solid; }
.statusbar a { color: white; }
.footer { border-top: 5px #349C01 solid; }
</style>
Of course you can also completely replace the “layout.html” and “py4web.css” files with your own.
Mobile development
Although the default layout.html is designed to be mobile-friendly, one may sometimes need to use different views when a page is visited by a mobile device.
To make developing for desktop and mobile devices easier, py4web
includes the @mobilize decorator. This decorator is applied to
actions that should have a normal view and a mobile view. This is
demonstrated here:
from gluon.contrib.user_agent_parser import mobilize
@mobilize
def index():
return dict()
Notice that the decorator must be imported before using it in a controller. When the “index” function is called from a regular browser (desktop computer), py4web will render the returned dictionary using the view “[controller]/index.html”. However, when it is called by a mobile device, the dictionary will be rendered by “[controller]/index.mobile.html”. Notice that mobile views have the “mobile.html” extension.
Alternatively you can apply the following logic to make all views mobile friendly:
if request.user_agent().is_mobile:
response.view.replace('.html', '.mobile.html')
The task of creating the «*.mobile.html» views is left to the developer but we strongly suggest using the “jQuery Mobile” plugin which makes the task very easy.
Functions in views
Consider this “layout.html”:
<html>
<body>
[[include]]
<div class="sidebar">
[[if 'mysidebar' in globals():]][[mysidebar()]][[else:]]
my default sidebar
[[pass]]
</div>
</body>
</html>
and this extending view
[[def mysidebar():]]
my new sidebar!!!
[[return]]
[[extend 'layout.html']]
Hello World!!!
Notice the function is defined before the [[extend...]] statement –
this results in the function being created before the “layout.html” code
is executed, so the function can be called anywhere within
“layout.html”, even before the [[include]]. Also notice the function
is included in the extended view without the = prefix.
The code generates the following output:
<html>
<body>
Hello World!!!
<div class="sidebar">
my new sidebar!!!
</div>
</body>
</html>
Notice that the function is defined in HTML (although it could also
contain Python code) so that response.write is used to write its
content (the function does not return the content). This is why the
layout calls the view function using [[mysidebar()]] rather than
[[=mysidebar()]]. Functions defined in this way can take arguments.
Blocks in views
The main way to make a view more modular is by using
[[block ...]]s and this mechanism is an alternative to the
mechanism discussed in the previous section.
To understand how this works, consider apps based on the scaffolding app
welcome, which has a view layout.html. This view is extended by the view
default/index.html via [[extend 'layout.html']]. The contents of
layout.html predefine certain blocks with certain default content, and
these are therefore included into default/index.html.
You can override these default content blocks by enclosing your new content inside the same block name. The location of the block in the layout.html is not changed, but the contents is.
Here is a simplifed version. Imagine this is “layout.html”:
<html>
<body>
[[include]]
<div class="sidebar">
[[block mysidebar]]
my default sidebar (this content to be replaced)
[[end]]
</div>
</body>
</html>
and this is a simple extending view default/index.html:
[[extend 'layout.html']]
Hello World!!!
[[block mysidebar]]
my new sidebar!!!
[[end]]
It generates the following output, where the content is provided by the over-riding block in the extending view, yet the enclosing DIV and class comes from layout.html. This allows consistency across views:
<html>
<body>
Hello World!!!
<div class="sidebar">
my new sidebar!!!
</div>
</body>
</html>
The real layout.html defines a number of useful blocks, and you can easily add more to match the layout your desire.
You can have many blocks, and if a block is present in the extended view
but not in the extending view, the content of the extended view is used.
Also, notice that unlike with functions, it is not necessary to define
blocks before the [[extend ...]] – even if defined after the
extend, they can be used to make substitutions anywhere in the
extended view.
Inside a block, you can use the expression [[super]] to include the
content of the parent. For example, if we replace the above extending
view with:
[[extend 'layout.html']]
Hello World!!!
[[block mysidebar]]
[[super]]
my new sidebar!!!
[[end]]
we get:
<html>
<body>
Hello World!!!
<div class="sidebar">
my default sidebar
my new sidebar!
</div>
</body>
</html>