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Helpers

Helpers are functions intended for usage in templates, to assist with common HTML and text manipulation, higher level constructs like a HTML tag builder (that safely escapes variables), and advanced functionality like Pagination of data sets.

The majority of the helpers available in Pylons are provided by the :mod:`webhelpers` package. Some of these helpers are also used in controllers to prepare data for use in the template by other helpers, such as the :func:`~webhelpers.rails.secure_form_tag` function which has a corresponding :func:`~pylons.decorators.secure.authenticate_form`.

To make individual helpers available for use in templates under :term:`h`, the appropriate functions need to be imported in :file:`lib/helpers.py`. All the functions available in this file are then available under :term:`h` just like any other module reference.

By customizing the :file:`lib/helpers.py` module you can quickly add custom functions and classes for use in your templates.

Helper functions are organized into modules by theme. All HTML generators are under the webhelpers_html package, except for a few third-party modules which are directly under webhelpers. The webhelpers modules are separately documented, see :mod:`webhelpers`.

Pagination

Note

The paginate module is not compatible to the deprecated pagination module that was provided with former versions of the Webhelpers package.

Purpose of a paginator

When you display large amounts of data like a result from an SQL query then usually you cannot display all the results on a single page. It would simply be too much. So you divide the data into smaller chunks. This is what a paginator does. It shows one page of chunk of data at a time. Imagine you are providing a company phonebook through the web and let the user search the entries. Assume the search result contains 23 entries. You may decide to display no more than 10 entries per page. The first page contains entries 1-10, the second 11-20 and the third 21-23. And you also show a navigational element like Page 1 of 3: [1] 2 3 that allows the user to switch between the available pages.

The Page class

The :mod:`webhelpers` package provides a paginate module that can be used for this purpose. It can create pages from simple Python lists as well as SQLAlchemy queries and SQLAlchemy select objects. The module provides a Page object that represents a single page of items from a larger result set. Such a Page mainly behaves like a list of items on that page. Let's take the above example of 23 items spread across 3 pages:

# Create a list of items from 1 to 23
>>> items = range(1,24)

# Import the paginate module
>>> import webhelpers.paginate

# Create a Page object from the 'items' for the second page
>>> page2 = webhelpers.paginate.Page(items, page=2, items_per_page=10)

# The Page object can be printed (__repr__) to show details on the page
>>> page2

    Page:
    Collection type:  <type 'list'>
    (Current) page:   2
    First item:       11
    Last item:        20
    First page:       1
    Last page:        3
    Previous page:    1
    Next page:        3
    Items per page:   10
    Number of items:  23
    Number of pages:  3

# Show the items on this page
>>> list(page2)

    [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]

# Print the items in a for loop
>>> for i in page2: print "This is entry", i

    This is entry 11
    This is entry 12
    This is entry 13
    This is entry 14
    This is entry 15
    This is entry 16
    This is entry 17
    This is entry 18
    This is entry 19
    This is entry 20

There are further parameters to invoking a Page object. Please see :class:`webhelpers.paginate.Page`

Note

Page numbers and item numbers start from 1. If you are accessing the items on the page by their index please note that the first item is item[1] instead of item[0].

Switching between pages using a pager

The user needs a way to get to another page. This is usually done with a list of links like Page 3 of 41 - 1 2 [3] 4 5 .. 41. Such a list can be created by the Page's :meth:`~webhelpers.paginate.Page.pager` method. Take the above example again:

>>> page2.pager()

    <a class="pager_link" href="/content?page=1">1</a>
    <span class="pager_curpage">2</span>
    <a class="pager_link" href="/content?page=3">3</a>

Without the HTML tags it looks like 1 [2] 3. The links point to a URL where the respective page is found. And the current page (2) is highlighted.

The appearance of a pager can be customized. By default the format string is ~2~ which means it shows adjacent pages from the current page with a maximal radius of 2. In a larger set this would look like 1 .. 34 35 [36] 37 38 .. 176. The radius of 2 means that two pages before and after the current page 36 are shown.

Several special variables can be used in the format string. See :meth:`~webhelpers.paginate.Page.pager` for a complete list. Some examples for a pager of 20 pages while being on page 10 currently:

>>> page.pager()

    1 .. 8 9 [10] 11 12 .. 20

>>> page.pager('~4~')

    1 .. 6 7 8 9 [10] 11 12 13 14 .. 20

>>> page.pager('Page $page of $page_count - ~3~')

    Page 10 of 20 - 1 .. 7 8 9 [10] 11 12 13 .. 20

>>> page.pager('$link_previous $link_next ~2~')

    < > 1 .. 8 9 [10] 11 12 .. 20

>>> page.pager('Items $first_item - $last_item / ~2~')

    Items 91 - 100 / 1 .. 8 9 [10] 11 12 .. 20

Paging over an SQLAlchemy query

If the data to page over comes from a database via SQLAlchemy then the paginate module can access a query object directly. This is useful when using ORM-mapped models. Example:

>>> employee_query = Session.query(Employee)
>>> page2 = webhelpers.paginate.Page(
        employee_query,
        page=2,
        items_per_page=10)
>>> for employee in page2: print employee.first_name

    John
    Jack
    Joseph
    Kay
    Lars
    Lynn
    Pamela
    Sandra
    Thomas
    Tim

The paginate module is smart enough to only query the database for the objects that are needed on this page. E.g. if a page consists of the items 11-20 then SQLAlchemy will be asked to fetch exactly that 10 rows through LIMIT and OFFSET in the actual SQL query. So you must not load the complete result set into memory and pass that. Instead always pass a query when creating a Page.

Paging over an SQLAlchemy select

SQLAlchemy also allows to run arbitrary SELECTs on database tables. This is useful for non-ORM queries. paginate can use such select objects, too. Example:

>>> selection = sqlalchemy.select([Employee.c.first_name])
>>> page2 = webhelpers.paginate.Page(
        selection,
        page=2,
        items_per_page=10,
        sqlalchemy_session=model.Session)
>>> for first_name in page2: print first_name

    John
    Jack
    Joseph
    Kay
    Lars
    Lynn
    Pamela
    Sandra
    Thomas
    Tim

The only difference to using SQLAlchemy query objects is that you need to pass an SQLAlchemy session via the sqlalchemy_session parameter. A bare select does not have a database connection assigned. But the session has.

Usage in a Pylons controller and template

A simple example to begin with.

Controller:

def list(self):
    c.employees = webhelpers.paginate.Page(
        model.Session.query(model.Employee),
        page = int(request.params['page']),
        items_per_page = 5)
    return render('/employees/list.mako')

Template:

${c.employees.pager('Page $page: $link_previous $link_next ~4~')}
<ul>
% for employee in c.employees:
    <li>${employee.first_name} ${employee.last_name}</li>
% endfor
</ul>

The pager() creates links to the previous URL and just sets the page parameter appropriately. That's why you need to pass the requested page number (request.params['page']) when you create a Page.

Partial updates with AJAX

Updating a page partially is easy. All it takes is a little Javascript that - instead of loading the complete page - updates just the part of the page containing the paginated items. The pager() method accepts an onclick parameter for that purpose. This value is added as an onclick parameter to the A-HREF tags. So the href parameter points to a URL that loads the complete page while the onclick parameter provides Javascript that loads a partial page. An example (using the jQuery Javascript library for simplification) may help explain that.

Controller:

def list(self):
    c.employees = webhelpers.paginate.Page(
        model.Session.query(model.Employee),
        page = int(request.params['page']),
        items_per_page = 5)
    if 'partial' in request.params:
        # Render the partial page
        return render('/employees/list-partial.mako')
    else:
        # Render the full page
        return render('/employees/list-full.mako')

Template list-full.mako:

<html>
    <head>
        ${webhelpers.html.tags.javascript_link('/public/jQuery.js')}
    </head>
    <body>
        <div id="page-area">
            <%include file="list-partial.mako"/>
        </div>
    </body>
</html>

Template list-partial.mako:

${c.employees.pager(
    'Page $page: $link_previous $link_next ~4~',
    onclick="$('#my-page-area').load('%s'); return false;")}
<ul>
% for employee in c.employees:
    <li>${employee.first_name} ${employee.last_name}</li>
% endfor
</ul>

To avoid code duplication in the template the full template includes the partial template. If a partial page load is requested then just the list-partial.mako gets rendered. And if a full page load is requested then the list-full.mako is rendered which in turn includes the list-partial.mako.

The %s variable in the onclick string gets replaced with a URL pointing to the respective page with a partial=1 added (the name of the parameter can be customized through the partial_param parameter). Example:

  • href parameter points to /employees/list?page=3
  • onclick parameter contains Javascript loading /employees/list?page=3&partial=1

jQuery's syntax to load a URL into a certain DOM object (e.g. a DIV) is simply:

$('#some-id').load('/the/url')

The advantage of this technique is that it degrades gracefully. If the user does not have Javascript enabled then a full page is loaded. And if Javascript works then a partial load is done through the onclick action.

Secure Form Tag Helpers

For prevention of Cross-site request forgery (CSRF) attacks.

Generates form tags that include client-specific authorization tokens to be verified by the destined web app.

Authorization tokens are stored in the client's session. The web app can then verify the request's submitted authorization token with the value in the client's session.

This ensures the request came from the originating page. See the wikipedia entry for Cross-site request forgery for more information.

Pylons provides an authenticate_form decorator that does this verification on the behalf of controllers.

These helpers depend on Pylons' session object. Most of them can be easily ported to another framework by changing the API calls.

The helpers are implemented in such a way that it should be easy for developers to create their own helpers if using helpers for AJAX calls.

:func:`authentication_token` returns the current authentication token, creating one and storing it in the session if it doesn't already exist.

:func:`auth_token_hidden_field` creates a hidden field containing the authentication token.

:func:`secure_form` is :func:`form` plus :func:`auth_token_hidden_field`.

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