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# htmlfill

author: | Ian Bicking <ianb@colorstudy.com>

Contents

- htmlfill
  - Introduction
  - Usage
    - Errors
    - Valid form templates

## Introduction

`formencode.htmlfill` is a library to fill out forms, both with default values and error messages. It’s like a template library, but more limited, and it can be used with the output from other templates. It has no prerequesites, and can be used without any other parts of FormEncode.

## Usage

The basic usage is something like this:

    >>> from formencode import htmlfill
    >>> form = '<input type="text" name="fname">'
    >>> defaults = {'fname': 'Joe'}
    >>> htmlfill.render(form, defaults)
    '<input type="text" name="fname" value="Joe">'

The parser looks for HTML input elements (including `select` and `textarea`) and fills in the defaults. The quintessential way to use this would be with a form submission that had errors – you can return the form to the user with the values they entered, in addition to errors.

See `formencode.htmlfill.render()` for more.

### Errors

Since errors are a common issue, this also has some features for filling the form with error messages. It defines two special tags for this purpose:

`<form:error name="field_name" format="formatter">`:
  This tag is eliminated completely if there is no error for the named field. Otherwise the error is passed through the given formatter (`"default"` if no `format` attribute is given).
`<form:iferror name="field_name">...</form:iferror>`:
  If the named field doesn’t have an error, everything between the tags will be eliminated. Use `name="not field_name"` to invert the behavior (i.e., include text only if there are no errors for the field).

Formatters are functions that take the error text as a single argument. (In the future they may take extra arguments as well.) They return a string that is inserted into the template. By default, the formatter returns:

    <span class="error-message">(message)</span><br>

In addition to explicit error tags, any leftover errors will be placed immediately above the associated input field.

The default formatters available to you:

`default`:
  HTML-quotes the error and wraps it in `<span class="error-message">`
`none`:
  Puts the error message in with no quoting of any kind. This allows you to put HTML in the error message, but might also expose you to cross-site scripting vulnerabilities.
`escape`:
  HTML-quotes the error, but doesn’t wrap it in anything.
`escapenl`:
  HTML-quotes the error, and translates newlines to `<br>`
`ignore`:
  Swallows the error, emitting nothing. You can use this when you never want an error for a field to display.

### Valid form templates

When you call `parser.close()` (also called by `render()`) the parser will check that you’ve fully used all the defaults and errors that were given in the constructor if you pass in `use_all_keys=True`. If there are leftover fields an `AssertionError` will be raised.

In most cases, htmlfill tries to keep the template the way it found it, without reformatting it too much. If HTMLParser chokes on the code, so will htmlfill.

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