34.1. "formatter" — Generic output formatting
*********************************************

This module supports two interface definitions, each with multiple
implementations.  The *formatter* interface is used by the
"HTMLParser" class of the "htmllib" module, and the *writer* interface
is required by the formatter interface.

Formatter objects transform an abstract flow of formatting events into
specific output events on writer objects.  Formatters manage several
stack structures to allow various properties of a writer object to be
changed and restored; writers need not be able to handle relative
changes nor any sort of “change back” operation.  Specific writer
properties which may be controlled via formatter objects are
horizontal alignment, font, and left margin indentations.  A mechanism
is provided which supports providing arbitrary, non-exclusive style
settings to a writer as well.  Additional interfaces facilitate
formatting events which are not reversible, such as paragraph
separation.

Writer objects encapsulate device interfaces.  Abstract devices, such
as file formats, are supported as well as physical devices.  The
provided implementations all work with abstract devices.  The
interface makes available mechanisms for setting the properties which
formatter objects manage and inserting data into the output.


34.1.1. The Formatter Interface
===============================

Interfaces to create formatters are dependent on the specific
formatter class being instantiated.  The interfaces described below
are the required interfaces which all formatters must support once
initialized.

One data element is defined at the module level:

formatter.AS_IS

   Value which can be used in the font specification passed to the
   "push_font()" method described below, or as the new value to any
   other "push_property()" method.  Pushing the "AS_IS" value allows
   the corresponding "pop_property()" method to be called without
   having to track whether the property was changed.

The following attributes are defined for formatter instance objects:

formatter.writer

   The writer instance with which the formatter interacts.

formatter.end_paragraph(blanklines)

   Close any open paragraphs and insert at least *blanklines* before
   the next paragraph.

formatter.add_line_break()

   Add a hard line break if one does not already exist.  This does not
   break the logical paragraph.

formatter.add_hor_rule(*args, **kw)

   Insert a horizontal rule in the output.  A hard break is inserted
   if there is data in the current paragraph, but the logical
   paragraph is not broken.  The arguments and keywords are passed on
   to the writer’s "send_line_break()" method.

formatter.add_flowing_data(data)

   Provide data which should be formatted with collapsed whitespace.
   Whitespace from preceding and successive calls to
   "add_flowing_data()" is considered as well when the whitespace
   collapse is performed.  The data which is passed to this method is
   expected to be word-wrapped by the output device.  Note that any
   word-wrapping still must be performed by the writer object due to
   the need to rely on device and font information.

formatter.add_literal_data(data)

   Provide data which should be passed to the writer unchanged.
   Whitespace, including newline and tab characters, are considered
   legal in the value of *data*.

formatter.add_label_data(format, counter)

   Insert a label which should be placed to the left of the current
   left margin. This should be used for constructing bulleted or
   numbered lists.  If the *format* value is a string, it is
   interpreted as a format specification for *counter*, which should
   be an integer. The result of this formatting becomes the value of
   the label; if *format* is not a string it is used as the label
   value directly. The label value is passed as the only argument to
   the writer’s "send_label_data()" method.  Interpretation of non-
   string label values is dependent on the associated writer.

   Format specifications are strings which, in combination with a
   counter value, are used to compute label values.  Each character in
   the format string is copied to the label value, with some
   characters recognized to indicate a transform on the counter value.
   Specifically, the character "'1'" represents the counter value
   formatter as an Arabic number, the characters "'A'" and "'a'"
   represent alphabetic representations of the counter value in upper
   and lower case, respectively, and "'I'" and "'i'" represent the
   counter value in Roman numerals, in upper and lower case.  Note
   that the alphabetic and roman transforms require that the counter
   value be greater than zero.

formatter.flush_softspace()

   Send any pending whitespace buffered from a previous call to
   "add_flowing_data()" to the associated writer object.  This should
   be called before any direct manipulation of the writer object.

formatter.push_alignment(align)

   Push a new alignment setting onto the alignment stack.  This may be
   "AS_IS" if no change is desired.  If the alignment value is changed
   from the previous setting, the writer’s "new_alignment()" method is
   called with the *align* value.

formatter.pop_alignment()

   Restore the previous alignment.

formatter.push_font((size, italic, bold, teletype))

   Change some or all font properties of the writer object.
   Properties which are not set to "AS_IS" are set to the values
   passed in while others are maintained at their current settings.
   The writer’s "new_font()" method is called with the fully resolved
   font specification.

formatter.pop_font()

   Restore the previous font.

formatter.push_margin(margin)

   Increase the number of left margin indentations by one, associating
   the logical tag *margin* with the new indentation.  The initial
   margin level is "0". Changed values of the logical tag must be true
   values; false values other than "AS_IS" are not sufficient to
   change the margin.

formatter.pop_margin()

   Restore the previous margin.

formatter.push_style(*styles)

   Push any number of arbitrary style specifications.  All styles are
   pushed onto the styles stack in order.  A tuple representing the
   entire stack, including "AS_IS" values, is passed to the writer’s
   "new_styles()" method.

formatter.pop_style([n=1])

   Pop the last *n* style specifications passed to "push_style()".  A
   tuple representing the revised stack, including "AS_IS" values, is
   passed to the writer’s "new_styles()" method.

formatter.set_spacing(spacing)

   Set the spacing style for the writer.

formatter.assert_line_data([flag=1])

   Inform the formatter that data has been added to the current
   paragraph out-of-band.  This should be used when the writer has
   been manipulated directly.  The optional *flag* argument can be set
   to false if the writer manipulations produced a hard line break at
   the end of the output.


34.1.2. Formatter Implementations
=================================

Two implementations of formatter objects are provided by this module.
Most applications may use one of these classes without modification or
subclassing.

class formatter.NullFormatter([writer])

   A formatter which does nothing.  If *writer* is omitted, a
   "NullWriter" instance is created.  No methods of the writer are
   called by "NullFormatter" instances.  Implementations should
   inherit from this class if implementing a writer interface but
   don’t need to inherit any implementation.

class formatter.AbstractFormatter(writer)

   The standard formatter.  This implementation has demonstrated wide
   applicability to many writers, and may be used directly in most
   circumstances.  It has been used to implement a full-featured World
   Wide Web browser.


34.1.3. The Writer Interface
============================

Interfaces to create writers are dependent on the specific writer
class being instantiated.  The interfaces described below are the
required interfaces which all writers must support once initialized.
Note that while most applications can use the "AbstractFormatter"
class as a formatter, the writer must typically be provided by the
application.

writer.flush()

   Flush any buffered output or device control events.

writer.new_alignment(align)

   Set the alignment style.  The *align* value can be any object, but
   by convention is a string or "None", where "None" indicates that
   the writer’s “preferred” alignment should be used. Conventional
   *align* values are "'left'", "'center'", "'right'", and
   "'justify'".

writer.new_font(font)

   Set the font style.  The value of *font* will be "None", indicating
   that the device’s default font should be used, or a tuple of the
   form "(size, italic, bold, teletype)".  Size will be a string
   indicating the size of font that should be used; specific strings
   and their interpretation must be defined by the application.  The
   *italic*, *bold*, and *teletype* values are Boolean values
   specifying which of those font attributes should be used.

writer.new_margin(margin, level)

   Set the margin level to the integer *level* and the logical tag to
   *margin*. Interpretation of the logical tag is at the writer’s
   discretion; the only restriction on the value of the logical tag is
   that it not be a false value for non-zero values of *level*.

writer.new_spacing(spacing)

   Set the spacing style to *spacing*.

writer.new_styles(styles)

   Set additional styles.  The *styles* value is a tuple of arbitrary
   values; the value "AS_IS" should be ignored.  The *styles* tuple
   may be interpreted either as a set or as a stack depending on the
   requirements of the application and writer implementation.

writer.send_line_break()

   Break the current line.

writer.send_paragraph(blankline)

   Produce a paragraph separation of at least *blankline* blank lines,
   or the equivalent.  The *blankline* value will be an integer.  Note
   that the implementation will receive a call to "send_line_break()"
   before this call if a line break is needed;  this method should not
   include ending the last line of the paragraph. It is only
   responsible for vertical spacing between paragraphs.

writer.send_hor_rule(*args, **kw)

   Display a horizontal rule on the output device.  The arguments to
   this method are entirely application- and writer-specific, and
   should be interpreted with care.  The method implementation may
   assume that a line break has already been issued via
   "send_line_break()".

writer.send_flowing_data(data)

   Output character data which may be word-wrapped and re-flowed as
   needed.  Within any sequence of calls to this method, the writer
   may assume that spans of multiple whitespace characters have been
   collapsed to single space characters.

writer.send_literal_data(data)

   Output character data which has already been formatted for display.
   Generally, this should be interpreted to mean that line breaks
   indicated by newline characters should be preserved and no new line
   breaks should be introduced.  The data may contain embedded newline
   and tab characters, unlike data provided to the
   "send_formatted_data()" interface.

writer.send_label_data(data)

   Set *data* to the left of the current left margin, if possible. The
   value of *data* is not restricted; treatment of non-string values
   is entirely application- and writer-dependent.  This method will
   only be called at the beginning of a line.


34.1.4. Writer Implementations
==============================

Three implementations of the writer object interface are provided as
examples by this module.  Most applications will need to derive new
writer classes from the "NullWriter" class.

class formatter.NullWriter

   A writer which only provides the interface definition; no actions
   are taken on any methods.  This should be the base class for all
   writers which do not need to inherit any implementation methods.

class formatter.AbstractWriter

   A writer which can be used in debugging formatters, but not much
   else.  Each method simply announces itself by printing its name and
   arguments on standard output.

class formatter.DumbWriter(file=None, maxcol=72)

   Simple writer class which writes output on the file object passed
   in as *file* or, if *file* is "None", on standard output.  The
   output is simply word-wrapped to the number of columns specified by
   *maxcol*.  This class is suitable for reflowing a sequence of
   paragraphs.
