cpython/Doc/lib/libformatter.tex

\section{\module{formatter} ---
         Generic output formatting}

\declaremodule{standard}{formatter}
\modulesynopsis{Generic output formatter and device interface.}



This module supports two interface definitions, each with multiple
implementations.  The \emph{formatter} interface is used by the
\class{HTMLParser} class of the \refmodule{htmllib} module, and the
\emph{writer} interface is required by the formatter interface.
\withsubitem{(class in htmllib)}{\ttindex{HTMLParser}}

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.


\subsection{The Formatter Interface \label{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:


\begin{datadesc}{AS_IS}
Value which can be used in the font specification passed to the
\code{push_font()} method described below, or as the new value to any
other \code{push_\var{property}()} method.  Pushing the \code{AS_IS}
value allows the corresponding \code{pop_\var{property}()} method to
be called without having to track whether the property was changed.
\end{datadesc}

The following attributes are defined for formatter instance objects:


\begin{memberdesc}[formatter]{writer}
The writer instance with which the formatter interacts.
\end{memberdesc}


\begin{methoddesc}[formatter]{end_paragraph}{blanklines}
Close any open paragraphs and insert at least \var{blanklines}
before the next paragraph.
\end{methoddesc}

\begin{methoddesc}[formatter]{add_line_break}{}
Add a hard line break if one does not already exist.  This does not
break the logical paragraph.
\end{methoddesc}

\begin{methoddesc}[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
\method{send_line_break()} method.
\end{methoddesc}

\begin{methoddesc}[formatter]{add_flowing_data}{data}
Provide data which should be formatted with collapsed whitespace.
Whitespace from preceding and successive calls to
\method{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.
\end{methoddesc}

\begin{methoddesc}[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 \var{data}.  
\end{methoddesc}

\begin{methoddesc}[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 \var{format} value is a string, it is interpreted as a
format specification for \var{counter}, which should be an integer.
The result of this formatting becomes the value of the label; if
\var{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
\method{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 \character{1} represents the counter value formatter as an
Arabic number, the characters \character{A} and \character{a}
represent alphabetic representations of the counter value in upper and
lower case, respectively, and \character{I} and \character{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.
\end{methoddesc}

\begin{methoddesc}[formatter]{flush_softspace}{}
Send any pending whitespace buffered from a previous call to
\method{add_flowing_data()} to the associated writer object.  This
should be called before any direct manipulation of the writer object.
\end{methoddesc}

\begin{methoddesc}[formatter]{push_alignment}{align}
Push a new alignment setting onto the alignment stack.  This may be
\constant{AS_IS} if no change is desired.  If the alignment value is
changed from the previous setting, the writer's \method{new_alignment()}
method is called with the \var{align} value.
\end{methoddesc}

\begin{methoddesc}[formatter]{pop_alignment}{}
Restore the previous alignment.
\end{methoddesc}

\begin{methoddesc}[formatter]{push_font}{\code{(}size, italic, bold, teletype\code{)}}
Change some or all font properties of the writer object.  Properties
which are not set to \constant{AS_IS} are set to the values passed in
while others are maintained at their current settings.  The writer's
\method{new_font()} method is called with the fully resolved font
specification.
\end{methoddesc}

\begin{methoddesc}[formatter]{pop_font}{}
Restore the previous font.
\end{methoddesc}

\begin{methoddesc}[formatter]{push_margin}{margin}
Increase the number of left margin indentations by one, associating
the logical tag \var{margin} with the new indentation.  The initial
margin level is \code{0}.  Changed values of the logical tag must be
true values; false values other than \constant{AS_IS} are not
sufficient to change the margin.
\end{methoddesc}

\begin{methoddesc}[formatter]{pop_margin}{}
Restore the previous margin.
\end{methoddesc}

\begin{methoddesc}[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 \constant{AS_IS} values, is passed to the
writer's \method{new_styles()} method.
\end{methoddesc}

\begin{methoddesc}[formatter]{pop_style}{\optional{n\code{ = 1}}}
Pop the last \var{n} style specifications passed to
\method{push_style()}.  A tuple representing the revised stack,
including \constant{AS_IS} values, is passed to the writer's
\method{new_styles()} method.
\end{methoddesc}

\begin{methoddesc}[formatter]{set_spacing}{spacing}
Set the spacing style for the writer.
\end{methoddesc}

\begin{methoddesc}[formatter]{assert_line_data}{\optional{flag\code{ = 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 \var{flag} argument can be set to false if
the writer manipulations produced a hard line break at the end of the
output.
\end{methoddesc}


\subsection{Formatter Implementations \label{formatter-impls}}

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

\begin{classdesc}{NullFormatter}{\optional{writer}}
A formatter which does nothing.  If \var{writer} is omitted, a
\class{NullWriter} instance is created.  No methods of the writer are
called by \class{NullFormatter} instances.  Implementations should
inherit from this class if implementing a writer interface but don't
need to inherit any implementation.
\end{classdesc}

\begin{classdesc}{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.
\end{classdesc}



\subsection{The Writer Interface \label{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
\class{AbstractFormatter} class as a formatter, the writer must
typically be provided by the application.


\begin{methoddesc}[writer]{flush}{}
Flush any buffered output or device control events.
\end{methoddesc}

\begin{methoddesc}[writer]{new_alignment}{align}
Set the alignment style.  The \var{align} value can be any object,
but by convention is a string or \code{None}, where \code{None}
indicates that the writer's ``preferred'' alignment should be used.
Conventional \var{align} values are \code{'left'}, \code{'center'},
\code{'right'}, and \code{'justify'}.
\end{methoddesc}

\begin{methoddesc}[writer]{new_font}{font}
Set the font style.  The value of \var{font} will be \code{None},
indicating that the device's default font should be used, or a tuple
of the form \code{(}\var{size}, \var{italic}, \var{bold},
\var{teletype}\code{)}.  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 \var{italic}, \var{bold}, and
\var{teletype} values are boolean indicators specifying which of those
font attributes should be used.
\end{methoddesc}

\begin{methoddesc}[writer]{new_margin}{margin, level}
Set the margin level to the integer \var{level} and the logical tag
to \var{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
\var{level}.
\end{methoddesc}

\begin{methoddesc}[writer]{new_spacing}{spacing}
Set the spacing style to \var{spacing}.
\end{methoddesc}

\begin{methoddesc}[writer]{new_styles}{styles}
Set additional styles.  The \var{styles} value is a tuple of
arbitrary values; the value \constant{AS_IS} should be ignored.  The
\var{styles} tuple may be interpreted either as a set or as a stack
depending on the requirements of the application and writer
implementation.
\end{methoddesc}

\begin{methoddesc}[writer]{send_line_break}{}
Break the current line.
\end{methoddesc}

\begin{methoddesc}[writer]{send_paragraph}{blankline}
Produce a paragraph separation of at least \var{blankline} blank
lines, or the equivalent.  The \var{blankline} value will be an
integer.  Note that the implementation will receive a call to
\method{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.
\end{methoddesc}

\begin{methoddesc}[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 \method{send_line_break()}.
\end{methoddesc}

\begin{methoddesc}[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.
\end{methoddesc}

\begin{methoddesc}[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
\method{send_formatted_data()} interface.
\end{methoddesc}

\begin{methoddesc}[writer]{send_label_data}{data}
Set \var{data} to the left of the current left margin, if possible.
The value of \var{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.
\end{methoddesc}


\subsection{Writer Implementations \label{writer-impls}}

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 \class{NullWriter} class.

\begin{classdesc}{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.
\end{classdesc}

\begin{classdesc}{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.
\end{classdesc}

\begin{classdesc}{DumbWriter}{\optional{file\optional{, maxcol\code{ = 72}}}}
Simple writer class which writes output on the file object passed in
as \var{file} or, if \var{file} is omitted, on standard output.  The
output is simply word-wrapped to the number of columns specified by
\var{maxcol}.  This class is suitable for reflowing a sequence of
paragraphs.
\end{classdesc}