/Doc/library/formatter.rst
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- :mod:`formatter` --- Generic output formatting
- ==============================================
- .. module:: formatter
- :synopsis: Generic output formatter and device interface.
- .. index:: single: HTMLParser (class in htmllib)
- This module supports two interface definitions, each with multiple
- implementations. The *formatter* interface is used by the :class:`HTMLParser`
- class of the :mod:`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.
- .. _formatter-interface:
- 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:
- .. data:: 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:
- .. attribute:: formatter.writer
- The writer instance with which the formatter interacts.
- .. method:: formatter.end_paragraph(blanklines)
- Close any open paragraphs and insert at least *blanklines* before the next
- paragraph.
- .. method:: formatter.add_line_break()
- Add a hard line break if one does not already exist. This does not break the
- logical paragraph.
- .. method:: 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 :meth:`send_line_break`
- method.
- .. method:: formatter.add_flowing_data(data)
- Provide data which should be formatted with collapsed whitespace. Whitespace
- from preceding and successive calls to :meth:`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.
- .. method:: 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*.
- .. method:: 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
- :meth:`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.
- .. method:: formatter.flush_softspace()
- Send any pending whitespace buffered from a previous call to
- :meth:`add_flowing_data` to the associated writer object. This should be called
- before any direct manipulation of the writer object.
- .. method:: formatter.push_alignment(align)
- Push a new alignment setting onto the alignment stack. This may be
- :const:`AS_IS` if no change is desired. If the alignment value is changed from
- the previous setting, the writer's :meth:`new_alignment` method is called with
- the *align* value.
- .. method:: formatter.pop_alignment()
- Restore the previous alignment.
- .. method:: formatter.push_font((size, italic, bold, teletype))
- Change some or all font properties of the writer object. Properties which are
- not set to :const:`AS_IS` are set to the values passed in while others are
- maintained at their current settings. The writer's :meth:`new_font` method is
- called with the fully resolved font specification.
- .. method:: formatter.pop_font()
- Restore the previous font.
- .. method:: 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
- :const:`AS_IS` are not sufficient to change the margin.
- .. method:: formatter.pop_margin()
- Restore the previous margin.
- .. method:: 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
- :const:`AS_IS` values, is passed to the writer's :meth:`new_styles` method.
- .. method:: formatter.pop_style([n=1])
- Pop the last *n* style specifications passed to :meth:`push_style`. A tuple
- representing the revised stack, including :const:`AS_IS` values, is passed to
- the writer's :meth:`new_styles` method.
- .. method:: formatter.set_spacing(spacing)
- Set the spacing style for the writer.
- .. method:: 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.
- .. _formatter-impls:
- 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:: NullFormatter([writer])
- A formatter which does nothing. If *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.
- .. class:: 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.
- .. _writer-interface:
- 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 :class:`AbstractFormatter` class as a formatter, the writer must
- typically be provided by the application.
- .. method:: writer.flush()
- Flush any buffered output or device control events.
- .. method:: 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'``.
- .. method:: 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.
- .. method:: 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*.
- .. method:: writer.new_spacing(spacing)
- Set the spacing style to *spacing*.
- .. method:: writer.new_styles(styles)
- Set additional styles. The *styles* value is a tuple of arbitrary values; the
- value :const:`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.
- .. method:: writer.send_line_break()
- Break the current line.
- .. method:: 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 :meth:`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.
- .. method:: 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 :meth:`send_line_break`.
- .. method:: 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.
- .. method:: 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 :meth:`send_formatted_data` interface.
- .. method:: 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.
- .. _writer-impls:
- 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
- :class:`NullWriter` class.
- .. class:: 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:: 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:: DumbWriter([file[, maxcol=72]])
- Simple writer class which writes output on the file object passed in as *file*
- or, if *file* is omitted, 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.