MCL/sf/adapt/qemu: comparison symbian-qemu-0.9.1-12/python-2.6.1/Doc/library/difflib.rst

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+:mod:`difflib` --- Helpers for computing deltas
+===============================================
+.. module:: difflib
+:synopsis: Helpers for computing differences between objects.
+.. moduleauthor:: Tim Peters <tim_one@users.sourceforge.net>
+.. sectionauthor:: Tim Peters <tim_one@users.sourceforge.net>
+.. Markup by Fred L. Drake, Jr. <fdrake@acm.org>
+.. testsetup::
+import sys
+from difflib import *
+.. versionadded:: 2.1
+This module provides classes and functions for comparing sequences. It
+can be used for example, for comparing files, and can produce difference
+information in various formats, including HTML and context and unified
+diffs. For comparing directories and files, see also, the :mod:`filecmp` module.
+.. class:: SequenceMatcher
+This is a flexible class for comparing pairs of sequences of any type, so long
+as the sequence elements are :term:`hashable`.  The basic algorithm predates, and is a
+little fancier than, an algorithm published in the late 1980's by Ratcliff and
+Obershelp under the hyperbolic name "gestalt pattern matching."  The idea is to
+find the longest contiguous matching subsequence that contains no "junk"
+elements (the Ratcliff and Obershelp algorithm doesn't address junk).  The same
+idea is then applied recursively to the pieces of the sequences to the left and
+to the right of the matching subsequence.  This does not yield minimal edit
+sequences, but does tend to yield matches that "look right" to people.
+**Timing:** The basic Ratcliff-Obershelp algorithm is cubic time in the worst
+case and quadratic time in the expected case. :class:`SequenceMatcher` is
+quadratic time for the worst case and has expected-case behavior dependent in a
+complicated way on how many elements the sequences have in common; best case
+time is linear.
+.. class:: Differ
+This is a class for comparing sequences of lines of text, and producing
+human-readable differences or deltas.  Differ uses :class:`SequenceMatcher`
+both to compare sequences of lines, and to compare sequences of characters
+within similar (near-matching) lines.
+Each line of a :class:`Differ` delta begins with a two-letter code:
++----------+-------------------------------------------+
+| Code     | Meaning                                   |
++==========+===========================================+
+| ``'- '`` | line unique to sequence 1                 |
++----------+-------------------------------------------+
+| ``'+ '`` | line unique to sequence 2                 |
++----------+-------------------------------------------+
+| ``'  '`` | line common to both sequences             |
++----------+-------------------------------------------+
+| ``'? '`` | line not present in either input sequence |
++----------+-------------------------------------------+
+Lines beginning with '``?``' attempt to guide the eye to intraline differences,
+and were not present in either input sequence. These lines can be confusing if
+the sequences contain tab characters.
+.. class:: HtmlDiff
+This class can be used to create an HTML table (or a complete HTML file
+containing the table) showing a side by side, line by line comparison of text
+with inter-line and intra-line change highlights.  The table can be generated in
+either full or contextual difference mode.
+The constructor for this class is:
+.. function:: __init__([tabsize][, wrapcolumn][, linejunk][, charjunk])
+Initializes instance of :class:`HtmlDiff`.
+*tabsize* is an optional keyword argument to specify tab stop spacing and
+defaults to ``8``.
+*wrapcolumn* is an optional keyword to specify column number where lines are
+broken and wrapped, defaults to ``None`` where lines are not wrapped.
+*linejunk* and *charjunk* are optional keyword arguments passed into ``ndiff()``
+(used by :class:`HtmlDiff` to generate the side by side HTML differences).  See
+``ndiff()`` documentation for argument default values and descriptions.
+The following methods are public:
+.. function:: make_file(fromlines, tolines [, fromdesc][, todesc][, context][, numlines])
+Compares *fromlines* and *tolines* (lists of strings) and returns a string which
+is a complete HTML file containing a table showing line by line differences with
+inter-line and intra-line changes highlighted.
+*fromdesc* and *todesc* are optional keyword arguments to specify from/to file
+column header strings (both default to an empty string).
+*context* and *numlines* are both optional keyword arguments. Set *context* to
+``True`` when contextual differences are to be shown, else the default is
+``False`` to show the full files. *numlines* defaults to ``5``.  When *context*
+is ``True`` *numlines* controls the number of context lines which surround the
+difference highlights.  When *context* is ``False`` *numlines* controls the
+number of lines which are shown before a difference highlight when using the
+"next" hyperlinks (setting to zero would cause the "next" hyperlinks to place
+the next difference highlight at the top of the browser without any leading
+context).
+.. function:: make_table(fromlines, tolines [, fromdesc][, todesc][, context][, numlines])
+Compares *fromlines* and *tolines* (lists of strings) and returns a string which
+is a complete HTML table showing line by line differences with inter-line and
+intra-line changes highlighted.
+The arguments for this method are the same as those for the :meth:`make_file`
+method.
+:file:`Tools/scripts/diff.py` is a command-line front-end to this class and
+contains a good example of its use.
+.. versionadded:: 2.4
+.. function:: context_diff(a, b[, fromfile][, tofile][, fromfiledate][, tofiledate][, n][, lineterm])
+Compare *a* and *b* (lists of strings); return a delta (a :term:`generator`
+generating the delta lines) in context diff format.
+Context diffs are a compact way of showing just the lines that have changed plus
+a few lines of context.  The changes are shown in a before/after style.  The
+number of context lines is set by *n* which defaults to three.
+By default, the diff control lines (those with ``***`` or ``---``) are created
+with a trailing newline.  This is helpful so that inputs created from
+:func:`file.readlines` result in diffs that are suitable for use with
+:func:`file.writelines` since both the inputs and outputs have trailing
+newlines.
+For inputs that do not have trailing newlines, set the *lineterm* argument to
+``""`` so that the output will be uniformly newline free.
+The context diff format normally has a header for filenames and modification
+times.  Any or all of these may be specified using strings for *fromfile*,
+*tofile*, *fromfiledate*, and *tofiledate*. The modification times are normally
+expressed in the format returned by :func:`time.ctime`.  If not specified, the
+strings default to blanks.
+>>> s1 = ['bacon\n', 'eggs\n', 'ham\n', 'guido\n']
+>>> s2 = ['python\n', 'eggy\n', 'hamster\n', 'guido\n']
+>>> for line in context_diff(s1, s2, fromfile='before.py', tofile='after.py'):
+...     sys.stdout.write(line)  # doctest: +NORMALIZE_WHITESPACE
+*** before.py
+--- after.py
+***************
+*** 1,4 ****
+! bacon
+! eggs
+! ham
+guido
+--- 1,4 ----
+! python
+! eggy
+! hamster
+guido
+See :ref:`difflib-interface` for a more detailed example.
+.. versionadded:: 2.3
+.. function:: get_close_matches(word, possibilities[, n][, cutoff])
+Return a list of the best "good enough" matches.  *word* is a sequence for which
+close matches are desired (typically a string), and *possibilities* is a list of
+sequences against which to match *word* (typically a list of strings).
+Optional argument *n* (default ``3``) is the maximum number of close matches to
+return; *n* must be greater than ``0``.
+Optional argument *cutoff* (default ``0.6``) is a float in the range [0, 1].
+Possibilities that don't score at least that similar to *word* are ignored.
+The best (no more than *n*) matches among the possibilities are returned in a
+list, sorted by similarity score, most similar first.
+>>> get_close_matches('appel', ['ape', 'apple', 'peach', 'puppy'])
+['apple', 'ape']
+>>> import keyword
+>>> get_close_matches('wheel', keyword.kwlist)
+['while']
+>>> get_close_matches('apple', keyword.kwlist)
+[]
+>>> get_close_matches('accept', keyword.kwlist)
+['except']
+.. function:: ndiff(a, b[, linejunk][, charjunk])
+Compare *a* and *b* (lists of strings); return a :class:`Differ`\ -style
+delta (a :term:`generator` generating the delta lines).
+Optional keyword parameters *linejunk* and *charjunk* are for filter functions
+(or ``None``):
+*linejunk*: A function that accepts a single string argument, and returns true
+if the string is junk, or false if not. The default is (``None``), starting with
+Python 2.3.  Before then, the default was the module-level function
+:func:`IS_LINE_JUNK`, which filters out lines without visible characters, except
+for at most one pound character (``'#'``). As of Python 2.3, the underlying
+:class:`SequenceMatcher` class does a dynamic analysis of which lines are so
+frequent as to constitute noise, and this usually works better than the pre-2.3
+default.
+*charjunk*: A function that accepts a character (a string of length 1), and
+returns if the character is junk, or false if not. The default is module-level
+function :func:`IS_CHARACTER_JUNK`, which filters out whitespace characters (a
+blank or tab; note: bad idea to include newline in this!).
+:file:`Tools/scripts/ndiff.py` is a command-line front-end to this function.
+>>> diff = ndiff('one\ntwo\nthree\n'.splitlines(1),
+...              'ore\ntree\nemu\n'.splitlines(1))
+>>> print ''.join(diff),
+- one
+?  ^
++ ore
+?  ^
+- two
+- three
+?  -
++ tree
++ emu
+.. function:: restore(sequence, which)
+Return one of the two sequences that generated a delta.
+Given a *sequence* produced by :meth:`Differ.compare` or :func:`ndiff`, extract
+lines originating from file 1 or 2 (parameter *which*), stripping off line
+prefixes.
+Example:
+>>> diff = ndiff('one\ntwo\nthree\n'.splitlines(1),
+...              'ore\ntree\nemu\n'.splitlines(1))
+>>> diff = list(diff) # materialize the generated delta into a list
+>>> print ''.join(restore(diff, 1)),
+one
+two
+three
+>>> print ''.join(restore(diff, 2)),
+ore
+tree
+emu
+.. function:: unified_diff(a, b[, fromfile][, tofile][, fromfiledate][, tofiledate][, n][, lineterm])
+Compare *a* and *b* (lists of strings); return a delta (a :term:`generator`
+generating the delta lines) in unified diff format.
+Unified diffs are a compact way of showing just the lines that have changed plus
+a few lines of context.  The changes are shown in a inline style (instead of
+separate before/after blocks).  The number of context lines is set by *n* which
+defaults to three.
+By default, the diff control lines (those with ``---``, ``+++``, or ``@@``) are
+created with a trailing newline.  This is helpful so that inputs created from
+:func:`file.readlines` result in diffs that are suitable for use with
+:func:`file.writelines` since both the inputs and outputs have trailing
+newlines.
+For inputs that do not have trailing newlines, set the *lineterm* argument to
+``""`` so that the output will be uniformly newline free.
+The context diff format normally has a header for filenames and modification
+times.  Any or all of these may be specified using strings for *fromfile*,
+*tofile*, *fromfiledate*, and *tofiledate*. The modification times are normally
+expressed in the format returned by :func:`time.ctime`.  If not specified, the
+strings default to blanks.
+>>> s1 = ['bacon\n', 'eggs\n', 'ham\n', 'guido\n']
+>>> s2 = ['python\n', 'eggy\n', 'hamster\n', 'guido\n']
+>>> for line in unified_diff(s1, s2, fromfile='before.py', tofile='after.py'):
+...     sys.stdout.write(line)   # doctest: +NORMALIZE_WHITESPACE
+--- before.py
++++ after.py
+@@ -1,4 +1,4 @@
+-bacon
+-eggs
+-ham
++python
++eggy
++hamster
+guido
+See :ref:`difflib-interface` for a more detailed example.
+.. versionadded:: 2.3
+.. function:: IS_LINE_JUNK(line)
+Return true for ignorable lines.  The line *line* is ignorable if *line* is
+blank or contains a single ``'#'``, otherwise it is not ignorable.  Used as a
+default for parameter *linejunk* in :func:`ndiff` before Python 2.3.
+.. function:: IS_CHARACTER_JUNK(ch)
+Return true for ignorable characters.  The character *ch* is ignorable if *ch*
+is a space or tab, otherwise it is not ignorable.  Used as a default for
+parameter *charjunk* in :func:`ndiff`.
+.. seealso::
+`Pattern Matching: The Gestalt Approach <http://www.ddj.com/184407970?pgno=5>`_
+Discussion of a similar algorithm by John W. Ratcliff and D. E. Metzener. This
+was published in `Dr. Dobb's Journal <http://www.ddj.com/>`_ in July, 1988.
+.. _sequence-matcher:
+SequenceMatcher Objects
+-----------------------
+The :class:`SequenceMatcher` class has this constructor:
+.. class:: SequenceMatcher([isjunk[, a[, b]]])
+Optional argument *isjunk* must be ``None`` (the default) or a one-argument
+function that takes a sequence element and returns true if and only if the
+element is "junk" and should be ignored. Passing ``None`` for *isjunk* is
+equivalent to passing ``lambda x: 0``; in other words, no elements are ignored.
+For example, pass::
+lambda x: x in " \t"
+if you're comparing lines as sequences of characters, and don't want to synch up
+on blanks or hard tabs.
+The optional arguments *a* and *b* are sequences to be compared; both default to
+empty strings.  The elements of both sequences must be :term:`hashable`.
+:class:`SequenceMatcher` objects have the following methods:
+.. method:: set_seqs(a, b)
+Set the two sequences to be compared.
+:class:`SequenceMatcher` computes and caches detailed information about the
+second sequence, so if you want to compare one sequence against many
+sequences, use :meth:`set_seq2` to set the commonly used sequence once and
+call :meth:`set_seq1` repeatedly, once for each of the other sequences.
+.. method:: set_seq1(a)
+Set the first sequence to be compared.  The second sequence to be compared
+is not changed.
+.. method:: set_seq2(b)
+Set the second sequence to be compared.  The first sequence to be compared
+is not changed.
+.. method:: find_longest_match(alo, ahi, blo, bhi)
+Find longest matching block in ``a[alo:ahi]`` and ``b[blo:bhi]``.
+If *isjunk* was omitted or ``None``, :meth:`find_longest_match` returns
+``(i, j, k)`` such that ``a[i:i+k]`` is equal to ``b[j:j+k]``, where ``alo
+<= i <= i+k <= ahi`` and ``blo <= j <= j+k <= bhi``. For all ``(i', j',
+k')`` meeting those conditions, the additional conditions ``k >= k'``, ``i
+<= i'``, and if ``i == i'``, ``j <= j'`` are also met. In other words, of
+all maximal matching blocks, return one that starts earliest in *a*, and
+of all those maximal matching blocks that start earliest in *a*, return
+the one that starts earliest in *b*.
+>>> s = SequenceMatcher(None, " abcd", "abcd abcd")
+>>> s.find_longest_match(0, 5, 0, 9)
+Match(a=0, b=4, size=5)
+If *isjunk* was provided, first the longest matching block is determined
+as above, but with the additional restriction that no junk element appears
+in the block.  Then that block is extended as far as possible by matching
+(only) junk elements on both sides. So the resulting block never matches
+on junk except as identical junk happens to be adjacent to an interesting
+match.
+Here's the same example as before, but considering blanks to be junk. That
+prevents ``' abcd'`` from matching the ``' abcd'`` at the tail end of the
+second sequence directly.  Instead only the ``'abcd'`` can match, and
+matches the leftmost ``'abcd'`` in the second sequence:
+>>> s = SequenceMatcher(lambda x: x==" ", " abcd", "abcd abcd")
+>>> s.find_longest_match(0, 5, 0, 9)
+Match(a=1, b=0, size=4)
+If no blocks match, this returns ``(alo, blo, 0)``.
+.. versionchanged:: 2.6
+This method returns a :term:`named tuple` ``Match(a, b, size)``.
+.. method:: get_matching_blocks()
+Return list of triples describing matching subsequences. Each triple is of
+the form ``(i, j, n)``, and means that ``a[i:i+n] == b[j:j+n]``.  The
+triples are monotonically increasing in *i* and *j*.
+The last triple is a dummy, and has the value ``(len(a), len(b), 0)``.  It
+is the only triple with ``n == 0``.  If ``(i, j, n)`` and ``(i', j', n')``
+are adjacent triples in the list, and the second is not the last triple in
+the list, then ``i+n != i'`` or ``j+n != j'``; in other words, adjacent
+triples always describe non-adjacent equal blocks.
+.. XXX Explain why a dummy is used!
+.. versionchanged:: 2.5
+The guarantee that adjacent triples always describe non-adjacent blocks
+was implemented.
+.. doctest::
+>>> s = SequenceMatcher(None, "abxcd", "abcd")
+>>> s.get_matching_blocks()
+[Match(a=0, b=0, size=2), Match(a=3, b=2, size=2), Match(a=5, b=4, size=0)]
+.. method:: get_opcodes()
+Return list of 5-tuples describing how to turn *a* into *b*. Each tuple is
+of the form ``(tag, i1, i2, j1, j2)``.  The first tuple has ``i1 == j1 ==
+0``, and remaining tuples have *i1* equal to the *i2* from the preceding
+tuple, and, likewise, *j1* equal to the previous *j2*.
+The *tag* values are strings, with these meanings:
++---------------+---------------------------------------------+
+| Value         | Meaning                                     |
++===============+=============================================+
+| ``'replace'`` | ``a[i1:i2]`` should be replaced by          |
+|               | ``b[j1:j2]``.                               |
++---------------+---------------------------------------------+
+| ``'delete'``  | ``a[i1:i2]`` should be deleted.  Note that  |
+|               | ``j1 == j2`` in this case.                  |
++---------------+---------------------------------------------+
+| ``'insert'``  | ``b[j1:j2]`` should be inserted at          |
+|               | ``a[i1:i1]``. Note that ``i1 == i2`` in     |
+|               | this case.                                  |
++---------------+---------------------------------------------+
+| ``'equal'``   | ``a[i1:i2] == b[j1:j2]`` (the sub-sequences |
+|               | are equal).                                 |
++---------------+---------------------------------------------+
+For example:
+>>> a = "qabxcd"
+>>> b = "abycdf"
+>>> s = SequenceMatcher(None, a, b)
+>>> for tag, i1, i2, j1, j2 in s.get_opcodes():
+...    print ("%7s a[%d:%d] (%s) b[%d:%d] (%s)" %
+...           (tag, i1, i2, a[i1:i2], j1, j2, b[j1:j2]))
+delete a[0:1] (q) b[0:0] ()
+equal a[1:3] (ab) b[0:2] (ab)
+replace a[3:4] (x) b[2:3] (y)
+equal a[4:6] (cd) b[3:5] (cd)
+insert a[6:6] () b[5:6] (f)
+.. method:: get_grouped_opcodes([n])
+Return a :term:`generator` of groups with up to *n* lines of context.
+Starting with the groups returned by :meth:`get_opcodes`, this method
+splits out smaller change clusters and eliminates intervening ranges which
+have no changes.
+The groups are returned in the same format as :meth:`get_opcodes`.
+.. versionadded:: 2.3
+.. method:: ratio()
+Return a measure of the sequences' similarity as a float in the range [0,
+1].
+Where T is the total number of elements in both sequences, and M is the
+number of matches, this is 2.0\*M / T. Note that this is ``1.0`` if the
+sequences are identical, and ``0.0`` if they have nothing in common.
+This is expensive to compute if :meth:`get_matching_blocks` or
+:meth:`get_opcodes` hasn't already been called, in which case you may want
+to try :meth:`quick_ratio` or :meth:`real_quick_ratio` first to get an
+upper bound.
+.. method:: quick_ratio()
+Return an upper bound on :meth:`ratio` relatively quickly.
+This isn't defined beyond that it is an upper bound on :meth:`ratio`, and
+is faster to compute.
+.. method:: real_quick_ratio()
+Return an upper bound on :meth:`ratio` very quickly.
+This isn't defined beyond that it is an upper bound on :meth:`ratio`, and
+is faster to compute than either :meth:`ratio` or :meth:`quick_ratio`.
+The three methods that return the ratio of matching to total characters can give
+different results due to differing levels of approximation, although
+:meth:`quick_ratio` and :meth:`real_quick_ratio` are always at least as large as
+:meth:`ratio`:
+>>> s = SequenceMatcher(None, "abcd", "bcde")
+>>> s.ratio()
+0.75
+>>> s.quick_ratio()
+0.75
+>>> s.real_quick_ratio()
+1.0
+.. _sequencematcher-examples:
+SequenceMatcher Examples
+------------------------
+This example compares two strings, considering blanks to be "junk:"
+>>> s = SequenceMatcher(lambda x: x == " ",
+...                     "private Thread currentThread;",
+...                     "private volatile Thread currentThread;")
+:meth:`ratio` returns a float in [0, 1], measuring the similarity of the
+sequences.  As a rule of thumb, a :meth:`ratio` value over 0.6 means the
+sequences are close matches:
+>>> print round(s.ratio(), 3)
+0.866
+If you're only interested in where the sequences match,
+:meth:`get_matching_blocks` is handy:
+>>> for block in s.get_matching_blocks():
+...     print "a[%d] and b[%d] match for %d elements" % block
+a[0] and b[0] match for 8 elements
+a[8] and b[17] match for 21 elements
+a[29] and b[38] match for 0 elements
+Note that the last tuple returned by :meth:`get_matching_blocks` is always a
+dummy, ``(len(a), len(b), 0)``, and this is the only case in which the last
+tuple element (number of elements matched) is ``0``.
+If you want to know how to change the first sequence into the second, use
+:meth:`get_opcodes`:
+>>> for opcode in s.get_opcodes():
+...     print "%6s a[%d:%d] b[%d:%d]" % opcode
+equal a[0:8] b[0:8]
+insert a[8:8] b[8:17]
+equal a[8:29] b[17:38]
+See also the function :func:`get_close_matches` in this module, which shows how
+simple code building on :class:`SequenceMatcher` can be used to do useful work.
+.. _differ-objects:
+Differ Objects
+--------------
+Note that :class:`Differ`\ -generated deltas make no claim to be **minimal**
+diffs. To the contrary, minimal diffs are often counter-intuitive, because they
+synch up anywhere possible, sometimes accidental matches 100 pages apart.
+Restricting synch points to contiguous matches preserves some notion of
+locality, at the occasional cost of producing a longer diff.
+The :class:`Differ` class has this constructor:
+.. class:: Differ([linejunk[, charjunk]])
+Optional keyword parameters *linejunk* and *charjunk* are for filter functions
+(or ``None``):
+*linejunk*: A function that accepts a single string argument, and returns true
+if the string is junk.  The default is ``None``, meaning that no line is
+considered junk.
+*charjunk*: A function that accepts a single character argument (a string of
+length 1), and returns true if the character is junk. The default is ``None``,
+meaning that no character is considered junk.
+:class:`Differ` objects are used (deltas generated) via a single method:
+.. method:: Differ.compare(a, b)
+Compare two sequences of lines, and generate the delta (a sequence of lines).
+Each sequence must contain individual single-line strings ending with newlines.
+Such sequences can be obtained from the :meth:`readlines` method of file-like
+objects.  The delta generated also consists of newline-terminated strings, ready
+to be printed as-is via the :meth:`writelines` method of a file-like object.
+.. _differ-examples:
+Differ Example
+--------------
+This example compares two texts. First we set up the texts, sequences of
+individual single-line strings ending with newlines (such sequences can also be
+obtained from the :meth:`readlines` method of file-like objects):
+>>> text1 = '''  1. Beautiful is better than ugly.
+...   2. Explicit is better than implicit.
+...   3. Simple is better than complex.
+...   4. Complex is better than complicated.
+... '''.splitlines(1)
+>>> len(text1)
+4
+>>> text1[0][-1]
+'\n'
+>>> text2 = '''  1. Beautiful is better than ugly.
+...   3.   Simple is better than complex.
+...   4. Complicated is better than complex.
+...   5. Flat is better than nested.
+... '''.splitlines(1)
+Next we instantiate a Differ object:
+>>> d = Differ()
+Note that when instantiating a :class:`Differ` object we may pass functions to
+filter out line and character "junk."  See the :meth:`Differ` constructor for
+details.
+Finally, we compare the two:
+>>> result = list(d.compare(text1, text2))
+``result`` is a list of strings, so let's pretty-print it:
+>>> from pprint import pprint
+>>> pprint(result)
+['    1. Beautiful is better than ugly.\n',
+'-   2. Explicit is better than implicit.\n',
+'-   3. Simple is better than complex.\n',
+'+   3.   Simple is better than complex.\n',
+'?     ++\n',
+'-   4. Complex is better than complicated.\n',
+'?            ^                     ---- ^\n',
+'+   4. Complicated is better than complex.\n',
+'?           ++++ ^                      ^\n',
+'+   5. Flat is better than nested.\n']
+As a single multi-line string it looks like this:
+>>> import sys
+>>> sys.stdout.writelines(result)
+1. Beautiful is better than ugly.
+-   2. Explicit is better than implicit.
+-   3. Simple is better than complex.
++   3.   Simple is better than complex.
+?     ++
+-   4. Complex is better than complicated.
+?            ^                     ---- ^
++   4. Complicated is better than complex.
+?           ++++ ^                      ^
++   5. Flat is better than nested.
+.. _difflib-interface:
+A command-line interface to difflib
+-----------------------------------
+This example shows how to use difflib to create a ``diff``-like utility.
+It is also contained in the Python source distribution, as
+:file:`Tools/scripts/diff.py`.
+.. testcode::
+""" Command line interface to difflib.py providing diffs in four formats:
+* ndiff:    lists every line and highlights interline changes.
+* context:  highlights clusters of changes in a before/after format.
+* unified:  highlights clusters of changes in an inline format.
+* html:     generates side by side comparison with change highlights.
+"""
+import sys, os, time, difflib, optparse
+def main():
+# Configure the option parser
+usage = "usage: %prog [options] fromfile tofile"
+parser = optparse.OptionParser(usage)
+parser.add_option("-c", action="store_true", default=False,
+help='Produce a context format diff (default)')
+parser.add_option("-u", action="store_true", default=False,
+help='Produce a unified format diff')
+hlp = 'Produce HTML side by side diff (can use -c and -l in conjunction)'
+parser.add_option("-m", action="store_true", default=False, help=hlp)
+parser.add_option("-n", action="store_true", default=False,
+help='Produce a ndiff format diff')
+parser.add_option("-l", "--lines", type="int", default=3,
+help='Set number of context lines (default 3)')
+(options, args) = parser.parse_args()
+if len(args) == 0:
+parser.print_help()
+sys.exit(1)
+if len(args) != 2:
+parser.error("need to specify both a fromfile and tofile")
+n = options.lines
+fromfile, tofile = args # as specified in the usage string
+# we're passing these as arguments to the diff function
+fromdate = time.ctime(os.stat(fromfile).st_mtime)
+todate = time.ctime(os.stat(tofile).st_mtime)
+fromlines = open(fromfile, 'U').readlines()
+tolines = open(tofile, 'U').readlines()
+if options.u:
+diff = difflib.unified_diff(fromlines, tolines, fromfile, tofile,
+fromdate, todate, n=n)
+elif options.n:
+diff = difflib.ndiff(fromlines, tolines)
+elif options.m:
+diff = difflib.HtmlDiff().make_file(fromlines, tolines, fromfile,
+tofile, context=options.c,
+numlines=n)
+else:
+diff = difflib.context_diff(fromlines, tolines, fromfile, tofile,
+fromdate, todate, n=n)
+# we're using writelines because diff is a generator
+sys.stdout.writelines(diff)
+if __name__ == '__main__':
+main()