652 lines
25 KiB
ReStructuredText
652 lines
25 KiB
ReStructuredText
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:mod:`difflib` --- Helpers for computing deltas
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===============================================
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.. module:: difflib
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:synopsis: Helpers for computing differences between objects.
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.. moduleauthor:: Tim Peters <tim_one@users.sourceforge.net>
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.. sectionauthor:: Tim Peters <tim_one@users.sourceforge.net>
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.. Markup by Fred L. Drake, Jr. <fdrake@acm.org>
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.. versionadded:: 2.1
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This module provides classes and functions for comparing sequences. It
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can be used for example, for comparing files, and can produce difference
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information in various formats, including HTML and context and unified
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diffs. For comparing directories and files, see also, the :mod:`filecmp` module.
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.. class:: SequenceMatcher
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This is a flexible class for comparing pairs of sequences of any type, so long
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as the sequence elements are :term:`hashable`. The basic algorithm predates, and is a
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little fancier than, an algorithm published in the late 1980's by Ratcliff and
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Obershelp under the hyperbolic name "gestalt pattern matching." The idea is to
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find the longest contiguous matching subsequence that contains no "junk"
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elements (the Ratcliff and Obershelp algorithm doesn't address junk). The same
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idea is then applied recursively to the pieces of the sequences to the left and
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to the right of the matching subsequence. This does not yield minimal edit
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sequences, but does tend to yield matches that "look right" to people.
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**Timing:** The basic Ratcliff-Obershelp algorithm is cubic time in the worst
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case and quadratic time in the expected case. :class:`SequenceMatcher` is
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quadratic time for the worst case and has expected-case behavior dependent in a
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complicated way on how many elements the sequences have in common; best case
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time is linear.
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.. class:: Differ
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This is a class for comparing sequences of lines of text, and producing
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human-readable differences or deltas. Differ uses :class:`SequenceMatcher`
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both to compare sequences of lines, and to compare sequences of characters
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within similar (near-matching) lines.
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Each line of a :class:`Differ` delta begins with a two-letter code:
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+----------+-------------------------------------------+
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| Code | Meaning |
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+==========+===========================================+
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| ``'- '`` | line unique to sequence 1 |
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+----------+-------------------------------------------+
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| ``'+ '`` | line unique to sequence 2 |
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+----------+-------------------------------------------+
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| ``' '`` | line common to both sequences |
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+----------+-------------------------------------------+
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| ``'? '`` | line not present in either input sequence |
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+----------+-------------------------------------------+
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Lines beginning with '``?``' attempt to guide the eye to intraline differences,
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and were not present in either input sequence. These lines can be confusing if
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the sequences contain tab characters.
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.. class:: HtmlDiff
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This class can be used to create an HTML table (or a complete HTML file
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containing the table) showing a side by side, line by line comparison of text
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with inter-line and intra-line change highlights. The table can be generated in
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either full or contextual difference mode.
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The constructor for this class is:
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.. function:: __init__([tabsize][, wrapcolumn][, linejunk][, charjunk])
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Initializes instance of :class:`HtmlDiff`.
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*tabsize* is an optional keyword argument to specify tab stop spacing and
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defaults to ``8``.
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*wrapcolumn* is an optional keyword to specify column number where lines are
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broken and wrapped, defaults to ``None`` where lines are not wrapped.
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*linejunk* and *charjunk* are optional keyword arguments passed into ``ndiff()``
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(used by :class:`HtmlDiff` to generate the side by side HTML differences). See
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``ndiff()`` documentation for argument default values and descriptions.
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The following methods are public:
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.. function:: make_file(fromlines, tolines [, fromdesc][, todesc][, context][, numlines])
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Compares *fromlines* and *tolines* (lists of strings) and returns a string which
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is a complete HTML file containing a table showing line by line differences with
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inter-line and intra-line changes highlighted.
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*fromdesc* and *todesc* are optional keyword arguments to specify from/to file
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column header strings (both default to an empty string).
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*context* and *numlines* are both optional keyword arguments. Set *context* to
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``True`` when contextual differences are to be shown, else the default is
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``False`` to show the full files. *numlines* defaults to ``5``. When *context*
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is ``True`` *numlines* controls the number of context lines which surround the
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difference highlights. When *context* is ``False`` *numlines* controls the
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number of lines which are shown before a difference highlight when using the
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"next" hyperlinks (setting to zero would cause the "next" hyperlinks to place
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the next difference highlight at the top of the browser without any leading
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context).
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.. function:: make_table(fromlines, tolines [, fromdesc][, todesc][, context][, numlines])
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Compares *fromlines* and *tolines* (lists of strings) and returns a string which
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is a complete HTML table showing line by line differences with inter-line and
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intra-line changes highlighted.
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The arguments for this method are the same as those for the :meth:`make_file`
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method.
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:file:`Tools/scripts/diff.py` is a command-line front-end to this class and
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contains a good example of its use.
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.. versionadded:: 2.4
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.. function:: context_diff(a, b[, fromfile][, tofile][, fromfiledate][, tofiledate][, n][, lineterm])
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Compare *a* and *b* (lists of strings); return a delta (a :term:`generator`
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generating the delta lines) in context diff format.
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Context diffs are a compact way of showing just the lines that have changed plus
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a few lines of context. The changes are shown in a before/after style. The
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number of context lines is set by *n* which defaults to three.
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By default, the diff control lines (those with ``***`` or ``---``) are created
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with a trailing newline. This is helpful so that inputs created from
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:func:`file.readlines` result in diffs that are suitable for use with
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:func:`file.writelines` since both the inputs and outputs have trailing
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newlines.
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For inputs that do not have trailing newlines, set the *lineterm* argument to
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``""`` so that the output will be uniformly newline free.
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The context diff format normally has a header for filenames and modification
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times. Any or all of these may be specified using strings for *fromfile*,
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*tofile*, *fromfiledate*, and *tofiledate*. The modification times are normally
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expressed in the format returned by :func:`time.ctime`. If not specified, the
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strings default to blanks.
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:file:`Tools/scripts/diff.py` is a command-line front-end for this function.
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.. versionadded:: 2.3
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.. function:: get_close_matches(word, possibilities[, n][, cutoff])
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Return a list of the best "good enough" matches. *word* is a sequence for which
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close matches are desired (typically a string), and *possibilities* is a list of
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sequences against which to match *word* (typically a list of strings).
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Optional argument *n* (default ``3``) is the maximum number of close matches to
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return; *n* must be greater than ``0``.
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Optional argument *cutoff* (default ``0.6``) is a float in the range [0, 1].
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Possibilities that don't score at least that similar to *word* are ignored.
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The best (no more than *n*) matches among the possibilities are returned in a
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list, sorted by similarity score, most similar first. ::
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>>> get_close_matches('appel', ['ape', 'apple', 'peach', 'puppy'])
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['apple', 'ape']
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>>> import keyword
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>>> get_close_matches('wheel', keyword.kwlist)
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['while']
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>>> get_close_matches('apple', keyword.kwlist)
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[]
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>>> get_close_matches('accept', keyword.kwlist)
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['except']
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.. function:: ndiff(a, b[, linejunk][, charjunk])
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Compare *a* and *b* (lists of strings); return a :class:`Differ`\ -style
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delta (a :term:`generator` generating the delta lines).
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Optional keyword parameters *linejunk* and *charjunk* are for filter functions
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(or ``None``):
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*linejunk*: A function that accepts a single string argument, and returns true
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if the string is junk, or false if not. The default is (``None``), starting with
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Python 2.3. Before then, the default was the module-level function
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:func:`IS_LINE_JUNK`, which filters out lines without visible characters, except
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for at most one pound character (``'#'``). As of Python 2.3, the underlying
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:class:`SequenceMatcher` class does a dynamic analysis of which lines are so
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frequent as to constitute noise, and this usually works better than the pre-2.3
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default.
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*charjunk*: A function that accepts a character (a string of length 1), and
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returns if the character is junk, or false if not. The default is module-level
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function :func:`IS_CHARACTER_JUNK`, which filters out whitespace characters (a
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blank or tab; note: bad idea to include newline in this!).
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:file:`Tools/scripts/ndiff.py` is a command-line front-end to this function. ::
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>>> diff = ndiff('one\ntwo\nthree\n'.splitlines(1),
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... 'ore\ntree\nemu\n'.splitlines(1))
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>>> print ''.join(diff),
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- one
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? ^
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+ ore
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? ^
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- two
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- three
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? -
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+ tree
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+ emu
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.. function:: restore(sequence, which)
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Return one of the two sequences that generated a delta.
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Given a *sequence* produced by :meth:`Differ.compare` or :func:`ndiff`, extract
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lines originating from file 1 or 2 (parameter *which*), stripping off line
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prefixes.
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Example::
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>>> diff = ndiff('one\ntwo\nthree\n'.splitlines(1),
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... 'ore\ntree\nemu\n'.splitlines(1))
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>>> diff = list(diff) # materialize the generated delta into a list
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>>> print ''.join(restore(diff, 1)),
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one
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two
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three
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>>> print ''.join(restore(diff, 2)),
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ore
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tree
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emu
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.. function:: unified_diff(a, b[, fromfile][, tofile][, fromfiledate][, tofiledate][, n][, lineterm])
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Compare *a* and *b* (lists of strings); return a delta (a :term:`generator`
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generating the delta lines) in unified diff format.
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Unified diffs are a compact way of showing just the lines that have changed plus
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a few lines of context. The changes are shown in a inline style (instead of
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separate before/after blocks). The number of context lines is set by *n* which
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defaults to three.
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By default, the diff control lines (those with ``---``, ``+++``, or ``@@``) are
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created with a trailing newline. This is helpful so that inputs created from
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:func:`file.readlines` result in diffs that are suitable for use with
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:func:`file.writelines` since both the inputs and outputs have trailing
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newlines.
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For inputs that do not have trailing newlines, set the *lineterm* argument to
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``""`` so that the output will be uniformly newline free.
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The context diff format normally has a header for filenames and modification
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times. Any or all of these may be specified using strings for *fromfile*,
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*tofile*, *fromfiledate*, and *tofiledate*. The modification times are normally
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expressed in the format returned by :func:`time.ctime`. If not specified, the
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strings default to blanks.
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:file:`Tools/scripts/diff.py` is a command-line front-end for this function.
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.. versionadded:: 2.3
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.. function:: IS_LINE_JUNK(line)
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Return true for ignorable lines. The line *line* is ignorable if *line* is
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blank or contains a single ``'#'``, otherwise it is not ignorable. Used as a
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default for parameter *linejunk* in :func:`ndiff` before Python 2.3.
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.. function:: IS_CHARACTER_JUNK(ch)
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Return true for ignorable characters. The character *ch* is ignorable if *ch*
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is a space or tab, otherwise it is not ignorable. Used as a default for
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parameter *charjunk* in :func:`ndiff`.
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.. seealso::
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`Pattern Matching: The Gestalt Approach <http://www.ddj.com/184407970?pgno=5>`_
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Discussion of a similar algorithm by John W. Ratcliff and D. E. Metzener. This
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was published in `Dr. Dobb's Journal <http://www.ddj.com/>`_ in July, 1988.
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.. _sequence-matcher:
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SequenceMatcher Objects
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-----------------------
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The :class:`SequenceMatcher` class has this constructor:
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.. class:: SequenceMatcher([isjunk[, a[, b]]])
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Optional argument *isjunk* must be ``None`` (the default) or a one-argument
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function that takes a sequence element and returns true if and only if the
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element is "junk" and should be ignored. Passing ``None`` for *isjunk* is
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equivalent to passing ``lambda x: 0``; in other words, no elements are ignored.
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For example, pass::
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lambda x: x in " \t"
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if you're comparing lines as sequences of characters, and don't want to synch up
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on blanks or hard tabs.
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The optional arguments *a* and *b* are sequences to be compared; both default to
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empty strings. The elements of both sequences must be :term:`hashable`.
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:class:`SequenceMatcher` objects have the following methods:
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.. method:: SequenceMatcher.set_seqs(a, b)
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Set the two sequences to be compared.
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:class:`SequenceMatcher` computes and caches detailed information about the
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second sequence, so if you want to compare one sequence against many sequences,
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use :meth:`set_seq2` to set the commonly used sequence once and call
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:meth:`set_seq1` repeatedly, once for each of the other sequences.
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.. method:: SequenceMatcher.set_seq1(a)
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Set the first sequence to be compared. The second sequence to be compared is
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not changed.
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.. method:: SequenceMatcher.set_seq2(b)
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Set the second sequence to be compared. The first sequence to be compared is
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not changed.
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.. method:: SequenceMatcher.find_longest_match(alo, ahi, blo, bhi)
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Find longest matching block in ``a[alo:ahi]`` and ``b[blo:bhi]``.
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If *isjunk* was omitted or ``None``, :meth:`find_longest_match` returns ``(i, j,
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k)`` such that ``a[i:i+k]`` is equal to ``b[j:j+k]``, where ``alo <= i <= i+k <=
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ahi`` and ``blo <= j <= j+k <= bhi``. For all ``(i', j', k')`` meeting those
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conditions, the additional conditions ``k >= k'``, ``i <= i'``, and if ``i ==
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i'``, ``j <= j'`` are also met. In other words, of all maximal matching blocks,
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return one that starts earliest in *a*, and of all those maximal matching blocks
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that start earliest in *a*, return the one that starts earliest in *b*. ::
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>>> s = SequenceMatcher(None, " abcd", "abcd abcd")
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>>> s.find_longest_match(0, 5, 0, 9)
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(0, 4, 5)
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If *isjunk* was provided, first the longest matching block is determined as
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above, but with the additional restriction that no junk element appears in the
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block. Then that block is extended as far as possible by matching (only) junk
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elements on both sides. So the resulting block never matches on junk except as
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identical junk happens to be adjacent to an interesting match.
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Here's the same example as before, but considering blanks to be junk. That
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prevents ``' abcd'`` from matching the ``' abcd'`` at the tail end of the second
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sequence directly. Instead only the ``'abcd'`` can match, and matches the
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leftmost ``'abcd'`` in the second sequence::
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>>> s = SequenceMatcher(lambda x: x==" ", " abcd", "abcd abcd")
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>>> s.find_longest_match(0, 5, 0, 9)
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(1, 0, 4)
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If no blocks match, this returns ``(alo, blo, 0)``.
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.. versionchanged:: 2.6
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This method returns a :term:`named tuple` ``Match(a, b, size)``.
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.. method:: SequenceMatcher.get_matching_blocks()
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Return list of triples describing matching subsequences. Each triple is of the
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form ``(i, j, n)``, and means that ``a[i:i+n] == b[j:j+n]``. The triples are
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monotonically increasing in *i* and *j*.
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The last triple is a dummy, and has the value ``(len(a), len(b), 0)``. It is
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the only triple with ``n == 0``. If ``(i, j, n)`` and ``(i', j', n')`` are
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adjacent triples in the list, and the second is not the last triple in the list,
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then ``i+n != i'`` or ``j+n != j'``; in other words, adjacent triples always
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describe non-adjacent equal blocks.
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.. XXX Explain why a dummy is used!
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.. versionchanged:: 2.5
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The guarantee that adjacent triples always describe non-adjacent blocks was
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implemented.
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::
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>>> s = SequenceMatcher(None, "abxcd", "abcd")
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>>> s.get_matching_blocks()
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[(0, 0, 2), (3, 2, 2), (5, 4, 0)]
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.. method:: SequenceMatcher.get_opcodes()
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Return list of 5-tuples describing how to turn *a* into *b*. Each tuple is of
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the form ``(tag, i1, i2, j1, j2)``. The first tuple has ``i1 == j1 == 0``, and
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remaining tuples have *i1* equal to the *i2* from the preceding tuple, and,
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likewise, *j1* equal to the previous *j2*.
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The *tag* values are strings, with these meanings:
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+---------------+---------------------------------------------+
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| Value | Meaning |
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+===============+=============================================+
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| ``'replace'`` | ``a[i1:i2]`` should be replaced by |
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| | ``b[j1:j2]``. |
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+---------------+---------------------------------------------+
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| ``'delete'`` | ``a[i1:i2]`` should be deleted. Note that |
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| | ``j1 == j2`` in this case. |
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+---------------+---------------------------------------------+
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| ``'insert'`` | ``b[j1:j2]`` should be inserted at |
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| | ``a[i1:i1]``. Note that ``i1 == i2`` in |
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| | this case. |
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+---------------+---------------------------------------------+
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| ``'equal'`` | ``a[i1:i2] == b[j1:j2]`` (the sub-sequences |
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| | are equal). |
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+---------------+---------------------------------------------+
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For example::
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>>> a = "qabxcd"
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>>> b = "abycdf"
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>>> s = SequenceMatcher(None, a, b)
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>>> for tag, i1, i2, j1, j2 in s.get_opcodes():
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... print ("%7s a[%d:%d] (%s) b[%d:%d] (%s)" %
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... (tag, i1, i2, a[i1:i2], j1, j2, b[j1:j2]))
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delete a[0:1] (q) b[0:0] ()
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equal a[1:3] (ab) b[0:2] (ab)
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replace a[3:4] (x) b[2:3] (y)
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equal a[4:6] (cd) b[3:5] (cd)
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insert a[6:6] () b[5:6] (f)
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.. method:: SequenceMatcher.get_grouped_opcodes([n])
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Return a :term:`generator` of groups with up to *n* lines of context.
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Starting with the groups returned by :meth:`get_opcodes`, this method splits out
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smaller change clusters and eliminates intervening ranges which have no changes.
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The groups are returned in the same format as :meth:`get_opcodes`.
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.. versionadded:: 2.3
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.. method:: SequenceMatcher.ratio()
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Return a measure of the sequences' similarity as a float in the range [0, 1].
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Where T is the total number of elements in both sequences, and M is the number
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of matches, this is 2.0\*M / T. Note that this is ``1.0`` if the sequences are
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identical, and ``0.0`` if they have nothing in common.
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This is expensive to compute if :meth:`get_matching_blocks` or
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: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:: SequenceMatcher.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:: SequenceMatcher.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 6 elements
|
|
a[14] and b[23] match for 15 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[8:14] b[17:23]
|
|
equal a[14:29] b[23:38]
|
|
|
|
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.
|
|
|