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 <div class="section" id="module-difflib">
<h1>7.4. <a class="reference internal" href="#module-difflib" title="difflib: Helpers for computing differences between objects."><tt class="xref py py-mod docutils literal">difflib</tt></a> &#8212; Helpers for computing deltas<a class="headerlink" href="#module-difflib" title="Permalink to this headline">¶</a></h1>

New in version 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 <a class="reference internal" href="filecmp.html#module-filecmp" title="filecmp: Compare files efficiently."><tt class="xref py py-mod docutils literal">filecmp</tt></a> module.
<dl class="class">
<dt id="difflib.SequenceMatcher">
class <tt class="descclassname">difflib.</tt><tt class="descname">SequenceMatcher</tt><a class="headerlink" href="#difflib.SequenceMatcher" title="Permalink to this definition">¶</a></dt>
<dd>This is a flexible class for comparing pairs of sequences of any type, so long
as the sequence elements are <a class="reference internal" href="../glossary.html#term-hashable">hashable</a>. The basic algorithm predates, and is a
little fancier than, an algorithm published in the late 1980&#8217;s by Ratcliff and
Obershelp under the hyperbolic name &#8220;gestalt pattern matching.&#8221; The idea is to
find the longest contiguous matching subsequence that contains no &#8220;junk&#8221;
elements (the Ratcliff and Obershelp algorithm doesn&#8217;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 &#8220;look right&#8221; to people.
Timing: The basic Ratcliff-Obershelp algorithm is cubic time in the worst
case and quadratic time in the expected case. <a class="reference internal" href="#difflib.SequenceMatcher" title="difflib.SequenceMatcher"><tt class="xref py py-class docutils literal">SequenceMatcher</tt></a> 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.
Automatic junk heuristic: <a class="reference internal" href="#difflib.SequenceMatcher" title="difflib.SequenceMatcher"><tt class="xref py py-class docutils literal">SequenceMatcher</tt></a> supports a heuristic that
automatically treats certain sequence items as junk. The heuristic counts how many
times each individual item appears in the sequence. If an item&#8217;s duplicates (after
the first one) account for more than 1% of the sequence and the sequence is at least
200 items long, this item is marked as &#8220;popular&#8221; and is treated as junk for
the purpose of sequence matching. This heuristic can be turned off by setting
the <tt class="docutils literal">autojunk</tt> argument to <tt class="docutils literal">False</tt> when creating the <a class="reference internal" href="#difflib.SequenceMatcher" title="difflib.SequenceMatcher"><tt class="xref py py-class docutils literal">SequenceMatcher</tt></a>.

New in version 2.7.1: The autojunk parameter.
</dd></dl>

<dl class="class">
<dt id="difflib.Differ">
class <tt class="descclassname">difflib.</tt><tt class="descname">Differ</tt><a class="headerlink" href="#difflib.Differ" title="Permalink to this definition">¶</a></dt>
<dd>This is a class for comparing sequences of lines of text, and producing
human-readable differences or deltas. Differ uses <a class="reference internal" href="#difflib.SequenceMatcher" title="difflib.SequenceMatcher"><tt class="xref py py-class docutils literal">SequenceMatcher</tt></a>
both to compare sequences of lines, and to compare sequences of characters
within similar (near-matching) lines.
Each line of a <a class="reference internal" href="#difflib.Differ" title="difflib.Differ"><tt class="xref py py-class docutils literal">Differ</tt></a> delta begins with a two-letter code:
<table border="1" class="docutils">
<colgroup>
<col width="19%" />
<col width="81%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Code</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="docutils literal">'- '</tt></td>
<td>line unique to sequence 1</td>
</tr>
<tr class="row-odd"><td><tt class="docutils literal">'+ '</tt></td>
<td>line unique to sequence 2</td>
</tr>
<tr class="row-even"><td><tt class="docutils literal">'&nbsp; '</tt></td>
<td>line common to both sequences</td>
</tr>
<tr class="row-odd"><td><tt class="docutils literal">'? '</tt></td>
<td>line not present in either input sequence</td>
</tr>
</tbody>
</table>
Lines beginning with &#8216;<tt class="docutils literal">?</tt>&#8216; 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.
</dd></dl>

<dl class="class">
<dt id="difflib.HtmlDiff">
class <tt class="descclassname">difflib.</tt><tt class="descname">HtmlDiff</tt><a class="headerlink" href="#difflib.HtmlDiff" title="Permalink to this definition">¶</a></dt>
<dd>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:
<dl class="function">
<dt id="difflib.HtmlDiff.__init__">
<tt class="descname">__init__</tt><big>(</big>tabsize=8, wrapcolumn=None, linejunk=None, charjunk=IS_CHARACTER_JUNK<big>)</big><a class="headerlink" href="#difflib.HtmlDiff.__init__" title="Permalink to this definition">¶</a></dt>
<dd>Initializes instance of <a class="reference internal" href="#difflib.HtmlDiff" title="difflib.HtmlDiff"><tt class="xref py py-class docutils literal">HtmlDiff</tt></a>.
tabsize is an optional keyword argument to specify tab stop spacing and
defaults to <tt class="docutils literal">8</tt>.
wrapcolumn is an optional keyword to specify column number where lines are
broken and wrapped, defaults to <tt class="docutils literal">None</tt> where lines are not wrapped.
linejunk and charjunk are optional keyword arguments passed into <tt class="docutils literal">ndiff()</tt>
(used by <a class="reference internal" href="#difflib.HtmlDiff" title="difflib.HtmlDiff"><tt class="xref py py-class docutils literal">HtmlDiff</tt></a> to generate the side by side HTML differences). See
<tt class="docutils literal">ndiff()</tt> documentation for argument default values and descriptions.
</dd></dl>

The following methods are public:
<dl class="function">
<dt id="difflib.HtmlDiff.make_file">
<tt class="descname">make_file</tt><big>(</big>fromlines, tolines [, fromdesc][, todesc][, context][, numlines]<big>)</big><a class="headerlink" href="#difflib.HtmlDiff.make_file" title="Permalink to this definition">¶</a></dt>
<dd>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
<tt class="docutils literal">True</tt> when contextual differences are to be shown, else the default is
<tt class="docutils literal">False</tt> to show the full files. numlines defaults to <tt class="docutils literal">5</tt>. When context
is <tt class="docutils literal">True</tt> numlines controls the number of context lines which surround the
difference highlights. When context is <tt class="docutils literal">False</tt> numlines controls the
number of lines which are shown before a difference highlight when using the
&#8220;next&#8221; hyperlinks (setting to zero would cause the &#8220;next&#8221; hyperlinks to place
the next difference highlight at the top of the browser without any leading
context).
</dd></dl>

<dl class="function">
<dt id="difflib.HtmlDiff.make_table">
<tt class="descname">make_table</tt><big>(</big>fromlines, tolines [, fromdesc][, todesc][, context][, numlines]<big>)</big><a class="headerlink" href="#difflib.HtmlDiff.make_table" title="Permalink to this definition">¶</a></dt>
<dd>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 <a class="reference internal" href="#difflib.HtmlDiff.make_file" title="difflib.HtmlDiff.make_file"><tt class="xref py py-meth docutils literal">make_file()</tt></a>
method.
</dd></dl>

<tt class="file docutils literal">Tools/scripts/diff.py</tt> is a command-line front-end to this class and
contains a good example of its use.

New in version 2.4.
</dd></dl>

<dl class="function">
<dt id="difflib.context_diff">
<tt class="descclassname">difflib.</tt><tt class="descname">context_diff</tt><big>(</big>a, b[, fromfile][, tofile][, fromfiledate][, tofiledate][, n][, lineterm]<big>)</big><a class="headerlink" href="#difflib.context_diff" title="Permalink to this definition">¶</a></dt>
<dd>Compare a and b (lists of strings); return a delta (a <a class="reference internal" href="../glossary.html#term-generator">generator</a>
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 <tt class="docutils literal">***</tt> or <tt class="docutils literal">---</tt>) are created
with a trailing newline. This is helpful so that inputs created from
<a class="reference internal" href="stdtypes.html#file.readlines" title="file.readlines"><tt class="xref py py-func docutils literal">file.readlines()</tt></a> result in diffs that are suitable for use with
<a class="reference internal" href="stdtypes.html#file.writelines" title="file.writelines"><tt class="xref py py-func docutils literal">file.writelines()</tt></a> since both the inputs and outputs have trailing
newlines.
For inputs that do not have trailing newlines, set the lineterm argument to
<tt class="docutils literal">&quot;&quot;</tt> 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 ISO 8601 format. If not specified, the
strings default to blanks.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; s1 = [&#39;bacon\n&#39;, &#39;eggs\n&#39;, &#39;ham\n&#39;, &#39;guido\n&#39;]
&gt;&gt;&gt; s2 = [&#39;python\n&#39;, &#39;eggy\n&#39;, &#39;hamster\n&#39;, &#39;guido\n&#39;]
&gt;&gt;&gt; for line in context_diff(s1, s2, fromfile=&#39;before.py&#39;, tofile=&#39;after.py&#39;):
... sys.stdout.write(line) 
*** before.py
--- after.py
***************
*** 1,4 ****
! bacon
! eggs
! ham
 guido
--- 1,4 ----
! python
! eggy
! hamster
 guido
</pre></div>
</div>
See <a class="reference internal" href="#difflib-interface">A command-line interface to difflib</a> for a more detailed example.

New in version 2.3.
</dd></dl>

<dl class="function">
<dt id="difflib.get_close_matches">
<tt class="descclassname">difflib.</tt><tt class="descname">get_close_matches</tt><big>(</big>word, possibilities[, n][, cutoff]<big>)</big><a class="headerlink" href="#difflib.get_close_matches" title="Permalink to this definition">¶</a></dt>
<dd>Return a list of the best &#8220;good enough&#8221; 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 <tt class="docutils literal">3</tt>) is the maximum number of close matches to
return; n must be greater than <tt class="docutils literal">0</tt>.
Optional argument cutoff (default <tt class="docutils literal">0.6</tt>) is a float in the range [0, 1].
Possibilities that don&#8217;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.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; get_close_matches(&#39;appel&#39;, [&#39;ape&#39;, &#39;apple&#39;, &#39;peach&#39;, &#39;puppy&#39;])
[&#39;apple&#39;, &#39;ape&#39;]
&gt;&gt;&gt; import keyword
&gt;&gt;&gt; get_close_matches(&#39;wheel&#39;, keyword.kwlist)
[&#39;while&#39;]
&gt;&gt;&gt; get_close_matches(&#39;apple&#39;, keyword.kwlist)
[]
&gt;&gt;&gt; get_close_matches(&#39;accept&#39;, keyword.kwlist)
[&#39;except&#39;]
</pre></div>
</div>
</dd></dl>

<dl class="function">
<dt id="difflib.ndiff">
<tt class="descclassname">difflib.</tt><tt class="descname">ndiff</tt><big>(</big>a, b[, linejunk][, charjunk]<big>)</big><a class="headerlink" href="#difflib.ndiff" title="Permalink to this definition">¶</a></dt>
<dd>Compare a and b (lists of strings); return a <a class="reference internal" href="#difflib.Differ" title="difflib.Differ"><tt class="xref py py-class docutils literal">Differ</tt></a>-style
delta (a <a class="reference internal" href="../glossary.html#term-generator">generator</a> generating the delta lines).
Optional keyword parameters linejunk and charjunk are for filter functions
(or <tt class="docutils literal">None</tt>):
linejunk: A function that accepts a single string argument, and returns true
if the string is junk, or false if not. The default is (<tt class="docutils literal">None</tt>), starting with
Python 2.3. Before then, the default was the module-level function
<a class="reference internal" href="#difflib.IS_LINE_JUNK" title="difflib.IS_LINE_JUNK"><tt class="xref py py-func docutils literal">IS_LINE_JUNK()</tt></a>, which filters out lines without visible characters, except
for at most one pound character (<tt class="docutils literal">'#'</tt>). As of Python 2.3, the underlying
<a class="reference internal" href="#difflib.SequenceMatcher" title="difflib.SequenceMatcher"><tt class="xref py py-class docutils literal">SequenceMatcher</tt></a> 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 <a class="reference internal" href="#difflib.IS_CHARACTER_JUNK" title="difflib.IS_CHARACTER_JUNK"><tt class="xref py py-func docutils literal">IS_CHARACTER_JUNK()</tt></a>, which filters out whitespace characters (a
blank or tab; note: bad idea to include newline in this!).
<tt class="file docutils literal">Tools/scripts/ndiff.py</tt> is a command-line front-end to this function.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; diff = ndiff(&#39;one\ntwo\nthree\n&#39;.splitlines(1),
... &#39;ore\ntree\nemu\n&#39;.splitlines(1))
&gt;&gt;&gt; print &#39;&#39;.join(diff),
- one
? ^
+ ore
? ^
- two
- three
? -
+ tree
+ emu
</pre></div>
</div>
</dd></dl>

<dl class="function">
<dt id="difflib.restore">
<tt class="descclassname">difflib.</tt><tt class="descname">restore</tt><big>(</big>sequence, which<big>)</big><a class="headerlink" href="#difflib.restore" title="Permalink to this definition">¶</a></dt>
<dd>Return one of the two sequences that generated a delta.
Given a sequence produced by <a class="reference internal" href="#difflib.Differ.compare" title="difflib.Differ.compare"><tt class="xref py py-meth docutils literal">Differ.compare()</tt></a> or <a class="reference internal" href="#difflib.ndiff" title="difflib.ndiff"><tt class="xref py py-func docutils literal">ndiff()</tt></a>, extract
lines originating from file 1 or 2 (parameter which), stripping off line
prefixes.
Example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; diff = ndiff(&#39;one\ntwo\nthree\n&#39;.splitlines(1),
... &#39;ore\ntree\nemu\n&#39;.splitlines(1))
&gt;&gt;&gt; diff = list(diff) # materialize the generated delta into a list
&gt;&gt;&gt; print &#39;&#39;.join(restore(diff, 1)),
one
two
three
&gt;&gt;&gt; print &#39;&#39;.join(restore(diff, 2)),
ore
tree
emu
</pre></div>
</div>
</dd></dl>

<dl class="function">
<dt id="difflib.unified_diff">
<tt class="descclassname">difflib.</tt><tt class="descname">unified_diff</tt><big>(</big>a, b[, fromfile][, tofile][, fromfiledate][, tofiledate][, n][, lineterm]<big>)</big><a class="headerlink" href="#difflib.unified_diff" title="Permalink to this definition">¶</a></dt>
<dd>Compare a and b (lists of strings); return a delta (a <a class="reference internal" href="../glossary.html#term-generator">generator</a>
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 <tt class="docutils literal">---</tt>, <tt class="docutils literal">+++</tt>, or <tt class="docutils literal">&#64;&#64;</tt>) are
created with a trailing newline. This is helpful so that inputs created from
<a class="reference internal" href="stdtypes.html#file.readlines" title="file.readlines"><tt class="xref py py-func docutils literal">file.readlines()</tt></a> result in diffs that are suitable for use with
<a class="reference internal" href="stdtypes.html#file.writelines" title="file.writelines"><tt class="xref py py-func docutils literal">file.writelines()</tt></a> since both the inputs and outputs have trailing
newlines.
For inputs that do not have trailing newlines, set the lineterm argument to
<tt class="docutils literal">&quot;&quot;</tt> 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 ISO 8601 format. If not specified, the
strings default to blanks.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; s1 = [&#39;bacon\n&#39;, &#39;eggs\n&#39;, &#39;ham\n&#39;, &#39;guido\n&#39;]
&gt;&gt;&gt; s2 = [&#39;python\n&#39;, &#39;eggy\n&#39;, &#39;hamster\n&#39;, &#39;guido\n&#39;]
&gt;&gt;&gt; for line in unified_diff(s1, s2, fromfile=&#39;before.py&#39;, tofile=&#39;after.py&#39;):
... sys.stdout.write(line) 
--- before.py
+++ after.py
@@ -1,4 +1,4 @@
-bacon
-eggs
-ham
+python
+eggy
+hamster
 guido
</pre></div>
</div>
See <a class="reference internal" href="#difflib-interface">A command-line interface to difflib</a> for a more detailed example.

New in version 2.3.
</dd></dl>

<dl class="function">
<dt id="difflib.IS_LINE_JUNK">
<tt class="descclassname">difflib.</tt><tt class="descname">IS_LINE_JUNK</tt><big>(</big>line<big>)</big><a class="headerlink" href="#difflib.IS_LINE_JUNK" title="Permalink to this definition">¶</a></dt>
<dd>Return true for ignorable lines. The line line is ignorable if line is
blank or contains a single <tt class="docutils literal">'#'</tt>, otherwise it is not ignorable. Used as a
default for parameter linejunk in <a class="reference internal" href="#difflib.ndiff" title="difflib.ndiff"><tt class="xref py py-func docutils literal">ndiff()</tt></a> before Python 2.3.
</dd></dl>

<dl class="function">
<dt id="difflib.IS_CHARACTER_JUNK">
<tt class="descclassname">difflib.</tt><tt class="descname">IS_CHARACTER_JUNK</tt><big>(</big>ch<big>)</big><a class="headerlink" href="#difflib.IS_CHARACTER_JUNK" title="Permalink to this definition">¶</a></dt>
<dd>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 <a class="reference internal" href="#difflib.ndiff" title="difflib.ndiff"><tt class="xref py py-func docutils literal">ndiff()</tt></a>.
</dd></dl>

<div class="admonition-see-also admonition seealso">
See also
<dl class="last docutils">
<dt><a class="reference external" href="http://www.ddj.com/184407970?pgno=5">Pattern Matching: The Gestalt Approach</a></dt>
<dd>Discussion of a similar algorithm by John W. Ratcliff and D. E. Metzener. This
was published in <a class="reference external" href="http://www.ddj.com/">Dr. Dobb&#8217;s Journal</a> in July, 1988.</dd>
</dl>
</div>
<div class="section" id="sequencematcher-objects">
<h2>7.4.1. SequenceMatcher Objects<a class="headerlink" href="#sequencematcher-objects" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="#difflib.SequenceMatcher" title="difflib.SequenceMatcher"><tt class="xref py py-class docutils literal">SequenceMatcher</tt></a> class has this constructor:
<dl class="class">
<dt>
class <tt class="descclassname">difflib.</tt><tt class="descname">SequenceMatcher</tt><big>(</big>isjunk=None, a='', b='', autojunk=True<big>)</big></dt>
<dd>Optional argument isjunk must be <tt class="docutils literal">None</tt> (the default) or a one-argument
function that takes a sequence element and returns true if and only if the
element is &#8220;junk&#8221; and should be ignored. Passing <tt class="docutils literal">None</tt> for isjunk is
equivalent to passing <tt class="docutils literal">lambda x: 0</tt>; in other words, no elements are ignored.
For example, pass:
<div class="highlight-python"><div class="highlight"><pre>lambda x: x in &quot; \t&quot;
</pre></div>
</div>
if you&#8217;re comparing lines as sequences of characters, and don&#8217;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 <a class="reference internal" href="../glossary.html#term-hashable">hashable</a>.
The optional argument autojunk can be used to disable the automatic junk
heuristic.

New in version 2.7.1: The autojunk parameter.
<a class="reference internal" href="#difflib.SequenceMatcher" title="difflib.SequenceMatcher"><tt class="xref py py-class docutils literal">SequenceMatcher</tt></a> objects have the following methods:
<dl class="method">
<dt id="difflib.SequenceMatcher.set_seqs">
<tt class="descname">set_seqs</tt><big>(</big>a, b<big>)</big><a class="headerlink" href="#difflib.SequenceMatcher.set_seqs" title="Permalink to this definition">¶</a></dt>
<dd>Set the two sequences to be compared.
</dd></dl>

<a class="reference internal" href="#difflib.SequenceMatcher" title="difflib.SequenceMatcher"><tt class="xref py py-class docutils literal">SequenceMatcher</tt></a> computes and caches detailed information about the
second sequence, so if you want to compare one sequence against many
sequences, use <a class="reference internal" href="#difflib.SequenceMatcher.set_seq2" title="difflib.SequenceMatcher.set_seq2"><tt class="xref py py-meth docutils literal">set_seq2()</tt></a> to set the commonly used sequence once and
call <a class="reference internal" href="#difflib.SequenceMatcher.set_seq1" title="difflib.SequenceMatcher.set_seq1"><tt class="xref py py-meth docutils literal">set_seq1()</tt></a> repeatedly, once for each of the other sequences.
<dl class="method">
<dt id="difflib.SequenceMatcher.set_seq1">
<tt class="descname">set_seq1</tt><big>(</big>a<big>)</big><a class="headerlink" href="#difflib.SequenceMatcher.set_seq1" title="Permalink to this definition">¶</a></dt>
<dd>Set the first sequence to be compared. The second sequence to be compared
is not changed.
</dd></dl>

<dl class="method">
<dt id="difflib.SequenceMatcher.set_seq2">
<tt class="descname">set_seq2</tt><big>(</big>b<big>)</big><a class="headerlink" href="#difflib.SequenceMatcher.set_seq2" title="Permalink to this definition">¶</a></dt>
<dd>Set the second sequence to be compared. The first sequence to be compared
is not changed.
</dd></dl>

<dl class="method">
<dt id="difflib.SequenceMatcher.find_longest_match">
<tt class="descname">find_longest_match</tt><big>(</big>alo, ahi, blo, bhi<big>)</big><a class="headerlink" href="#difflib.SequenceMatcher.find_longest_match" title="Permalink to this definition">¶</a></dt>
<dd>Find longest matching block in <tt class="docutils literal">a[alo:ahi]</tt> and <tt class="docutils literal">b[blo:bhi]</tt>.
If isjunk was omitted or <tt class="docutils literal">None</tt>, <a class="reference internal" href="#difflib.SequenceMatcher.find_longest_match" title="difflib.SequenceMatcher.find_longest_match"><tt class="xref py py-meth docutils literal">find_longest_match()</tt></a> returns
<tt class="docutils literal">(i, j, k)</tt> such that <tt class="docutils literal">a[i:i+k]</tt> is equal to <tt class="docutils literal">b[j:j+k]</tt>, where <tt class="docutils literal">alo
&lt;= i &lt;= i+k &lt;= ahi</tt> and <tt class="docutils literal">blo &lt;= j &lt;= j+k &lt;= bhi</tt>. For all <tt class="docutils literal">(i', j',
k')</tt> meeting those conditions, the additional conditions <tt class="docutils literal">k &gt;= k'</tt>, <tt class="docutils literal">i
&lt;= i'</tt>, and if <tt class="docutils literal">i == i'</tt>, <tt class="docutils literal">j &lt;= j'</tt> 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.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; s = SequenceMatcher(None, &quot; abcd&quot;, &quot;abcd abcd&quot;)
&gt;&gt;&gt; s.find_longest_match(0, 5, 0, 9)
Match(a=0, b=4, size=5)
</pre></div>
</div>
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&#8217;s the same example as before, but considering blanks to be junk. That
prevents <tt class="docutils literal">' abcd'</tt> from matching the <tt class="docutils literal">' abcd'</tt> at the tail end of the
second sequence directly. Instead only the <tt class="docutils literal">'abcd'</tt> can match, and
matches the leftmost <tt class="docutils literal">'abcd'</tt> in the second sequence:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; s = SequenceMatcher(lambda x: x==&quot; &quot;, &quot; abcd&quot;, &quot;abcd abcd&quot;)
&gt;&gt;&gt; s.find_longest_match(0, 5, 0, 9)
Match(a=1, b=0, size=4)
</pre></div>
</div>
If no blocks match, this returns <tt class="docutils literal">(alo, blo, 0)</tt>.

Changed in version 2.6: This method returns a <a class="reference internal" href="../glossary.html#term-named-tuple">named tuple</a> <tt class="docutils literal">Match(a, b, size)</tt>.
</dd></dl>

<dl class="method">
<dt id="difflib.SequenceMatcher.get_matching_blocks">
<tt class="descname">get_matching_blocks</tt><big>(</big><big>)</big><a class="headerlink" href="#difflib.SequenceMatcher.get_matching_blocks" title="Permalink to this definition">¶</a></dt>
<dd>Return list of triples describing matching subsequences. Each triple is of
the form <tt class="docutils literal">(i, j, n)</tt>, and means that <tt class="docutils literal">a[i:i+n] == b[j:j+n]</tt>. The
triples are monotonically increasing in i and j.
The last triple is a dummy, and has the value <tt class="docutils literal">(len(a), len(b), 0)</tt>. It
is the only triple with <tt class="docutils literal">n == 0</tt>. If <tt class="docutils literal">(i, j, n)</tt> and <tt class="docutils literal">(i', j', n')</tt>
are adjacent triples in the list, and the second is not the last triple in
the list, then <tt class="docutils literal">i+n != i'</tt> or <tt class="docutils literal">j+n != j'</tt>; in other words, adjacent
triples always describe non-adjacent equal blocks.

Changed in version 2.5: The guarantee that adjacent triples always describe non-adjacent blocks
was implemented.
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; s = SequenceMatcher(None, &quot;abxcd&quot;, &quot;abcd&quot;)
&gt;&gt;&gt; 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)]
</pre></div>
</div>
</dd></dl>

<dl class="method">
<dt id="difflib.SequenceMatcher.get_opcodes">
<tt class="descname">get_opcodes</tt><big>(</big><big>)</big><a class="headerlink" href="#difflib.SequenceMatcher.get_opcodes" title="Permalink to this definition">¶</a></dt>
<dd>Return list of 5-tuples describing how to turn a into b. Each tuple is
of the form <tt class="docutils literal">(tag, i1, i2, j1, j2)</tt>. The first tuple has <tt class="docutils literal">i1 == j1 ==
0</tt>, 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:
<table border="1" class="docutils">
<colgroup>
<col width="25%" />
<col width="75%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Value</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="docutils literal">'replace'</tt></td>
<td><tt class="docutils literal">a[i1:i2]</tt> should be replaced by
<tt class="docutils literal">b[j1:j2]</tt>.</td>
</tr>
<tr class="row-odd"><td><tt class="docutils literal">'delete'</tt></td>
<td><tt class="docutils literal">a[i1:i2]</tt> should be deleted. Note that
<tt class="docutils literal">j1 == j2</tt> in this case.</td>
</tr>
<tr class="row-even"><td><tt class="docutils literal">'insert'</tt></td>
<td><tt class="docutils literal">b[j1:j2]</tt> should be inserted at
<tt class="docutils literal">a[i1:i1]</tt>. Note that <tt class="docutils literal">i1 == i2</tt> in
this case.</td>
</tr>
<tr class="row-odd"><td><tt class="docutils literal">'equal'</tt></td>
<td><tt class="docutils literal">a[i1:i2] == b[j1:j2]</tt> (the sub-sequences
are equal).</td>
</tr>
</tbody>
</table>
For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a = &quot;qabxcd&quot;
&gt;&gt;&gt; b = &quot;abycdf&quot;
&gt;&gt;&gt; s = SequenceMatcher(None, a, b)
&gt;&gt;&gt; for tag, i1, i2, j1, j2 in s.get_opcodes():
... print (&quot;%7s a[%d:%d] (%s) b[%d:%d] (%s)&quot; %
... (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)
</pre></div>
</div>
</dd></dl>

<dl class="method">
<dt id="difflib.SequenceMatcher.get_grouped_opcodes">
<tt class="descname">get_grouped_opcodes</tt><big>(</big>[n]<big>)</big><a class="headerlink" href="#difflib.SequenceMatcher.get_grouped_opcodes" title="Permalink to this definition">¶</a></dt>
<dd>Return a <a class="reference internal" href="../glossary.html#term-generator">generator</a> of groups with up to n lines of context.
Starting with the groups returned by <a class="reference internal" href="#difflib.SequenceMatcher.get_opcodes" title="difflib.SequenceMatcher.get_opcodes"><tt class="xref py py-meth docutils literal">get_opcodes()</tt></a>, this method
splits out smaller change clusters and eliminates intervening ranges which
have no changes.
The groups are returned in the same format as <a class="reference internal" href="#difflib.SequenceMatcher.get_opcodes" title="difflib.SequenceMatcher.get_opcodes"><tt class="xref py py-meth docutils literal">get_opcodes()</tt></a>.

New in version 2.3.
</dd></dl>

<dl class="method">
<dt id="difflib.SequenceMatcher.ratio">
<tt class="descname">ratio</tt><big>(</big><big>)</big><a class="headerlink" href="#difflib.SequenceMatcher.ratio" title="Permalink to this definition">¶</a></dt>
<dd>Return a measure of the sequences&#8217; 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 <tt class="docutils literal">1.0</tt> if the
sequences are identical, and <tt class="docutils literal">0.0</tt> if they have nothing in common.
This is expensive to compute if <a class="reference internal" href="#difflib.SequenceMatcher.get_matching_blocks" title="difflib.SequenceMatcher.get_matching_blocks"><tt class="xref py py-meth docutils literal">get_matching_blocks()</tt></a> or
<a class="reference internal" href="#difflib.SequenceMatcher.get_opcodes" title="difflib.SequenceMatcher.get_opcodes"><tt class="xref py py-meth docutils literal">get_opcodes()</tt></a> hasn&#8217;t already been called, in which case you may want
to try <a class="reference internal" href="#difflib.SequenceMatcher.quick_ratio" title="difflib.SequenceMatcher.quick_ratio"><tt class="xref py py-meth docutils literal">quick_ratio()</tt></a> or <a class="reference internal" href="#difflib.SequenceMatcher.real_quick_ratio" title="difflib.SequenceMatcher.real_quick_ratio"><tt class="xref py py-meth docutils literal">real_quick_ratio()</tt></a> first to get an
upper bound.
</dd></dl>

<dl class="method">
<dt id="difflib.SequenceMatcher.quick_ratio">
<tt class="descname">quick_ratio</tt><big>(</big><big>)</big><a class="headerlink" href="#difflib.SequenceMatcher.quick_ratio" title="Permalink to this definition">¶</a></dt>
<dd>Return an upper bound on <a class="reference internal" href="#difflib.SequenceMatcher.ratio" title="difflib.SequenceMatcher.ratio"><tt class="xref py py-meth docutils literal">ratio()</tt></a> relatively quickly.
</dd></dl>

<dl class="method">
<dt id="difflib.SequenceMatcher.real_quick_ratio">
<tt class="descname">real_quick_ratio</tt><big>(</big><big>)</big><a class="headerlink" href="#difflib.SequenceMatcher.real_quick_ratio" title="Permalink to this definition">¶</a></dt>
<dd>Return an upper bound on <a class="reference internal" href="#difflib.SequenceMatcher.ratio" title="difflib.SequenceMatcher.ratio"><tt class="xref py py-meth docutils literal">ratio()</tt></a> very quickly.
</dd></dl>

</dd></dl>

The three methods that return the ratio of matching to total characters can give
different results due to differing levels of approximation, although
<tt class="xref py py-meth docutils literal">quick_ratio()</tt> and <tt class="xref py py-meth docutils literal">real_quick_ratio()</tt> are always at least as large as
<tt class="xref py py-meth docutils literal">ratio()</tt>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; s = SequenceMatcher(None, &quot;abcd&quot;, &quot;bcde&quot;)
&gt;&gt;&gt; s.ratio()
0.75
&gt;&gt;&gt; s.quick_ratio()
0.75
&gt;&gt;&gt; s.real_quick_ratio()
1.0
</pre></div>
</div>
</div>
<div class="section" id="sequencematcher-examples">
<h2>7.4.2. SequenceMatcher Examples<a class="headerlink" href="#sequencematcher-examples" title="Permalink to this headline">¶</a></h2>
This example compares two strings, considering blanks to be &#8220;junk:&#8221;
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; s = SequenceMatcher(lambda x: x == &quot; &quot;,
... &quot;private Thread currentThread;&quot;,
... &quot;private volatile Thread currentThread;&quot;)
</pre></div>
</div>
<tt class="xref py py-meth docutils literal">ratio()</tt> returns a float in [0, 1], measuring the similarity of the
sequences. As a rule of thumb, a <tt class="xref py py-meth docutils literal">ratio()</tt> value over 0.6 means the
sequences are close matches:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; print round(s.ratio(), 3)
0.866
</pre></div>
</div>
If you&#8217;re only interested in where the sequences match,
<tt class="xref py py-meth docutils literal">get_matching_blocks()</tt> is handy:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; for block in s.get_matching_blocks():
... print &quot;a[%d] and b[%d] match for %d elements&quot; % 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
</pre></div>
</div>
Note that the last tuple returned by <tt class="xref py py-meth docutils literal">get_matching_blocks()</tt> is always a
dummy, <tt class="docutils literal">(len(a), len(b), 0)</tt>, and this is the only case in which the last
tuple element (number of elements matched) is <tt class="docutils literal">0</tt>.
If you want to know how to change the first sequence into the second, use
<tt class="xref py py-meth docutils literal">get_opcodes()</tt>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; for opcode in s.get_opcodes():
... print &quot;%6s a[%d:%d] b[%d:%d]&quot; % opcode
 equal a[0:8] b[0:8]
insert a[8:8] b[8:17]
 equal a[8:29] b[17:38]
</pre></div>
</div>
<div class="admonition-see-also admonition seealso">
See also
<ul class="last simple">
<li>The <a class="reference internal" href="#difflib.get_close_matches" title="difflib.get_close_matches"><tt class="xref py py-func docutils literal">get_close_matches()</tt></a> function in this module which shows how
simple code building on <a class="reference internal" href="#difflib.SequenceMatcher" title="difflib.SequenceMatcher"><tt class="xref py py-class docutils literal">SequenceMatcher</tt></a> can be used to do useful
work.</li>
<li><a class="reference external" href="http://code.activestate.com/recipes/576729/">Simple version control recipe</a> for a small application
built with <a class="reference internal" href="#difflib.SequenceMatcher" title="difflib.SequenceMatcher"><tt class="xref py py-class docutils literal">SequenceMatcher</tt></a>.</li>
</ul>
</div>
</div>
<div class="section" id="differ-objects">
<h2>7.4.3. Differ Objects<a class="headerlink" href="#differ-objects" title="Permalink to this headline">¶</a></h2>
Note that <a class="reference internal" href="#difflib.Differ" title="difflib.Differ"><tt class="xref py py-class docutils literal">Differ</tt></a>-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 <a class="reference internal" href="#difflib.Differ" title="difflib.Differ"><tt class="xref py py-class docutils literal">Differ</tt></a> class has this constructor:
<dl class="class">
<dt>
class <tt class="descclassname">difflib.</tt><tt class="descname">Differ</tt><big>(</big>[linejunk[, charjunk]]<big>)</big></dt>
<dd>Optional keyword parameters linejunk and charjunk are for filter functions
(or <tt class="docutils literal">None</tt>):
linejunk: A function that accepts a single string argument, and returns true
if the string is junk. The default is <tt class="docutils literal">None</tt>, 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 <tt class="docutils literal">None</tt>,
meaning that no character is considered junk.
<a class="reference internal" href="#difflib.Differ" title="difflib.Differ"><tt class="xref py py-class docutils literal">Differ</tt></a> objects are used (deltas generated) via a single method:
<dl class="method">
<dt id="difflib.Differ.compare">
<tt class="descname">compare</tt><big>(</big>a, b<big>)</big><a class="headerlink" href="#difflib.Differ.compare" title="Permalink to this definition">¶</a></dt>
<dd>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 <tt class="xref py py-meth docutils literal">readlines()</tt> method of file-like
objects. The delta generated also consists of newline-terminated strings, ready
to be printed as-is via the <tt class="xref py py-meth docutils literal">writelines()</tt> method of a file-like object.
</dd></dl>

</dd></dl>

</div>
<div class="section" id="differ-example">
<h2>7.4.4. Differ Example<a class="headerlink" href="#differ-example" title="Permalink to this headline">¶</a></h2>
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 <tt class="xref py py-meth docutils literal">readlines()</tt> method of file-like objects):
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; text1 = &#39;&#39;&#39; 1. Beautiful is better than ugly.
... 2. Explicit is better than implicit.
... 3. Simple is better than complex.
... 4. Complex is better than complicated.
... &#39;&#39;&#39;.splitlines(1)
&gt;&gt;&gt; len(text1)
4
&gt;&gt;&gt; text1[0][-1]
&#39;\n&#39;
&gt;&gt;&gt; text2 = &#39;&#39;&#39; 1. Beautiful is better than ugly.
... 3. Simple is better than complex.
... 4. Complicated is better than complex.
... 5. Flat is better than nested.
... &#39;&#39;&#39;.splitlines(1)
</pre></div>
</div>
Next we instantiate a Differ object:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; d = Differ()
</pre></div>
</div>
Note that when instantiating a <a class="reference internal" href="#difflib.Differ" title="difflib.Differ"><tt class="xref py py-class docutils literal">Differ</tt></a> object we may pass functions to
filter out line and character &#8220;junk.&#8221; See the <a class="reference internal" href="#difflib.Differ" title="difflib.Differ"><tt class="xref py py-meth docutils literal">Differ()</tt></a> constructor for
details.
Finally, we compare the two:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; result = list(d.compare(text1, text2))
</pre></div>
</div>
<tt class="docutils literal">result</tt> is a list of strings, so let&#8217;s pretty-print it:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; from pprint import pprint
&gt;&gt;&gt; pprint(result)
[&#39; 1. Beautiful is better than ugly.\n&#39;,
 &#39;- 2. Explicit is better than implicit.\n&#39;,
 &#39;- 3. Simple is better than complex.\n&#39;,
 &#39;+ 3. Simple is better than complex.\n&#39;,
 &#39;? ++\n&#39;,
 &#39;- 4. Complex is better than complicated.\n&#39;,
 &#39;? ^ ---- ^\n&#39;,
 &#39;+ 4. Complicated is better than complex.\n&#39;,
 &#39;? ++++ ^ ^\n&#39;,
 &#39;+ 5. Flat is better than nested.\n&#39;]
</pre></div>
</div>
As a single multi-line string it looks like this:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; import sys
&gt;&gt;&gt; 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.
</pre></div>
</div>
</div>
<div class="section" id="a-command-line-interface-to-difflib">
<h2>7.4.5. A command-line interface to difflib<a class="headerlink" href="#a-command-line-interface-to-difflib" title="Permalink to this headline">¶</a></h2>
This example shows how to use difflib to create a <tt class="docutils literal">diff</tt>-like utility.
It is also contained in the Python source distribution, as
<tt class="file docutils literal">Tools/scripts/diff.py</tt>.
<div class="highlight-python"><div class="highlight"><pre>&quot;&quot;&quot; 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.

&quot;&quot;&quot;

import sys, os, time, difflib, optparse

def main():
 # Configure the option parser
 usage = &quot;usage: %prog [options] fromfile tofile&quot;
 parser = optparse.OptionParser(usage)
 parser.add_option(&quot;-c&quot;, action=&quot;store_true&quot;, default=False,
 help=&#39;Produce a context format diff (default)&#39;)
 parser.add_option(&quot;-u&quot;, action=&quot;store_true&quot;, default=False,
 help=&#39;Produce a unified format diff&#39;)
 hlp = &#39;Produce HTML side by side diff (can use -c and -l in conjunction)&#39;
 parser.add_option(&quot;-m&quot;, action=&quot;store_true&quot;, default=False, help=hlp)
 parser.add_option(&quot;-n&quot;, action=&quot;store_true&quot;, default=False,
 help=&#39;Produce a ndiff format diff&#39;)
 parser.add_option(&quot;-l&quot;, &quot;--lines&quot;, type=&quot;int&quot;, default=3,
 help=&#39;Set number of context lines (default 3)&#39;)
 (options, args) = parser.parse_args()

if len(args) == 0:
 parser.print_help()
 sys.exit(1)
 if len(args) != 2:
 parser.error(&quot;need to specify both a fromfile and tofile&quot;)

n = options.lines
 fromfile, tofile = args # as specified in the usage string

# we&#39;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, &#39;U&#39;).readlines()
 tolines = open(tofile, &#39;U&#39;).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&#39;re using writelines because diff is a generator
 sys.stdout.writelines(diff)

if __name__ == &#39;__main__&#39;:
 main()
</pre></div>
</div>
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">7.4. <tt class="docutils literal">difflib</tt> &#8212; Helpers for computing deltas</a><ul>
<li><a class="reference internal" href="#sequencematcher-objects">7.4.1. SequenceMatcher Objects</a></li>
<li><a class="reference internal" href="#sequencematcher-examples">7.4.2. SequenceMatcher Examples</a></li>
<li><a class="reference internal" href="#differ-objects">7.4.3. Differ Objects</a></li>
<li><a class="reference internal" href="#differ-example">7.4.4. Differ Example</a></li>
<li><a class="reference internal" href="#a-command-line-interface-to-difflib">7.4.5. A command-line interface to difflib</a></li>
</ul>
</li>
</ul>

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