difflib.rst



:mod:`difflib` --- Helpers for computing deltas
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.
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.
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.
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:
The following methods are public:
:file:`Tools/scripts/diff.py` is a command-line front-end to this class and
contains a good example of its use.

SequenceMatcher Objects
The :class:`SequenceMatcher` class has this constructor:
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:
: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.
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
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]


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:
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:

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.


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`.