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gh-118150: warn in doc about results asymmetry for difflib junk by Lenormju · Pull Request #153892 · python/cpython · GitHub

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gh-118150: warn in doc about results asymmetry for difflib junk #153892

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Merged
encukou merged 4 commits into from
Jul 24, 2026
Conversation 20 Commits 4 Checks 40 Files changed
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gh-118150: warn in doc about results asymmetry for difflib junk #153892

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f6e54c2
docs: warn about results asymmetry for difflib junk
Lenormju Jul 18, 2026
39e47c3
merge and reword the whole junk mechanism
Lenormju Jul 18, 2026
1d9605e
remove all notes, keep only junk explanation at the top
Lenormju Jul 18, 2026
0793740
update NEWS to the current content of the PR
Lenormju Jul 18, 2026
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remove all notes, keep only junk explanation at the top
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Lenormju committed Jul 18, 2026
commit 1d9605ea67cfad182e9d91d293cc1237c3a2ae8f
57 changes: 0 additions & 57 deletions Doc/library/difflib.rst
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Expand Up @@ -194,11 +194,6 @@ diffs. For comparing directories and files, see also, the :mod:`filecmp` module.

See :ref:`difflib-interface` for a more detailed example.

.. note::

This uses a :class:`SequenceMatcher` internally, so swapping *a* and
*b* can produce different results.


.. function:: get_close_matches(word, possibilities, n=3, cutoff=0.6)

Expand Down Expand Up @@ -260,11 +255,6 @@ diffs. For comparing directories and files, see also, the :mod:`filecmp` module.
+ tree
+ emu

.. note::

This uses a :class:`SequenceMatcher` internally, so swapping *a* and
*b* can produce different results.


.. function:: restore(sequence, which)

Expand Down Expand Up @@ -334,11 +324,6 @@ diffs. For comparing directories and files, see also, the :mod:`filecmp` module.

See :ref:`difflib-interface` for a more detailed example.

.. note::

This uses a :class:`SequenceMatcher` internally, so swapping *a* and
*b* can produce different results.

.. versionchanged:: 3.15
Added the *color* parameter.

Expand Down Expand Up @@ -417,18 +402,6 @@ The :class:`SequenceMatcher` class has this constructor:
of positions where they occur. All three are reset whenever *b* is reset
with :meth:`set_seqs` or :meth:`set_seq2`.

.. note::

Both *isjunk* and the *autojunk* heuristic only consider *b* for filtering
out items. Consequently, swapping *a* and *b* can change which items are treated
as junk, which can in turn change the matching blocks found by
:meth:`get_matching_blocks` and :meth:`get_opcodes`, the value returned by
:meth:`ratio`, and the diffs produced by :func:`unified_diff`,
:func:`context_diff`, and :func:`ndiff`, all of which build a
:class:`SequenceMatcher` internally.
A custom *isjunk* has this effect at any sequence length, and the automatic
*autojunk* heuristic has the same effect for sequences of 200 or more items.

.. versionadded:: 3.2
The *bjunk* and *bpopular* attributes.

Expand Down Expand Up @@ -518,21 +491,6 @@ The :class:`SequenceMatcher` class has this constructor:
>>> 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)]

.. note::

Caution: the result may depend on the order of the arguments. For
instance::

>>> SequenceMatcher(None, 'tide', 'diet').get_matching_blocks()
[Match(a=0, b=3, size=1), Match(a=4, b=4, size=0)]
>>> SequenceMatcher(None, 'diet', 'tide').get_matching_blocks()
[Match(a=0, b=2, size=1), Match(a=2, b=3, size=1), Match(a=4, b=4, size=0)]

This comes from the matching algorithm itself. Another cause for order
causing different outputs, sometimes wildly, is the junk filtering
heuristic (a custom *isjunk* at any sequence length, or the
automatic *autojunk* heuristic for sequences of 200 or more
items). See the :class:`SequenceMatcher` for details.

.. method:: get_opcodes()

Expand Down Expand Up @@ -574,11 +532,6 @@ The :class:`SequenceMatcher` class has this constructor:
equal a[4:6] --> b[3:5] 'cd' --> 'cd'
insert a[6:6] --> b[5:6] '' --> 'f'

.. note::

This is derived from :meth:`get_matching_blocks`, and so shares
its order-dependence; see the note there.


.. method:: get_grouped_opcodes(n=3)

Expand All @@ -590,11 +543,6 @@ The :class:`SequenceMatcher` class has this constructor:

The groups are returned in the same format as :meth:`get_opcodes`.

.. note::

This is derived from :meth:`get_matching_blocks`, and so shares
its order-dependence; see the note there.


.. method:: ratio()

Expand All @@ -610,11 +558,6 @@ The :class:`SequenceMatcher` class has this constructor:
to try :meth:`quick_ratio` or :meth:`real_quick_ratio` first to get an
upper bound.

.. note::

This is derived from :meth:`get_matching_blocks`, and so shares
its order-dependence; see the note there.


.. method:: quick_ratio()

Expand Down
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