[Python-checkins] peps: According to Knuth, it is better to break *before* a binary operator.

[guido.van.rossum]

https://hg.python.org/peps/rev/3857909d7956
changeset:   6283:3857909d7956
user:        Guido van Rossum <guido at python.org>
date:        Fri Apr 15 09:43:52 2016 -0700
summary:
  According to Knuth, it is better to break *before* a binary operator.

files:
  pep-0008.txt |  46 +++++++++++++++++++++++++++------------
  1 files changed, 32 insertions(+), 14 deletions(-)


diff --git a/pep-0008.txt b/pep-0008.txt
--- a/pep-0008.txt
+++ b/pep-0008.txt
@@ -151,6 +151,9 @@
             that_is_another_thing):
         do_something()
 
+(Also see the discussion of whether to break before or after binary
+operators below.)
+
 The closing brace/bracket/parenthesis on multi-line constructs may
 either line up under the first non-whitespace character of the last
 line of list, as in::
@@ -244,20 +247,33 @@
 
 Another such case is with ``assert`` statements.
 
-Make sure to indent the continued line appropriately.  The preferred
-place to break around a binary operator is *after* the operator, not
-before it.  Some examples::
+Make sure to indent the continued line appropriately.
+
+
+Should a line break before or after a binary operator?
+------------------------------------------------------
+
+For decades the recommended style has been to break after binary
+operators.  However, recent reseach unearthed recommendations by
+Donald Knuth to break *before* binary operators, in his writings about
+typesetting [3]_.  Therefore it is permissible to break before or
+after a binary operator, as long as the convention is consistent
+locally.  For new code Knuth's style is suggested.
+
+Some examples of code beaking before binary Boolean operators::
 
     class Rectangle(Blob):
 
         def __init__(self, width, height,
                      color='black', emphasis=None, highlight=0):
-            if (width == 0 and height == 0 and
-                    color == 'red' and emphasis == 'strong' or
-                    highlight > 100):
+            if (width == 0
+                and height == 0
+                and color == 'red'
+                and emphasis == 'strong'
+                or highlight > 100):
                 raise ValueError("sorry, you lose")
-            if width == 0 and height == 0 and (color == 'red' or
-                                               emphasis is None):
+            if (width == 0 and height == 0
+                and (color == 'red' or emphasis is None)):
                 raise ValueError("I don't think so -- values are %s, %s" %
                                  (width, height))
             Blob.__init__(self, width, height,
@@ -709,7 +725,7 @@
 - ``UPPERCASE``
 - ``UPPER_CASE_WITH_UNDERSCORES``
 - ``CapitalizedWords`` (or CapWords, or CamelCase -- so named because
-  of the bumpy look of its letters [3]_).  This is also sometimes known
+  of the bumpy look of its letters [4]_).  This is also sometimes known
   as StudlyCaps.
 
   Note: When using abbreviations in CapWords, capitalize all the
@@ -1286,11 +1302,11 @@
   PEP 484 recommends the use of stub files: .pyi files that are read
   by the type checker in preference of the corresponding .py files.
   Stub files can be distributed with a library, or separately (with
-  the library author's permission) through the typeshed repo [4]_.
+  the library author's permission) through the typeshed repo [5]_.
 
 - For code that needs to be backwards compatible, function annotations
   can be added in the form of comments.  See the relevant section of
-  PEP 484 [5]_.
+  PEP 484 [6]_.
 
 
 .. rubric:: Footnotes
@@ -1311,12 +1327,14 @@
 .. [2] Barry's GNU Mailman style guide
        http://barry.warsaw.us/software/STYLEGUIDE.txt
 
-.. [3] http://www.wikipedia.com/wiki/CamelCase
+.. [3] http://rhodesmill.org/brandon/slides/2012-11-pyconca/#laying-down-the-law
 
-.. [4] Typeshed repo
+.. [4] http://www.wikipedia.com/wiki/CamelCase
+
+.. [5] Typeshed repo
    https://github.com/python/typeshed
 
-.. [5] Suggested syntax for Python 2.7 and straddling code
+.. [6] Suggested syntax for Python 2.7 and straddling code
    https://www.python.org/dev/peps/pep-0484/#suggested-syntax-for-python-2-7-and-straddling-code
 
 

-- 
Repository URL: https://hg.python.org/peps