pythongh-81263: Add assignment expressions to help (python#124641)

* Add assignment expression (:=) to `help`

* Update index for Assignment Expressions to include pair of `assignment; expression`

Doc/reference/expressions.rst

@@ -1807,6 +1807,7 @@ returns a boolean value regardless of the type of its argument
  single: assignment expression
  single: walrus operator
  single: named expression
+ pair: assignment; expression
 
 Assignment expressions
 ======================

Lib/pydoc.py

@@ -1861,6 +1861,7 @@ class Helper:
  ':': 'SLICINGS DICTIONARYLITERALS',
  '@': 'def class',
  '\\': 'STRINGS',
+ ':=': 'ASSIGNMENTEXPRESSIONS',
  '_': 'PRIVATENAMES',
  '__': 'PRIVATENAMES SPECIALMETHODS',
  '`': 'BACKQUOTES',
@@ -1954,6 +1955,7 @@ class Helper:
  'ASSERTION': 'assert',
  'ASSIGNMENT': ('assignment', 'AUGMENTEDASSIGNMENT'),
  'AUGMENTEDASSIGNMENT': ('augassign', 'NUMBERMETHODS'),
+ 'ASSIGNMENTEXPRESSIONS': ('assignment-expressions', ''),
  'DELETION': 'del',
  'RETURNING': 'return',
  'IMPORTING': 'import',

Lib/pydoc_data/topics.py

@@ -421,6 +421,34 @@
  'some expressions (like un-parenthesized tuple expressions) '
  'caused a\n'
  'syntax error.\n',
+ 'assignment-expressions': 'Assignment expressions\n'
+ '**********************\n'
+ '\n'
+ 'An assignment expression (sometimes also called a “named expression”'
+ '\nor “walrus”) assigns an expression to an identifier, while also\n'
+ 'returning the value of the expression.\n'
+ '\n'
+ 'One common use case is when handling matched regular expressions:\n'
+ '\n'
+ ' if matching := pattern.search(data):\n'
+ ' do_something(matching)\n'
+ '\n'
+ 'Or, when processing a file stream in chunks:\n'
+ '\n'
+ ' while chunk := file.read(9000):\n'
+ ' process(chunk)\n'
+ '\n'
+ 'Assignment expressions must be surrounded by parentheses when used as\n'
+ 'expression statements and when used as sub-expressions in slicing,\n'
+ 'conditional, lambda, keyword-argument, and comprehension-if\n'
+ 'expressions and in assert, with, and assignment statements. In all\n'
+ 'other places where they can be used, parentheses are not required,\n'
+ 'including in if and while statements.\n'
+ '\n'
+ 'Added in version 3.8.\n'
+ 'See also:\n'
+ '\n'
+ ' **PEP 572** - Assignment Expressions\n',
  'async': 'Coroutines\n'
  '**********\n'
  '\n'