bpo-42128: Add documentation for pattern matching (PEP 634) (#24664)

This is a first edition, ready to go out with the implementation. We'll iterate during the rest of the period leading up to 3.10.0.

Co-authored-by: Carol Willing <carolcode@willingconsulting.com>
Co-authored-by: Fidget-Spinner <28750310+Fidget-Spinner@users.noreply.github.com>
Co-authored-by: Brandt Bucher <brandt@python.org>
Co-authored-by: Raymond Hettinger <1623689+rhettinger@users.noreply.github.com>
Co-authored-by: Guido van Rossum <guido@python.org>

1 files changed, 169 insertions, 0 deletions

diff --git a/Doc/tutorial/controlflow.rst b/Doc/tutorial/controlflow.rst
index 9ee18f7..277e5c1 100644
--- a/Doc/tutorial/controlflow.rst
+++ b/Doc/tutorial/controlflow.rst
@@ -36,6 +36,9 @@ to avoid excessive indentation.  An  :keyword:`!if` ... :keyword:`!elif` ...
 :keyword:`!elif` ... sequence is a substitute for the ``switch`` or
 ``case`` statements found in other languages.
 
+If you're comparing the same value to several constants, or checking for specific types or
+attributes, you may also find the :keyword:`!match` statement useful. For more
+details see :ref:`tut-match`.
 
 .. _tut-for:
 
@@ -246,6 +249,172 @@ at a more abstract level.  The :keyword:`!pass` is silently ignored::
    ...     pass   # Remember to implement this!
    ...
 
+
+.. _tut-match:
+
+:keyword:`!match` Statements
+============================
+
+A match statement takes an expression and compares its value to successive
+patterns given as one or more case blocks.  This is superficially
+similar to a switch statement in C, Java or JavaScript (and many
+other languages), but it can also extract components (sequence elements or
+object attributes) from the value into variables.
+
+The simplest form compares a subject value against one or more literals::
+
+    def http_error(status):
+        match status:
+            case 400:
+                return "Bad request"
+            case 404:
+                return "Not found"
+            case 418:
+                return "I'm a teapot"
+            case _:
+                return "Something's wrong with the Internet"
+
+Note the last block: the "variable name" ``_`` acts as a *wildcard* and
+never fails to match. If no case matches, none of the branches is executed.
+
+You can combine several literals in a single pattern using ``|`` ("or")::
+
+            case 401 | 403 | 404:
+                return "Not allowed"
+
+Patterns can look like unpacking assignments, and can be used to bind
+variables::
+
+    # point is an (x, y) tuple
+    match point:
+        case (0, 0):
+            print("Origin")
+        case (0, y):
+            print(f"Y={y}")
+        case (x, 0):
+            print(f"X={x}")
+        case (x, y):
+            print(f"X={x}, Y={y}")
+        case _:
+            raise ValueError("Not a point")
+
+Study that one carefully!  The first pattern has two literals, and can
+be thought of as an extension of the literal pattern shown above.  But
+the next two patterns combine a literal and a variable, and the
+variable *binds* a value from the subject (``point``).  The fourth
+pattern captures two values, which makes it conceptually similar to
+the unpacking assignment ``(x, y) = point``.
+
+If you are using classes to structure your data
+you can use the class name followed by an argument list resembling a
+constructor, but with the ability to capture attributes into variables::
+
+    class Point:
+        x: int
+        y: int
+
+    def where_is(point):
+        match point:
+            case Point(x=0, y=0):
+                print("Origin")
+            case Point(x=0, y=y):
+                print(f"Y={y}")
+            case Point(x=x, y=0):
+                print(f"X={x}")
+            case Point():
+                print("Somewhere else")
+            case _:
+                print("Not a point")
+
+You can use positional parameters with some builtin classes that provide an
+ordering for their attributes (e.g. dataclasses). You can also define a specific
+position for attributes in patterns by setting the ``__match_args__`` special
+attribute in your classes. If it's set to ("x", "y"), the following patterns are all
+equivalent (and all bind the ``y`` attribute to the ``var`` variable)::
+
+    Point(1, var)
+    Point(1, y=var)
+    Point(x=1, y=var)
+    Point(y=var, x=1)
+
+A recommended way to read patterns is to look at them as an extended form of what you
+would put on the left of an assignment, to understand which variables would be set to
+what.
+Only the standalone names (like ``var`` above) are assigned to by a match statement.
+Dotted names (like ``foo.bar``), attribute names (the ``x=`` and ``y=`` above) or class names
+(recognized by the "(...)" next to them like ``Point`` above) are never assigned to.
+
+Patterns can be arbitrarily nested.  For example, if we have a short
+list of points, we could match it like this::
+
+    match points:
+        case []:
+            print("No points")
+        case [Point(0, 0)]:
+            print("The origin")
+        case [Point(x, y)]:
+            print(f"Single point {x}, {y}")
+        case [Point(0, y1), Point(0, y2)]:
+            print(f"Two on the Y axis at {y1}, {y2}")
+        case _:
+            print("Something else")
+
+We can add an ``if`` clause to a pattern, known as a "guard".  If the
+guard is false, ``match`` goes on to try the next case block.  Note
+that value capture happens before the guard is evaluated::
+
+    match point:
+        case Point(x, y) if x == y:
+            print(f"Y=X at {x}")
+        case Point(x, y):
+            print(f"Not on the diagonal")
+
+Several other key features of this statement:
+
+- Like unpacking assignments, tuple and list patterns have exactly the
+  same meaning and actually match arbitrary sequences.  An important
+  exception is that they don't match iterators or strings.
+
+- Sequence patterns support extended unpacking: ``[x, y, *rest]`` and ``(x, y,
+  *rest)`` work similar to unpacking assignments.  The
+  name after ``*`` may also be ``_``, so ``(x, y, *_)`` matches a sequence
+  of at least two items without binding the remaining items.
+
+- Mapping patterns: ``{"bandwidth": b, "latency": l}`` captures the
+  ``"bandwidth"`` and ``"latency"`` values from a dictionary.  Unlike sequence
+  patterns, extra keys are ignored.  An unpacking like ``**rest`` is also
+  supported.  (But ``**_`` would be redundant, so it not allowed.)
+
+- Subpatterns may be captured using the ``as`` keyword::
+
+      case (Point(x1, y1), Point(x2, y2) as p2): ...
+
+  will capture the second element of the input as ``p2`` (as long as the input is
+  a sequence of two points)
+
+- Most literals are compared by equality, however the singletons ``True``,
+  ``False`` and ``None`` are compared by identity.
+
+- Patterns may use named constants.  These must be dotted names
+  to prevent them from being interpreted as capture variable::
+
+      from enum import Enum
+      class Color(Enum):
+          RED = 0
+          GREEN = 1
+          BLUE = 2
+
+      match color:
+          case Color.RED:
+              print("I see red!")
+          case Color.GREEN:
+              print("Grass is green")
+          case Color.BLUE:
+              print("I'm feeling the blues :(")
+
+For a more detailed explanation and additional examples, you can look into
+:pep:`636` which is written in a tutorial format.
+
 .. _tut-functions:
 
 Defining Functions