black/parser: partial support for pattern matching (#2586)

Partial implementation for #2242. Only works when explicitly stated -t py310.

Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>

CHANGES.md

@@ -6,6 +6,9 @@
 
 - Warn about Python 2 deprecation in more cases by improving Python 2 only syntax
   detection (#2592)
+- Add partial support for the match statement. As it's experimental, it's only enabled
+  when `--target-version py310` is explicitly specified (#2586)
+- Add support for parenthesized with (#2586)
 
 ## 21.10b0
 

src/black/linegen.py

@@ -126,7 +126,7 @@ def visit_stmt(
         """Visit a statement.
 
         This implementation is shared for `if`, `while`, `for`, `try`, `except`,
-        `def`, `with`, `class`, `assert` and assignments.
+        `def`, `with`, `class`, `assert`, `match`, `case` and assignments.
 
         The relevant Python language `keywords` for a given statement will be
         NAME leaves within it. This methods puts those on a separate line.
@@ -292,6 +292,10 @@ def __post_init__(self) -> None:
         self.visit_async_funcdef = self.visit_async_stmt
         self.visit_decorated = self.visit_decorators
 
+        # PEP 634
+        self.visit_match_stmt = partial(v, keywords={"match"}, parens=Ø)
+        self.visit_case_block = partial(v, keywords={"case"}, parens=Ø)
+
 
 def transform_line(
     line: Line, mode: Mode, features: Collection[Feature] = ()

src/black/mode.py

@@ -20,6 +20,7 @@ class TargetVersion(Enum):
     PY37 = 7
     PY38 = 8
     PY39 = 9
+    PY310 = 10
 
     def is_python2(self) -> bool:
         return self is TargetVersion.PY27
@@ -39,6 +40,7 @@ class Feature(Enum):
     ASSIGNMENT_EXPRESSIONS = 8
     POS_ONLY_ARGUMENTS = 9
     RELAXED_DECORATORS = 10
+    PATTERN_MATCHING = 11
     FORCE_OPTIONAL_PARENTHESES = 50
 
     # temporary for Python 2 deprecation
@@ -108,6 +110,9 @@ class Feature(Enum):
         Feature.RELAXED_DECORATORS,
         Feature.POS_ONLY_ARGUMENTS,
     },
+    TargetVersion.PY310: {
+        Feature.PATTERN_MATCHING,
+    },
 }
 
 

src/black/parsing.py

@@ -59,6 +59,9 @@ def get_grammars(target_versions: Set[TargetVersion]) -> List[Grammar]:
 
     # Python 3-compatible code, so only try Python 3 grammar.
     grammars = []
+    if supports_feature(target_versions, Feature.PATTERN_MATCHING):
+        # Python 3.10+
+        grammars.append(pygram.python_grammar_soft_keywords)
     # If we have to parse both, try to parse async as a keyword first
     if not supports_feature(target_versions, Feature.ASYNC_IDENTIFIERS):
         # Python 3.7+

src/blib2to3/Grammar.txt

@@ -105,7 +105,7 @@ global_stmt: ('global' | 'nonlocal') NAME (',' NAME)*
 exec_stmt: 'exec' expr ['in' test [',' test]]
 assert_stmt: 'assert' test [',' test]
 
-compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated | async_stmt
+compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated | async_stmt | match_stmt
 async_stmt: ASYNC (funcdef | with_stmt | for_stmt)
 if_stmt: 'if' namedexpr_test ':' suite ('elif' namedexpr_test ':' suite)* ['else' ':' suite]
 while_stmt: 'while' namedexpr_test ':' suite ['else' ':' suite]
@@ -115,9 +115,8 @@ try_stmt: ('try' ':' suite
 	    ['else' ':' suite]
 	    ['finally' ':' suite] |
 	   'finally' ':' suite))
-with_stmt: 'with' with_item (',' with_item)*  ':' suite
-with_item: test ['as' expr]
-with_var: 'as' expr
+with_stmt: 'with' asexpr_test (',' asexpr_test)*  ':' suite
+
 # NB compile.c makes sure that the default except clause is last
 except_clause: 'except' [test [(',' | 'as') test]]
 suite: simple_stmt | NEWLINE INDENT stmt+ DEDENT
@@ -131,7 +130,15 @@ testlist_safe: old_test [(',' old_test)+ [',']]
 old_test: or_test | old_lambdef
 old_lambdef: 'lambda' [varargslist] ':' old_test
 
-namedexpr_test: test [':=' test]
+namedexpr_test: asexpr_test [':=' asexpr_test]
+
+# This is actually not a real rule, though since the parser is very
+# limited in terms of the strategy about match/case rules, we are inserting
+# a virtual case (<expr> as <expr>) as a valid expression. Unless a better
+# approach is thought, the only side effect of this seem to be just allowing
+# more stuff to be parser (which would fail on the ast).
+asexpr_test: test ['as' test]
+
 test: or_test ['if' or_test 'else' test] | lambdef
 or_test: and_test ('or' and_test)*
 and_test: not_test ('and' not_test)*
@@ -213,3 +220,27 @@ encoding_decl: NAME
 
 yield_expr: 'yield' [yield_arg]
 yield_arg: 'from' test | testlist_star_expr
+
+
+# 3.10 match statement definition
+
+# PS: normally the grammar is much much more restricted, but
+# at this moment for not trying to bother much with encoding the
+# exact same DSL in a LL(1) parser, we will just accept an expression
+# and let the ast.parse() step of the safe mode to reject invalid
+# grammar.
+
+# The reason why it is more restricted is that, patterns are some
+# sort of a DSL (more advanced than our LHS on assignments, but
+# still in a very limited python subset). They are not really
+# expressions, but who cares. If we can parse them, that is enough
+# to reformat them.
+
+match_stmt: "match" subject_expr ':' NEWLINE INDENT case_block+ DEDENT
+subject_expr: namedexpr_test
+
+# cases
+case_block: "case" patterns [guard] ':' suite
+guard: 'if' namedexpr_test
+patterns: pattern ['as' pattern]
+pattern: (expr|star_expr) (',' (expr|star_expr))* [',']

src/blib2to3/pgen2/driver.py

@@ -28,19 +28,92 @@
     List,
     Optional,
     Text,
+    Iterator,
     Tuple,
+    TypeVar,
+    Generic,
     Union,
 )
+from dataclasses import dataclass, field
 
 # Pgen imports
 from . import grammar, parse, token, tokenize, pgen
 from logging import Logger
 from blib2to3.pytree import _Convert, NL
 from blib2to3.pgen2.grammar import Grammar
+from contextlib import contextmanager
 
 Path = Union[str, "os.PathLike[str]"]
 
 
+@dataclass
+class ReleaseRange:
+    start: int
+    end: Optional[int] = None
+    tokens: List[Any] = field(default_factory=list)
+
+    def lock(self) -> None:
+        total_eaten = len(self.tokens)
+        self.end = self.start + total_eaten
+
+
+class TokenProxy:
+    def __init__(self, generator: Any) -> None:
+        self._tokens = generator
+        self._counter = 0
+        self._release_ranges: List[ReleaseRange] = []
+
+    @contextmanager
+    def release(self) -> Iterator["TokenProxy"]:
+        release_range = ReleaseRange(self._counter)
+        self._release_ranges.append(release_range)
+        try:
+            yield self
+        finally:
+            # Lock the last release range to the final position that
+            # has been eaten.
+            release_range.lock()
+
+    def eat(self, point: int) -> Any:
+        eaten_tokens = self._release_ranges[-1].tokens
+        if point < len(eaten_tokens):
+            return eaten_tokens[point]
+        else:
+            while point >= len(eaten_tokens):
+                token = next(self._tokens)
+                eaten_tokens.append(token)
+            return token
+
+    def __iter__(self) -> "TokenProxy":
+        return self
+
+    def __next__(self) -> Any:
+        # If the current position is already compromised (looked up)
+        # return the eaten token, if not just go further on the given
+        # token producer.
+        for release_range in self._release_ranges:
+            assert release_range.end is not None
+
+            start, end = release_range.start, release_range.end
+            if start <= self._counter < end:
+                token = release_range.tokens[self._counter - start]
+                break
+        else:
+            token = next(self._tokens)
+        self._counter += 1
+        return token
+
+    def can_advance(self, to: int) -> bool:
+        # Try to eat, fail if it can't. The eat operation is cached
+        # so there wont be any additional cost of eating here
+        try:
+            self.eat(to)
+        except StopIteration:
+            return False
+        else:
+            return True
+
+
 class Driver(object):
     def __init__(
         self,
@@ -57,14 +130,18 @@ def __init__(
     def parse_tokens(self, tokens: Iterable[Any], debug: bool = False) -> NL:
         """Parse a series of tokens and return the syntax tree."""
         # XXX Move the prefix computation into a wrapper around tokenize.
+        proxy = TokenProxy(tokens)
+
         p = parse.Parser(self.grammar, self.convert)
-        p.setup()
+        p.setup(proxy=proxy)
+
         lineno = 1
         column = 0
         indent_columns = []
         type = value = start = end = line_text = None
         prefix = ""
-        for quintuple in tokens:
+
+        for quintuple in proxy:
             type, value, start, end, line_text = quintuple
             if start != (lineno, column):
                 assert (lineno, column) <= start, ((lineno, column), start)

src/blib2to3/pgen2/grammar.py

@@ -89,6 +89,7 @@ def __init__(self) -> None:
         self.dfas: Dict[int, DFAS] = {}
         self.labels: List[Label] = [(0, "EMPTY")]
         self.keywords: Dict[str, int] = {}
+        self.soft_keywords: Dict[str, int] = {}
         self.tokens: Dict[int, int] = {}
         self.symbol2label: Dict[str, int] = {}
         self.start = 256
@@ -136,6 +137,7 @@ def copy(self: _P) -> _P:
             "number2symbol",
             "dfas",
             "keywords",
+            "soft_keywords",
             "tokens",
             "symbol2label",
         ):

src/blib2to3/pgen2/parse.py

@@ -9,22 +9,31 @@
 how this parsing engine works.
 
 """
+import copy
+from contextlib import contextmanager
 
 # Local imports
-from . import token
+from . import grammar, token, tokenize
 from typing import (
+    cast,
+    Any,
     Optional,
     Text,
     Union,
     Tuple,
     Dict,
     List,
+    Iterator,
     Callable,
     Set,
+    TYPE_CHECKING,
 )
 from blib2to3.pgen2.grammar import Grammar
 from blib2to3.pytree import NL, Context, RawNode, Leaf, Node
 
+if TYPE_CHECKING:
+    from blib2to3.driver import TokenProxy
+
 
 Results = Dict[Text, NL]
 Convert = Callable[[Grammar, RawNode], Union[Node, Leaf]]
@@ -37,6 +46,61 @@ def lam_sub(grammar: Grammar, node: RawNode) -> NL:
     return Node(type=node[0], children=node[3], context=node[2])
 
 
+class Recorder:
+    def __init__(self, parser: "Parser", ilabels: List[int], context: Context) -> None:
+        self.parser = parser
+        self._ilabels = ilabels
+        self.context = context  # not really matter
+
+        self._dead_ilabels: Set[int] = set()
+        self._start_point = copy.deepcopy(self.parser.stack)
+        self._points = {ilabel: copy.deepcopy(self._start_point) for ilabel in ilabels}
+
+    @property
+    def ilabels(self) -> Set[int]:
+        return self._dead_ilabels.symmetric_difference(self._ilabels)
+
+    @contextmanager
+    def switch_to(self, ilabel: int) -> Iterator[None]:
+        self.parser.stack = self._points[ilabel]
+        try:
+            yield
+        except ParseError:
+            self._dead_ilabels.add(ilabel)
+        finally:
+            self.parser.stack = self._start_point
+
+    def add_token(
+        self, tok_type: int, tok_val: Optional[Text], raw: bool = False
+    ) -> None:
+        func: Callable[..., Any]
+        if raw:
+            func = self.parser._addtoken
+        else:
+            func = self.parser.addtoken
+
+        for ilabel in self.ilabels:
+            with self.switch_to(ilabel):
+                args = [tok_type, tok_val, self.context]
+                if raw:
+                    args.insert(0, ilabel)
+                func(*args)
+
+    def determine_route(
+        self, value: Optional[Text] = None, force: bool = False
+    ) -> Optional[int]:
+        alive_ilabels = self.ilabels
+        if len(alive_ilabels) == 0:
+            *_, most_successful_ilabel = self._dead_ilabels
+            raise ParseError("bad input", most_successful_ilabel, value, self.context)
+
+        ilabel, *rest = alive_ilabels
+        if force or not rest:
+            return ilabel
+        else:
+            return None
+
+
 class ParseError(Exception):
     """Exception to signal the parser is stuck."""
 
@@ -114,7 +178,7 @@ def __init__(self, grammar: Grammar, convert: Optional[Convert] = None) -> None:
         self.grammar = grammar
         self.convert = convert or lam_sub
 
-    def setup(self, start: Optional[int] = None) -> None:
+    def setup(self, proxy: "TokenProxy", start: Optional[int] = None) -> None:
         """Prepare for parsing.
 
         This *must* be called before starting to parse.
@@ -137,11 +201,55 @@ def setup(self, start: Optional[int] = None) -> None:
         self.stack: List[Tuple[DFAS, int, RawNode]] = [stackentry]
         self.rootnode: Optional[NL] = None
         self.used_names: Set[str] = set()
+        self.proxy = proxy
 
     def addtoken(self, type: int, value: Optional[Text], context: Context) -> bool:
         """Add a token; return True iff this is the end of the program."""
         # Map from token to label
-        ilabel = self.classify(type, value, context)
+        ilabels = self.classify(type, value, context)
+        assert len(ilabels) >= 1
+
+        # If we have only one state to advance, we'll directly
+        # take it as is.
+        if len(ilabels) == 1:
+            [ilabel] = ilabels
+            return self._addtoken(ilabel, type, value, context)
+
+        # If there are multiple states which we can advance (only
+        # happen under soft-keywords), then we will try all of them
+        # in parallel and as soon as one state can reach further than
+        # the rest, we'll choose that one. This is a pretty hacky
+        # and hopefully temporary algorithm.
+        #
+        # For a more detailed explanation, check out this post:
+        # https://tree.science/what-the-backtracking.html
+
+        with self.proxy.release() as proxy:
+            counter, force = 0, False
+            recorder = Recorder(self, ilabels, context)
+            recorder.add_token(type, value, raw=True)
+
+            next_token_value = value
+            while recorder.determine_route(next_token_value) is None:
+                if not proxy.can_advance(counter):
+                    force = True
+                    break
+
+                next_token_type, next_token_value, *_ = proxy.eat(counter)
+                if next_token_type == tokenize.OP:
+                    next_token_type = grammar.opmap[cast(str, next_token_value)]
+
+                recorder.add_token(next_token_type, next_token_value)
+                counter += 1
+
+            ilabel = cast(int, recorder.determine_route(next_token_value, force=force))
+            assert ilabel is not None
+
+        return self._addtoken(ilabel, type, value, context)
+
+    def _addtoken(
+        self, ilabel: int, type: int, value: Optional[Text], context: Context
+    ) -> bool:
         # Loop until the token is shifted; may raise exceptions
         while True:
             dfa, state, node = self.stack[-1]
@@ -185,20 +293,29 @@ def addtoken(self, type: int, value: Optional[Text], context: Context) -> bool:
                     # No success finding a transition
                     raise ParseError("bad input", type, value, context)
 
-    def classify(self, type: int, value: Optional[Text], context: Context) -> int:
-        """Turn a token into a label.  (Internal)"""
+    def classify(self, type: int, value: Optional[Text], context: Context) -> List[int]:
+        """Turn a token into a label.  (Internal)
+
+        Depending on whether the value is a soft-keyword or not,
+        this function may return multiple labels to choose from."""
         if type == token.NAME:
             # Keep a listing of all used names
             assert value is not None
             self.used_names.add(value)
             # Check for reserved words
-            ilabel = self.grammar.keywords.get(value)
-            if ilabel is not None:
-                return ilabel
+            if value in self.grammar.keywords:
+                return [self.grammar.keywords[value]]
+            elif value in self.grammar.soft_keywords:
+                assert type in self.grammar.tokens
+                return [
+                    self.grammar.soft_keywords[value],
+                    self.grammar.tokens[type],
+                ]
+
         ilabel = self.grammar.tokens.get(type)
         if ilabel is None:
             raise ParseError("bad token", type, value, context)
-        return ilabel
+        return [ilabel]
 
     def shift(
         self, type: int, value: Optional[Text], newstate: int, context: Context

src/blib2to3/pgen2/pgen.py

@@ -115,12 +115,17 @@ def make_label(self, c: PgenGrammar, label: Text) -> int:
             assert label[0] in ('"', "'"), label
             value = eval(label)
             if value[0].isalpha():
+                if label[0] == '"':
+                    keywords = c.soft_keywords
+                else:
+                    keywords = c.keywords
+
                 # A keyword
-                if value in c.keywords:
-                    return c.keywords[value]
+                if value in keywords:
+                    return keywords[value]
                 else:
                     c.labels.append((token.NAME, value))
-                    c.keywords[value] = ilabel
+                    keywords[value] = ilabel
                     return ilabel
             else:
                 # An operator (any non-numeric token)

src/blib2to3/pygram.py

@@ -39,12 +39,14 @@ class _python_symbols(Symbols):
     arglist: int
     argument: int
     arith_expr: int
+    asexpr_test: int
     assert_stmt: int
     async_funcdef: int
     async_stmt: int
     atom: int
     augassign: int
     break_stmt: int
+    case_block: int
     classdef: int
     comp_for: int
     comp_if: int
@@ -74,6 +76,7 @@ class _python_symbols(Symbols):
     for_stmt: int
     funcdef: int
     global_stmt: int
+    guard: int
     if_stmt: int
     import_as_name: int
     import_as_names: int
@@ -82,6 +85,7 @@ class _python_symbols(Symbols):
     import_stmt: int
     lambdef: int
     listmaker: int
+    match_stmt: int
     namedexpr_test: int
     not_test: int
     old_comp_for: int
@@ -92,6 +96,8 @@ class _python_symbols(Symbols):
     or_test: int
     parameters: int
     pass_stmt: int
+    pattern: int
+    patterns: int
     power: int
     print_stmt: int
     raise_stmt: int
@@ -101,6 +107,7 @@ class _python_symbols(Symbols):
     single_input: int
     sliceop: int
     small_stmt: int
+    subject_expr: int
     star_expr: int
     stmt: int
     subscript: int
@@ -124,9 +131,7 @@ class _python_symbols(Symbols):
     vfplist: int
     vname: int
     while_stmt: int
-    with_item: int
     with_stmt: int
-    with_var: int
     xor_expr: int
     yield_arg: int
     yield_expr: int
@@ -149,6 +154,7 @@ class _pattern_symbols(Symbols):
 python_grammar_no_print_statement_no_exec_statement_async_keywords: Grammar
 python_grammar_no_exec_statement: Grammar
 pattern_grammar: Grammar
+python_grammar_soft_keywords: Grammar
 
 python_symbols: _python_symbols
 pattern_symbols: _pattern_symbols
@@ -159,6 +165,7 @@ def initialize(cache_dir: Union[str, "os.PathLike[str]", None] = None) -> None:
     global python_grammar_no_print_statement
     global python_grammar_no_print_statement_no_exec_statement
     global python_grammar_no_print_statement_no_exec_statement_async_keywords
+    global python_grammar_soft_keywords
     global python_symbols
     global pattern_grammar
     global pattern_symbols
@@ -171,6 +178,8 @@ def initialize(cache_dir: Union[str, "os.PathLike[str]", None] = None) -> None:
 
     # Python 2
     python_grammar = driver.load_packaged_grammar("blib2to3", _GRAMMAR_FILE, cache_dir)
+    soft_keywords = python_grammar.soft_keywords.copy()
+    python_grammar.soft_keywords.clear()
 
     python_symbols = _python_symbols(python_grammar)
 
@@ -191,6 +200,12 @@ def initialize(cache_dir: Union[str, "os.PathLike[str]", None] = None) -> None:
         True
     )
 
+    # Python 3.10+
+    python_grammar_soft_keywords = (
+        python_grammar_no_print_statement_no_exec_statement_async_keywords.copy()
+    )
+    python_grammar_soft_keywords.soft_keywords = soft_keywords
+
     pattern_grammar = driver.load_packaged_grammar(
         "blib2to3", _PATTERN_GRAMMAR_FILE, cache_dir
     )

tests/data/parenthesized_context_managers.py

@@ -0,0 +1,21 @@
+with (CtxManager() as example):
+    ...
+
+with (CtxManager1(), CtxManager2()):
+    ...
+
+with (CtxManager1() as example, CtxManager2()):
+    ...
+
+with (CtxManager1(), CtxManager2() as example):
+    ...
+
+with (CtxManager1() as example1, CtxManager2() as example2):
+    ...
+
+with (
+    CtxManager1() as example1,
+    CtxManager2() as example2,
+    CtxManager3() as example3,
+):
+    ...

tests/data/pattern_matching_complex.py

@@ -0,0 +1,144 @@
+# Cases sampled from Lib/test/test_patma.py
+
+# case black_test_patma_098
+match x:
+    case -0j:
+        y = 0
+# case black_test_patma_142
+match x:
+    case bytes(z):
+        y = 0
+# case black_test_patma_073
+match x:
+    case 0 if 0:
+        y = 0
+    case 0 if 1:
+        y = 1
+# case black_test_patma_006
+match 3:
+    case 0 | 1 | 2 | 3:
+        x = True
+# case black_test_patma_049
+match x:
+    case [0, 1] | [1, 0]:
+        y = 0
+# case black_check_sequence_then_mapping
+match x:
+    case [*_]:
+        return "seq"
+    case {}:
+        return "map"
+# case black_test_patma_035
+match x:
+    case {0: [1, 2, {}]}:
+        y = 0
+    case {0: [1, 2, {}] | True} | {1: [[]]} | {0: [1, 2, {}]} | [] | "X" | {}:
+        y = 1
+    case []:
+        y = 2
+# case black_test_patma_107
+match x:
+    case 0.25 + 1.75j:
+        y = 0
+# case black_test_patma_097
+match x:
+    case -0j:
+        y = 0
+# case black_test_patma_007
+match 4:
+    case 0 | 1 | 2 | 3:
+        x = True
+# case black_test_patma_154
+match x:
+    case 0 if x:
+        y = 0
+# case black_test_patma_134
+match x:
+    case {1: 0}:
+        y = 0
+    case {0: 0}:
+        y = 1
+    case {**z}:
+        y = 2
+# case black_test_patma_185
+match Seq():
+    case [*_]:
+        y = 0
+# case black_test_patma_063
+match x:
+    case 1:
+        y = 0
+    case 1:
+        y = 1
+# case black_test_patma_248
+match x:
+    case {"foo": bar}:
+        y = bar
+# case black_test_patma_019
+match (0, 1, 2):
+    case [0, 1, *x, 2]:
+        y = 0
+# case black_test_patma_052
+match x:
+    case [0]:
+        y = 0
+    case [1, 0] if (x := x[:0]):
+        y = 1
+    case [1, 0]:
+        y = 2
+# case black_test_patma_191
+match w:
+    case [x, y, *_]:
+        z = 0
+# case black_test_patma_110
+match x:
+    case -0.25 - 1.75j:
+        y = 0
+# case black_test_patma_151
+match (x,):
+    case [y]:
+        z = 0
+# case black_test_patma_114
+match x:
+    case A.B.C.D:
+        y = 0
+# case black_test_patma_232
+match x:
+    case None:
+        y = 0
+# case black_test_patma_058
+match x:
+    case 0:
+        y = 0
+# case black_test_patma_233
+match x:
+    case False:
+        y = 0
+# case black_test_patma_078
+match x:
+    case []:
+        y = 0
+    case [""]:
+        y = 1
+    case "":
+        y = 2
+# case black_test_patma_156
+match x:
+    case z:
+        y = 0
+# case black_test_patma_189
+match w:
+    case [x, y, *rest]:
+        z = 0
+# case black_test_patma_042
+match x:
+    case (0 as z) | (1 as z) | (2 as z) if z == x % 2:
+        y = 0
+# case black_test_patma_034
+match x:
+    case {0: [1, 2, {}]}:
+        y = 0
+    case {0: [1, 2, {}] | False} | {1: [[]]} | {0: [1, 2, {}]} | [] | "X" | {}:
+        y = 1
+    case []:
+        y = 2

tests/data/pattern_matching_simple.py

@@ -0,0 +1,92 @@
+# Cases sampled from PEP 636 examples
+
+match command.split():
+    case [action, obj]:
+        ...  # interpret action, obj
+
+match command.split():
+    case [action]:
+        ...  # interpret single-verb action
+    case [action, obj]:
+        ...  # interpret action, obj
+
+match command.split():
+    case ["quit"]:
+        print("Goodbye!")
+        quit_game()
+    case ["look"]:
+        current_room.describe()
+    case ["get", obj]:
+        character.get(obj, current_room)
+    case ["go", direction]:
+        current_room = current_room.neighbor(direction)
+    # The rest of your commands go here
+
+match command.split():
+    case ["drop", *objects]:
+        for obj in objects:
+            character.drop(obj, current_room)
+    # The rest of your commands go here
+
+match command.split():
+    case ["quit"]:
+        pass
+    case ["go", direction]:
+        print("Going:", direction)
+    case ["drop", *objects]:
+        print("Dropping: ", *objects)
+    case _:
+        print(f"Sorry, I couldn't understand {command!r}")
+
+match command.split():
+    case ["north"] | ["go", "north"]:
+        current_room = current_room.neighbor("north")
+    case ["get", obj] | ["pick", "up", obj] | ["pick", obj, "up"]:
+        ...  # Code for picking up the given object
+
+match command.split():
+    case ["go", ("north" | "south" | "east" | "west")]:
+        current_room = current_room.neighbor(...)
+        # how do I know which direction to go?
+
+match command.split():
+    case ["go", ("north" | "south" | "east" | "west") as direction]:
+        current_room = current_room.neighbor(direction)
+
+match command.split():
+    case ["go", direction] if direction in current_room.exits:
+        current_room = current_room.neighbor(direction)
+    case ["go", _]:
+        print("Sorry, you can't go that way")
+
+match event.get():
+    case Click(position=(x, y)):
+        handle_click_at(x, y)
+    case KeyPress(key_name="Q") | Quit():
+        game.quit()
+    case KeyPress(key_name="up arrow"):
+        game.go_north()
+    case KeyPress():
+        pass  # Ignore other keystrokes
+    case other_event:
+        raise ValueError(f"Unrecognized event: {other_event}")
+
+match event.get():
+    case Click((x, y), button=Button.LEFT):  # This is a left click
+        handle_click_at(x, y)
+    case Click():
+        pass  # ignore other clicks
+
+
+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")

tests/test_format.py

@@ -70,6 +70,11 @@
     "percent_precedence",
 ]
 
+PY310_CASES = [
+    "pattern_matching_simple",
+    "pattern_matching_complex",
+    "parenthesized_context_managers",
+]
 
 SOURCES = [
     "src/black/__init__.py",
@@ -187,6 +192,13 @@ def test_pep_570() -> None:
     assert_format(source, expected, minimum_version=(3, 8))
 
 
+@pytest.mark.parametrize("filename", PY310_CASES)
+def test_python_310(filename: str) -> None:
+    source, expected = read_data(filename)
+    mode = black.Mode(target_versions={black.TargetVersion.PY310})
+    assert_format(source, expected, mode, minimum_version=(3, 10))
+
+
 def test_docstring_no_string_normalization() -> None:
     """Like test_docstring but with string normalization off."""
     source, expected = read_data("docstring_no_string_normalization")