parser: force a `ValueError` to bytes before passing to `error.ParseError`

I'm not sure what changed before pytype 09-09-2021 (from 04-15-2021), but this
started getting flagged.  I think there's a pytype bug here, because I don't
see how `.lower()` can be getting called on a `ValueError` after it is forced to
a byte string.  That's suppressed for now to make progress.

This fixes:

    File "/mnt/c/Users/Matt/hg/mercurial/parser.py", line 219, in unescapestr: Function bytestr.__init__ was called with the wrong arguments [wrong-arg-types]
             Expected: (self, ints: Iterable[int])
      Actually passed: (self, ints: ValueError)
      Attributes of protocol Iterable[int] are not implemented on ValueError: __iter__
    File "/mnt/c/Users/Matt/hg/mercurial/parser.py", line 219, in unescapestr: No attribute 'lower' on ValueError [attribute-error]
      In Union[ValueError, mercurial.pycompat.bytestr]

Differential Revision: https://phab.mercurial-scm.org/D11471

mercurial/parser.py

@@ -1,743 +1,746 b''
 # parser.py - simple top-down operator precedence parser for mercurial
 #
 # Copyright 2010 Olivia Mackall <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 # see http://effbot.org/zone/simple-top-down-parsing.htm and
 # http://eli.thegreenplace.net/2010/01/02/top-down-operator-precedence-parsing/
 # for background
 
 # takes a tokenizer and elements
 # tokenizer is an iterator that returns (type, value, pos) tuples
 # elements is a mapping of types to binding strength, primary, prefix, infix
 # and suffix actions
 # an action is a tree node name, a tree label, and an optional match
 # __call__(program) parses program into a labeled tree
 
 from __future__ import absolute_import, print_function
 
 from .i18n import _
 from . import (
     error,
-    pycompat,
     util,
 )
 from .utils import stringutil
 
 
 class parser(object):
     def __init__(self, elements, methods=None):
         self._elements = elements
         self._methods = methods
         self.current = None
 
     def _advance(self):
         """advance the tokenizer"""
         t = self.current
         self.current = next(self._iter, None)
         return t
 
     def _hasnewterm(self):
         """True if next token may start new term"""
         return any(self._elements[self.current[0]][1:3])
 
     def _match(self, m):
         """make sure the tokenizer matches an end condition"""
         if self.current[0] != m:
             raise error.ParseError(
                 _(b"unexpected token: %s") % self.current[0], self.current[2]
             )
         self._advance()
 
     def _parseoperand(self, bind, m=None):
         """gather right-hand-side operand until an end condition or binding
         met"""
         if m and self.current[0] == m:
             expr = None
         else:
             expr = self._parse(bind)
         if m:
             self._match(m)
         return expr
 
     def _parse(self, bind=0):
         token, value, pos = self._advance()
         # handle prefix rules on current token, take as primary if unambiguous
         primary, prefix = self._elements[token][1:3]
         if primary and not (prefix and self._hasnewterm()):
             expr = (primary, value)
         elif prefix:
             expr = (prefix[0], self._parseoperand(*prefix[1:]))
         else:
             raise error.ParseError(_(b"not a prefix: %s") % token, pos)
         # gather tokens until we meet a lower binding strength
         while bind < self._elements[self.current[0]][0]:
             token, value, pos = self._advance()
             # handle infix rules, take as suffix if unambiguous
             infix, suffix = self._elements[token][3:]
             if suffix and not (infix and self._hasnewterm()):
                 expr = (suffix, expr)
             elif infix:
                 expr = (infix[0], expr, self._parseoperand(*infix[1:]))
             else:
                 raise error.ParseError(_(b"not an infix: %s") % token, pos)
         return expr
 
     def parse(self, tokeniter):
         """generate a parse tree from tokens"""
         self._iter = tokeniter
         self._advance()
         res = self._parse()
         token, value, pos = self.current
         return res, pos
 
     def eval(self, tree):
         """recursively evaluate a parse tree using node methods"""
         if not isinstance(tree, tuple):
             return tree
         return self._methods[tree[0]](*[self.eval(t) for t in tree[1:]])
 
     def __call__(self, tokeniter):
         """parse tokens into a parse tree and evaluate if methods given"""
         t = self.parse(tokeniter)
         if self._methods:
             return self.eval(t)
         return t
 
 
 def splitargspec(spec):
     """Parse spec of function arguments into (poskeys, varkey, keys, optkey)
 
     >>> splitargspec(b'')
     ([], None, [], None)
     >>> splitargspec(b'foo bar')
     ([], None, ['foo', 'bar'], None)
     >>> splitargspec(b'foo *bar baz **qux')
     (['foo'], 'bar', ['baz'], 'qux')
     >>> splitargspec(b'*foo')
     ([], 'foo', [], None)
     >>> splitargspec(b'**foo')
     ([], None, [], 'foo')
     """
     optkey = None
     pre, sep, post = spec.partition(b'**')
     if sep:
         posts = post.split()
         if not posts:
             raise error.ProgrammingError(b'no **optkey name provided')
         if len(posts) > 1:
             raise error.ProgrammingError(b'excessive **optkey names provided')
         optkey = posts[0]
 
     pre, sep, post = pre.partition(b'*')
     pres = pre.split()
     posts = post.split()
     if sep:
         if not posts:
             raise error.ProgrammingError(b'no *varkey name provided')
         return pres, posts[0], posts[1:], optkey
     return [], None, pres, optkey
 
 
 def buildargsdict(trees, funcname, argspec, keyvaluenode, keynode):
     """Build dict from list containing positional and keyword arguments
 
     Arguments are specified by a tuple of ``(poskeys, varkey, keys, optkey)``
     where
 
     - ``poskeys``: list of names of positional arguments
     - ``varkey``: optional argument name that takes up remainder
     - ``keys``: list of names that can be either positional or keyword arguments
     - ``optkey``: optional argument name that takes up excess keyword arguments
 
     If ``varkey`` specified, all ``keys`` must be given as keyword arguments.
 
     Invalid keywords, too few positional arguments, or too many positional
     arguments are rejected, but missing keyword arguments are just omitted.
     """
     poskeys, varkey, keys, optkey = argspec
     kwstart = next(
         (i for i, x in enumerate(trees) if x and x[0] == keyvaluenode),
         len(trees),
     )
     if kwstart < len(poskeys):
         raise error.ParseError(
             _(b"%(func)s takes at least %(nargs)d positional arguments")
             % {b'func': funcname, b'nargs': len(poskeys)}
         )
     if not varkey and kwstart > len(poskeys) + len(keys):
         raise error.ParseError(
             _(b"%(func)s takes at most %(nargs)d positional arguments")
             % {b'func': funcname, b'nargs': len(poskeys) + len(keys)}
         )
     args = util.sortdict()
     # consume positional arguments
     for k, x in zip(poskeys, trees[:kwstart]):
         args[k] = x
     if varkey:
         args[varkey] = trees[len(args) : kwstart]
     else:
         for k, x in zip(keys, trees[len(args) : kwstart]):
             args[k] = x
     # remainder should be keyword arguments
     if optkey:
         args[optkey] = util.sortdict()
     for x in trees[kwstart:]:
         if not x or x[0] != keyvaluenode or x[1][0] != keynode:
             raise error.ParseError(
                 _(b"%(func)s got an invalid argument") % {b'func': funcname}
             )
         k = x[1][1]
         if k in keys:
             d = args
         elif not optkey:
             raise error.ParseError(
                 _(b"%(func)s got an unexpected keyword argument '%(key)s'")
                 % {b'func': funcname, b'key': k}
             )
         else:
             d = args[optkey]
         if k in d:
             raise error.ParseError(
                 _(
                     b"%(func)s got multiple values for keyword "
                     b"argument '%(key)s'"
                 )
                 % {b'func': funcname, b'key': k}
             )
         d[k] = x[2]
     return args
 
 
 def unescapestr(s):
     try:
         return stringutil.unescapestr(s)
     except ValueError as e:
         # mangle Python's exception into our format
-        raise error.ParseError(pycompat.bytestr(e).lower())
+        # TODO: remove this suppression.  For some reason, pytype 2021.09.09
+        #   thinks .lower() is being called on Union[ValueError, bytes].
+        # pytype: disable=attribute-error
+        raise error.ParseError(stringutil.forcebytestr(e).lower())
+        # pytype: enable=attribute-error
 
 
 def _prettyformat(tree, leafnodes, level, lines):
     if not isinstance(tree, tuple):
         lines.append((level, stringutil.pprint(tree)))
     elif tree[0] in leafnodes:
         rs = map(stringutil.pprint, tree[1:])
         lines.append((level, b'(%s %s)' % (tree[0], b' '.join(rs))))
     else:
         lines.append((level, b'(%s' % tree[0]))
         for s in tree[1:]:
             _prettyformat(s, leafnodes, level + 1, lines)
         lines[-1:] = [(lines[-1][0], lines[-1][1] + b')')]
 
 
 def prettyformat(tree, leafnodes):
     lines = []
     _prettyformat(tree, leafnodes, 0, lines)
     output = b'\n'.join((b'  ' * l + s) for l, s in lines)
     return output
 
 
 def simplifyinfixops(tree, targetnodes):
     """Flatten chained infix operations to reduce usage of Python stack
 
     >>> from . import pycompat
     >>> def f(tree):
     ...     s = prettyformat(simplifyinfixops(tree, (b'or',)), (b'symbol',))
     ...     print(pycompat.sysstr(s))
     >>> f((b'or',
     ...     (b'or',
     ...       (b'symbol', b'1'),
     ...       (b'symbol', b'2')),
     ...     (b'symbol', b'3')))
     (or
       (symbol '1')
       (symbol '2')
       (symbol '3'))
     >>> f((b'func',
     ...     (b'symbol', b'p1'),
     ...     (b'or',
     ...       (b'or',
     ...         (b'func',
     ...           (b'symbol', b'sort'),
     ...           (b'list',
     ...             (b'or',
     ...               (b'or',
     ...                 (b'symbol', b'1'),
     ...                 (b'symbol', b'2')),
     ...               (b'symbol', b'3')),
     ...             (b'negate',
     ...               (b'symbol', b'rev')))),
     ...         (b'and',
     ...           (b'symbol', b'4'),
     ...           (b'group',
     ...             (b'or',
     ...               (b'or',
     ...                 (b'symbol', b'5'),
     ...                 (b'symbol', b'6')),
     ...               (b'symbol', b'7'))))),
     ...       (b'symbol', b'8'))))
     (func
       (symbol 'p1')
       (or
         (func
           (symbol 'sort')
           (list
             (or
               (symbol '1')
               (symbol '2')
               (symbol '3'))
             (negate
               (symbol 'rev'))))
         (and
           (symbol '4')
           (group
             (or
               (symbol '5')
               (symbol '6')
               (symbol '7'))))
         (symbol '8')))
     """
     if not isinstance(tree, tuple):
         return tree
     op = tree[0]
     if op not in targetnodes:
         return (op,) + tuple(simplifyinfixops(x, targetnodes) for x in tree[1:])
 
     # walk down left nodes taking each right node. no recursion to left nodes
     # because infix operators are left-associative, i.e. left tree is deep.
     # e.g. '1 + 2 + 3' -> (+ (+ 1 2) 3) -> (+ 1 2 3)
     simplified = []
     x = tree
     while x[0] == op:
         l, r = x[1:]
         simplified.append(simplifyinfixops(r, targetnodes))
         x = l
     simplified.append(simplifyinfixops(x, targetnodes))
     simplified.append(op)
     return tuple(reversed(simplified))
 
 
 def _buildtree(template, placeholder, replstack):
     if template == placeholder:
         return replstack.pop()
     if not isinstance(template, tuple):
         return template
     return tuple(_buildtree(x, placeholder, replstack) for x in template)
 
 
 def buildtree(template, placeholder, *repls):
     """Create new tree by substituting placeholders by replacements
 
     >>> _ = (b'symbol', b'_')
     >>> def f(template, *repls):
     ...     return buildtree(template, _, *repls)
     >>> f((b'func', (b'symbol', b'only'), (b'list', _, _)),
     ...   ('symbol', '1'), ('symbol', '2'))
     ('func', ('symbol', 'only'), ('list', ('symbol', '1'), ('symbol', '2')))
     >>> f((b'and', _, (b'not', _)), (b'symbol', b'1'), (b'symbol', b'2'))
     ('and', ('symbol', '1'), ('not', ('symbol', '2')))
     """
     if not isinstance(placeholder, tuple):
         raise error.ProgrammingError(b'placeholder must be a node tuple')
     replstack = list(reversed(repls))
     r = _buildtree(template, placeholder, replstack)
     if replstack:
         raise error.ProgrammingError(b'too many replacements')
     return r
 
 
 def _matchtree(pattern, tree, placeholder, incompletenodes, matches):
     if pattern == tree:
         return True
     if not isinstance(pattern, tuple) or not isinstance(tree, tuple):
         return False
     if pattern == placeholder and tree[0] not in incompletenodes:
         matches.append(tree)
         return True
     if len(pattern) != len(tree):
         return False
     return all(
         _matchtree(p, x, placeholder, incompletenodes, matches)
         for p, x in zip(pattern, tree)
     )
 
 
 def matchtree(pattern, tree, placeholder=None, incompletenodes=()):
     """If a tree matches the pattern, return a list of the tree and nodes
     matched with the placeholder; Otherwise None
 
     >>> def f(pattern, tree):
     ...     m = matchtree(pattern, tree, _, {b'keyvalue', b'list'})
     ...     if m:
     ...         return m[1:]
 
     >>> _ = (b'symbol', b'_')
     >>> f((b'func', (b'symbol', b'ancestors'), _),
     ...   (b'func', (b'symbol', b'ancestors'), (b'symbol', b'1')))
     [('symbol', '1')]
     >>> f((b'func', (b'symbol', b'ancestors'), _),
     ...   (b'func', (b'symbol', b'ancestors'), None))
     >>> f((b'range', (b'dagrange', _, _), _),
     ...   (b'range',
     ...     (b'dagrange', (b'symbol', b'1'), (b'symbol', b'2')),
     ...     (b'symbol', b'3')))
     [('symbol', '1'), ('symbol', '2'), ('symbol', '3')]
 
     The placeholder does not match the specified incomplete nodes because
     an incomplete node (e.g. argument list) cannot construct an expression.
 
     >>> f((b'func', (b'symbol', b'ancestors'), _),
     ...   (b'func', (b'symbol', b'ancestors'),
     ...     (b'list', (b'symbol', b'1'), (b'symbol', b'2'))))
 
     The placeholder may be omitted, but which shouldn't match a None node.
 
     >>> _ = None
     >>> f((b'func', (b'symbol', b'ancestors'), None),
     ...   (b'func', (b'symbol', b'ancestors'), (b'symbol', b'0')))
     """
     if placeholder is not None and not isinstance(placeholder, tuple):
         raise error.ProgrammingError(b'placeholder must be a node tuple')
     matches = [tree]
     if _matchtree(pattern, tree, placeholder, incompletenodes, matches):
         return matches
 
 
 def parseerrordetail(inst):
     """Compose error message from specified ParseError object"""
     if inst.location is not None:
         return _(b'at %d: %s') % (inst.location, inst.message)
     else:
         return inst.message
 
 
 class alias(object):
     """Parsed result of alias"""
 
     def __init__(self, name, args, err, replacement):
         self.name = name
         self.args = args
         self.error = err
         self.replacement = replacement
         # whether own `error` information is already shown or not.
         # this avoids showing same warning multiple times at each
         # `expandaliases`.
         self.warned = False
 
 
 class basealiasrules(object):
     """Parsing and expansion rule set of aliases
 
     This is a helper for fileset/revset/template aliases. A concrete rule set
     should be made by sub-classing this and implementing class/static methods.
 
     It supports alias expansion of symbol and function-call styles::
 
         # decl = defn
         h = heads(default)
         b($1) = ancestors($1) - ancestors(default)
     """
 
     # typically a config section, which will be included in error messages
     _section = None
     # tag of symbol node
     _symbolnode = b'symbol'
 
     def __new__(cls):
         raise TypeError(b"'%s' is not instantiatable" % cls.__name__)
 
     @staticmethod
     def _parse(spec):
         """Parse an alias name, arguments and definition"""
         raise NotImplementedError
 
     @staticmethod
     def _trygetfunc(tree):
         """Return (name, args) if tree is a function; otherwise None"""
         raise NotImplementedError
 
     @classmethod
     def _builddecl(cls, decl):
         """Parse an alias declaration into ``(name, args, errorstr)``
 
         This function analyzes the parsed tree. The parsing rule is provided
         by ``_parse()``.
 
         - ``name``: of declared alias (may be ``decl`` itself at error)
         - ``args``: list of argument names (or None for symbol declaration)
         - ``errorstr``: detail about detected error (or None)
 
         >>> sym = lambda x: (b'symbol', x)
         >>> symlist = lambda *xs: (b'list',) + tuple(sym(x) for x in xs)
         >>> func = lambda n, a: (b'func', sym(n), a)
         >>> parsemap = {
         ...     b'foo': sym(b'foo'),
         ...     b'$foo': sym(b'$foo'),
         ...     b'foo::bar': (b'dagrange', sym(b'foo'), sym(b'bar')),
         ...     b'foo()': func(b'foo', None),
         ...     b'$foo()': func(b'$foo', None),
         ...     b'foo($1, $2)': func(b'foo', symlist(b'$1', b'$2')),
         ...     b'foo(bar_bar, baz.baz)':
         ...         func(b'foo', symlist(b'bar_bar', b'baz.baz')),
         ...     b'foo(bar($1, $2))':
         ...         func(b'foo', func(b'bar', symlist(b'$1', b'$2'))),
         ...     b'foo($1, $2, nested($1, $2))':
         ...         func(b'foo', (symlist(b'$1', b'$2') +
         ...                      (func(b'nested', symlist(b'$1', b'$2')),))),
         ...     b'foo("bar")': func(b'foo', (b'string', b'bar')),
         ...     b'foo($1, $2': error.ParseError(b'unexpected token: end', 10),
         ...     b'foo("bar': error.ParseError(b'unterminated string', 5),
         ...     b'foo($1, $2, $1)': func(b'foo', symlist(b'$1', b'$2', b'$1')),
         ... }
         >>> def parse(expr):
         ...     x = parsemap[expr]
         ...     if isinstance(x, Exception):
         ...         raise x
         ...     return x
         >>> def trygetfunc(tree):
         ...     if not tree or tree[0] != b'func' or tree[1][0] != b'symbol':
         ...         return None
         ...     if not tree[2]:
         ...         return tree[1][1], []
         ...     if tree[2][0] == b'list':
         ...         return tree[1][1], list(tree[2][1:])
         ...     return tree[1][1], [tree[2]]
         >>> class aliasrules(basealiasrules):
         ...     _parse = staticmethod(parse)
         ...     _trygetfunc = staticmethod(trygetfunc)
         >>> builddecl = aliasrules._builddecl
         >>> builddecl(b'foo')
         ('foo', None, None)
         >>> builddecl(b'$foo')
         ('$foo', None, "invalid symbol '$foo'")
         >>> builddecl(b'foo::bar')
         ('foo::bar', None, 'invalid format')
         >>> builddecl(b'foo()')
         ('foo', [], None)
         >>> builddecl(b'$foo()')
         ('$foo()', None, "invalid function '$foo'")
         >>> builddecl(b'foo($1, $2)')
         ('foo', ['$1', '$2'], None)
         >>> builddecl(b'foo(bar_bar, baz.baz)')
         ('foo', ['bar_bar', 'baz.baz'], None)
         >>> builddecl(b'foo($1, $2, nested($1, $2))')
         ('foo($1, $2, nested($1, $2))', None, 'invalid argument list')
         >>> builddecl(b'foo(bar($1, $2))')
         ('foo(bar($1, $2))', None, 'invalid argument list')
         >>> builddecl(b'foo("bar")')
         ('foo("bar")', None, 'invalid argument list')
         >>> builddecl(b'foo($1, $2')
         ('foo($1, $2', None, 'at 10: unexpected token: end')
         >>> builddecl(b'foo("bar')
         ('foo("bar', None, 'at 5: unterminated string')
         >>> builddecl(b'foo($1, $2, $1)')
         ('foo', None, 'argument names collide with each other')
         """
         try:
             tree = cls._parse(decl)
         except error.ParseError as inst:
             return (decl, None, parseerrordetail(inst))
 
         if tree[0] == cls._symbolnode:
             # "name = ...." style
             name = tree[1]
             if name.startswith(b'$'):
                 return (decl, None, _(b"invalid symbol '%s'") % name)
             return (name, None, None)
 
         func = cls._trygetfunc(tree)
         if func:
             # "name(arg, ....) = ...." style
             name, args = func
             if name.startswith(b'$'):
                 return (decl, None, _(b"invalid function '%s'") % name)
             if any(t[0] != cls._symbolnode for t in args):
                 return (decl, None, _(b"invalid argument list"))
             if len(args) != len(set(args)):
                 return (
                     name,
                     None,
                     _(b"argument names collide with each other"),
                 )
             return (name, [t[1] for t in args], None)
 
         return (decl, None, _(b"invalid format"))
 
     @classmethod
     def _relabelargs(cls, tree, args):
         """Mark alias arguments as ``_aliasarg``"""
         if not isinstance(tree, tuple):
             return tree
         op = tree[0]
         if op != cls._symbolnode:
             return (op,) + tuple(cls._relabelargs(x, args) for x in tree[1:])
 
         assert len(tree) == 2
         sym = tree[1]
         if sym in args:
             op = b'_aliasarg'
         elif sym.startswith(b'$'):
             raise error.ParseError(_(b"invalid symbol '%s'") % sym)
         return (op, sym)
 
     @classmethod
     def _builddefn(cls, defn, args):
         """Parse an alias definition into a tree and marks substitutions
 
         This function marks alias argument references as ``_aliasarg``. The
         parsing rule is provided by ``_parse()``.
 
         ``args`` is a list of alias argument names, or None if the alias
         is declared as a symbol.
 
         >>> from . import pycompat
         >>> parsemap = {
         ...     b'$1 or foo': (b'or', (b'symbol', b'$1'), (b'symbol', b'foo')),
         ...     b'$1 or $bar':
         ...         (b'or', (b'symbol', b'$1'), (b'symbol', b'$bar')),
         ...     b'$10 or baz':
         ...         (b'or', (b'symbol', b'$10'), (b'symbol', b'baz')),
         ...     b'"$1" or "foo"':
         ...         (b'or', (b'string', b'$1'), (b'string', b'foo')),
         ... }
         >>> class aliasrules(basealiasrules):
         ...     _parse = staticmethod(parsemap.__getitem__)
         ...     _trygetfunc = staticmethod(lambda x: None)
         >>> builddefn = aliasrules._builddefn
         >>> def pprint(tree):
         ...     s = prettyformat(tree, (b'_aliasarg', b'string', b'symbol'))
         ...     print(pycompat.sysstr(s))
         >>> args = [b'$1', b'$2', b'foo']
         >>> pprint(builddefn(b'$1 or foo', args))
         (or
           (_aliasarg '$1')
           (_aliasarg 'foo'))
         >>> try:
         ...     builddefn(b'$1 or $bar', args)
         ... except error.ParseError as inst:
         ...     print(pycompat.sysstr(parseerrordetail(inst)))
         invalid symbol '$bar'
         >>> args = [b'$1', b'$10', b'foo']
         >>> pprint(builddefn(b'$10 or baz', args))
         (or
           (_aliasarg '$10')
           (symbol 'baz'))
         >>> pprint(builddefn(b'"$1" or "foo"', args))
         (or
           (string '$1')
           (string 'foo'))
         """
         tree = cls._parse(defn)
         if args:
             args = set(args)
         else:
             args = set()
         return cls._relabelargs(tree, args)
 
     @classmethod
     def build(cls, decl, defn):
         """Parse an alias declaration and definition into an alias object"""
         repl = efmt = None
         name, args, err = cls._builddecl(decl)
         if err:
             efmt = _(b'bad declaration of %(section)s "%(name)s": %(error)s')
         else:
             try:
                 repl = cls._builddefn(defn, args)
             except error.ParseError as inst:
                 err = parseerrordetail(inst)
                 efmt = _(b'bad definition of %(section)s "%(name)s": %(error)s')
         if err:
             err = efmt % {
                 b'section': cls._section,
                 b'name': name,
                 b'error': err,
             }
         return alias(name, args, err, repl)
 
     @classmethod
     def buildmap(cls, items):
         """Parse a list of alias (name, replacement) pairs into a dict of
         alias objects"""
         aliases = {}
         for decl, defn in items:
             a = cls.build(decl, defn)
             aliases[a.name] = a
         return aliases
 
     @classmethod
     def _getalias(cls, aliases, tree):
         """If tree looks like an unexpanded alias, return (alias, pattern-args)
         pair. Return None otherwise.
         """
         if not isinstance(tree, tuple):
             return None
         if tree[0] == cls._symbolnode:
             name = tree[1]
             a = aliases.get(name)
             if a and a.args is None:
                 return a, None
         func = cls._trygetfunc(tree)
         if func:
             name, args = func
             a = aliases.get(name)
             if a and a.args is not None:
                 return a, args
         return None
 
     @classmethod
     def _expandargs(cls, tree, args):
         """Replace _aliasarg instances with the substitution value of the
         same name in args, recursively.
         """
         if not isinstance(tree, tuple):
             return tree
         if tree[0] == b'_aliasarg':
             sym = tree[1]
             return args[sym]
         return tuple(cls._expandargs(t, args) for t in tree)
 
     @classmethod
     def _expand(cls, aliases, tree, expanding, cache):
         if not isinstance(tree, tuple):
             return tree
         r = cls._getalias(aliases, tree)
         if r is None:
             return tuple(
                 cls._expand(aliases, t, expanding, cache) for t in tree
             )
         a, l = r
         if a.error:
             raise error.Abort(a.error)
         if a in expanding:
             raise error.ParseError(
                 _(b'infinite expansion of %(section)s "%(name)s" detected')
                 % {b'section': cls._section, b'name': a.name}
             )
         # get cacheable replacement tree by expanding aliases recursively
         expanding.append(a)
         if a.name not in cache:
             cache[a.name] = cls._expand(
                 aliases, a.replacement, expanding, cache
             )
         result = cache[a.name]
         expanding.pop()
         if a.args is None:
             return result
         # substitute function arguments in replacement tree
         if len(l) != len(a.args):
             raise error.ParseError(
                 _(b'invalid number of arguments: %d') % len(l)
             )
         l = [cls._expand(aliases, t, [], cache) for t in l]
         return cls._expandargs(result, dict(zip(a.args, l)))
 
     @classmethod
     def expand(cls, aliases, tree):
         """Expand aliases in tree, recursively.
 
         'aliases' is a dictionary mapping user defined aliases to alias objects.
         """
         return cls._expand(aliases, tree, [], {})