diff --git a/contrib/import-checker.py b/contrib/import-checker.py
--- a/contrib/import-checker.py
+++ b/contrib/import-checker.py
@@ -10,7 +10,7 @@
 # Import a minimal set of stdlib modules needed for list_stdlib_modules()
 # to work when run from a virtualenv.  The modules were chosen empirically
 # so that the return value matches the return value without virtualenv.
-if True: # disable lexical sorting checks
+if True:  # disable lexical sorting checks
     try:
         import BaseHTTPServer as basehttpserver
     except ImportError:
@@ -45,9 +45,7 @@
 )
 
 # Whitelist of symbols that can be directly imported.
-directsymbols = (
-    'demandimport',
-)
+directsymbols = ('demandimport',)
 
 # Modules that must be aliased because they are commonly confused with
 # common variables and can create aliasing and readability issues.
@@ -55,6 +53,7 @@
     'ui': 'uimod',
 }
 
+
 def usingabsolute(root):
     """Whether absolute imports are being used."""
     if sys.version_info[0] >= 3:
@@ -69,6 +68,7 @@
 
     return False
 
+
 def walklocal(root):
     """Recursively yield all descendant nodes but not in a different scope"""
     todo = collections.deque(ast.iter_child_nodes(root))
@@ -80,6 +80,7 @@
             todo.extend(ast.iter_child_nodes(node))
         yield node, newscope
 
+
 def dotted_name_of_path(path):
     """Given a relative path to a source file, return its dotted module name.
 
@@ -89,11 +90,12 @@
     'zlib'
     """
     parts = path.replace(os.sep, '/').split('/')
-    parts[-1] = parts[-1].split('.', 1)[0] # remove .py and .so and .ARCH.so
+    parts[-1] = parts[-1].split('.', 1)[0]  # remove .py and .so and .ARCH.so
     if parts[-1].endswith('module'):
         parts[-1] = parts[-1][:-6]
     return '.'.join(parts)
 
+
 def fromlocalfunc(modulename, localmods):
     """Get a function to examine which locally defined module the
     target source imports via a specified name.
@@ -162,6 +164,7 @@
     prefix = '.'.join(modulename.split('.')[:-1])
     if prefix:
         prefix += '.'
+
     def fromlocal(name, level=0):
         # name is false value when relative imports are used.
         if not name:
@@ -173,8 +176,9 @@
                 # Check relative name first.
                 candidates = [prefix + name, name]
             else:
-                candidates = ['.'.join(modulename.split('.')[:-level]) +
-                              '.' + name]
+                candidates = [
+                    '.'.join(modulename.split('.')[:-level]) + '.' + name
+                ]
 
         for n in candidates:
             if n in localmods:
@@ -183,18 +187,21 @@
             if dottedpath in localmods:
                 return (n, dottedpath, True)
         return False
+
     return fromlocal
 
+
 def populateextmods(localmods):
     """Populate C extension modules based on pure modules"""
     newlocalmods = set(localmods)
     for n in localmods:
         if n.startswith('mercurial.pure.'):
-            m = n[len('mercurial.pure.'):]
+            m = n[len('mercurial.pure.') :]
             newlocalmods.add('mercurial.cext.' + m)
             newlocalmods.add('mercurial.cffi._' + m)
     return newlocalmods
 
+
 def list_stdlib_modules():
     """List the modules present in the stdlib.
 
@@ -230,13 +237,13 @@
     for m in ['msvcrt', '_winreg']:
         yield m
     yield '__builtin__'
-    yield 'builtins' # python3 only
-    yield 'importlib.abc' # python3 only
-    yield 'importlib.machinery' # python3 only
-    yield 'importlib.util' # python3 only
+    yield 'builtins'  # python3 only
+    yield 'importlib.abc'  # python3 only
+    yield 'importlib.machinery'  # python3 only
+    yield 'importlib.util'  # python3 only
     for m in 'fcntl', 'grp', 'pwd', 'termios':  # Unix only
         yield m
-    for m in 'cPickle', 'datetime': # in Python (not C) on PyPy
+    for m in 'cPickle', 'datetime':  # in Python (not C) on PyPy
         yield m
     for m in ['cffi']:
         yield m
@@ -262,14 +269,17 @@
     for libpath in sys.path:
         # We want to walk everything in sys.path that starts with something in
         # stdlib_prefixes, but not directories from the hg sources.
-        if (os.path.abspath(libpath).startswith(sourceroot)
-            or not any(libpath.startswith(p) for p in stdlib_prefixes)):
+        if os.path.abspath(libpath).startswith(sourceroot) or not any(
+            libpath.startswith(p) for p in stdlib_prefixes
+        ):
             continue
         for top, dirs, files in os.walk(libpath):
             for i, d in reversed(list(enumerate(dirs))):
-                if (not os.path.exists(os.path.join(top, d, '__init__.py'))
-                    or top == libpath and d in ('hgdemandimport', 'hgext',
-                                                'mercurial')):
+                if (
+                    not os.path.exists(os.path.join(top, d, '__init__.py'))
+                    or top == libpath
+                    and d in ('hgdemandimport', 'hgext', 'mercurial')
+                ):
                     del dirs[i]
             for name in files:
                 if not name.endswith(('.py', '.so', '.pyc', '.pyo', '.pyd')):
@@ -278,12 +288,14 @@
                     full_path = top
                 else:
                     full_path = os.path.join(top, name)
-                rel_path = full_path[len(libpath) + 1:]
+                rel_path = full_path[len(libpath) + 1 :]
                 mod = dotted_name_of_path(rel_path)
                 yield mod
 
+
 stdlib_modules = set(list_stdlib_modules())
 
+
 def imported_modules(source, modulename, f, localmods, ignore_nested=False):
     """Given the source of a file as a string, yield the names
     imported by that file.
@@ -381,6 +393,7 @@
                 # lookup
                 yield dottedpath
 
+
 def verify_import_convention(module, source, localmods):
     """Verify imports match our established coding convention.
 
@@ -398,6 +411,7 @@
     else:
         return verify_stdlib_on_own_line(root)
 
+
 def verify_modern_convention(module, root, localmods, root_col_offset=0):
     """Verify a file conforms to the modern import convention rules.
 
@@ -441,19 +455,24 @@
     seenlevels = set()
 
     for node, newscope in walklocal(root):
+
         def msg(fmt, *args):
             return (fmt % args, node.lineno)
+
         if newscope:
             # Check for local imports in function
-            for r in verify_modern_convention(module, node, localmods,
-                                              node.col_offset + 4):
+            for r in verify_modern_convention(
+                module, node, localmods, node.col_offset + 4
+            ):
                 yield r
         elif isinstance(node, ast.Import):
             # Disallow "import foo, bar" and require separate imports
             # for each module.
             if len(node.names) > 1:
-                yield msg('multiple imported names: %s',
-                          ', '.join(n.name for n in node.names))
+                yield msg(
+                    'multiple imported names: %s',
+                    ', '.join(n.name for n in node.names),
+                )
 
             name = node.names[0].name
             asname = node.names[0].asname
@@ -463,16 +482,20 @@
             # Ignore sorting rules on imports inside blocks.
             if node.col_offset == root_col_offset:
                 if lastname and name < lastname and laststdlib == stdlib:
-                    yield msg('imports not lexically sorted: %s < %s',
-                              name, lastname)
+                    yield msg(
+                        'imports not lexically sorted: %s < %s', name, lastname
+                    )
 
             lastname = name
             laststdlib = stdlib
 
             # stdlib imports should be before local imports.
             if stdlib and seenlocal and node.col_offset == root_col_offset:
-                yield msg('stdlib import "%s" follows local import: %s',
-                          name, seenlocal)
+                yield msg(
+                    'stdlib import "%s" follows local import: %s',
+                    name,
+                    seenlocal,
+                )
 
             if not stdlib:
                 seenlocal = name
@@ -483,13 +506,16 @@
                 yield msg('import should be relative: %s', name)
 
             if name in requirealias and asname != requirealias[name]:
-                yield msg('%s module must be "as" aliased to %s',
-                          name, requirealias[name])
+                yield msg(
+                    '%s module must be "as" aliased to %s',
+                    name,
+                    requirealias[name],
+                )
 
         elif isinstance(node, ast.ImportFrom):
             # Resolve the full imported module name.
             if node.level > 0:
-                fullname = '.'.join(module.split('.')[:-node.level])
+                fullname = '.'.join(module.split('.')[: -node.level])
                 if node.module:
                     fullname += '.%s' % node.module
             else:
@@ -506,7 +532,8 @@
                 if not fullname or (
                     fullname in stdlib_modules
                     and fullname not in localmods
-                    and fullname + '.__init__' not in localmods):
+                    and fullname + '.__init__' not in localmods
+                ):
                     yield msg('relative import of stdlib module')
                 else:
                     seenlocal = fullname
@@ -516,19 +543,24 @@
             found = fromlocal(node.module, node.level)
             if found and found[2]:  # node.module is a package
                 prefix = found[0] + '.'
-                symbols = (n.name for n in node.names
-                           if not fromlocal(prefix + n.name))
+                symbols = (
+                    n.name for n in node.names if not fromlocal(prefix + n.name)
+                )
             else:
                 symbols = (n.name for n in node.names)
             symbols = [sym for sym in symbols if sym not in directsymbols]
             if node.module and node.col_offset == root_col_offset:
                 if symbols and fullname not in allowsymbolimports:
-                    yield msg('direct symbol import %s from %s',
-                              ', '.join(symbols), fullname)
+                    yield msg(
+                        'direct symbol import %s from %s',
+                        ', '.join(symbols),
+                        fullname,
+                    )
 
                 if symbols and seennonsymbollocal:
-                    yield msg('symbol import follows non-symbol import: %s',
-                              fullname)
+                    yield msg(
+                        'symbol import follows non-symbol import: %s', fullname
+                    )
             if not symbols and fullname not in stdlib_modules:
                 seennonsymbollocal = True
 
@@ -536,15 +568,19 @@
                 assert node.level
 
                 # Only allow 1 group per level.
-                if (node.level in seenlevels
-                    and node.col_offset == root_col_offset):
-                    yield msg('multiple "from %s import" statements',
-                              '.' * node.level)
+                if (
+                    node.level in seenlevels
+                    and node.col_offset == root_col_offset
+                ):
+                    yield msg(
+                        'multiple "from %s import" statements', '.' * node.level
+                    )
 
                 # Higher-level groups come before lower-level groups.
                 if any(node.level > l for l in seenlevels):
-                    yield msg('higher-level import should come first: %s',
-                              fullname)
+                    yield msg(
+                        'higher-level import should come first: %s', fullname
+                    )
 
                 seenlevels.add(node.level)
 
@@ -554,14 +590,23 @@
 
             for n in node.names:
                 if lastentryname and n.name < lastentryname:
-                    yield msg('imports from %s not lexically sorted: %s < %s',
-                              fullname, n.name, lastentryname)
+                    yield msg(
+                        'imports from %s not lexically sorted: %s < %s',
+                        fullname,
+                        n.name,
+                        lastentryname,
+                    )
 
                 lastentryname = n.name
 
                 if n.name in requirealias and n.asname != requirealias[n.name]:
-                    yield msg('%s from %s must be "as" aliased to %s',
-                              n.name, fullname, requirealias[n.name])
+                    yield msg(
+                        '%s from %s must be "as" aliased to %s',
+                        n.name,
+                        fullname,
+                        requirealias[n.name],
+                    )
+
 
 def verify_stdlib_on_own_line(root):
     """Given some python source, verify that stdlib imports are done
@@ -580,13 +625,20 @@
             for n in node.names:
                 from_stdlib[n.name in stdlib_modules].append(n.name)
             if from_stdlib[True] and from_stdlib[False]:
-                yield ('mixed imports\n   stdlib:    %s\n   relative:  %s' %
-                       (', '.join(sorted(from_stdlib[True])),
-                        ', '.join(sorted(from_stdlib[False]))), node.lineno)
+                yield (
+                    'mixed imports\n   stdlib:    %s\n   relative:  %s'
+                    % (
+                        ', '.join(sorted(from_stdlib[True])),
+                        ', '.join(sorted(from_stdlib[False])),
+                    ),
+                    node.lineno,
+                )
+
 
 class CircularImport(Exception):
     pass
 
+
 def checkmod(mod, imports):
     shortest = {}
     visit = [[mod]]
@@ -601,6 +653,7 @@
                     continue
                 visit.append(path + [i])
 
+
 def rotatecycle(cycle):
     """arrange a cycle so that the lexicographically first module listed first
 
@@ -611,6 +664,7 @@
     idx = cycle.index(lowest)
     return cycle[idx:] + cycle[:idx] + [lowest]
 
+
 def find_cycles(imports):
     """Find cycles in an already-loaded import graph.
 
@@ -634,9 +688,11 @@
             cycles.add(" -> ".join(rotatecycle(cycle)))
     return cycles
 
+
 def _cycle_sortkey(c):
     return len(c), c
 
+
 def embedded(f, modname, src):
     """Extract embedded python code
 
@@ -678,6 +734,7 @@
             modname = modname.decode('utf8')
         yield code, "%s[%d]" % (modname, starts), name, starts - 1
 
+
 def sources(f, modname):
     """Yields possibly multiple sources from a filepath
 
@@ -698,6 +755,7 @@
             for script, modname, t, line in embedded(f, modname, src):
                 yield script, modname.encode('utf8'), t, line
 
+
 def main(argv):
     if len(argv) < 2 or (argv[1] == '-' and len(argv) > 2):
         print('Usage: %s {-|file [file] [file] ...}')
@@ -719,15 +777,19 @@
         for src, modname, name, line in sources(source_path, localmodname):
             try:
                 used_imports[modname] = sorted(
-                    imported_modules(src, modname, name, localmods,
-                                     ignore_nested=True))
-                for error, lineno in verify_import_convention(modname, src,
-                                                              localmods):
+                    imported_modules(
+                        src, modname, name, localmods, ignore_nested=True
+                    )
+                )
+                for error, lineno in verify_import_convention(
+                    modname, src, localmods
+                ):
                     any_errors = True
                     print('%s:%d: %s' % (source_path, lineno + line, error))
             except SyntaxError as e:
-                print('%s:%d: SyntaxError: %s' %
-                      (source_path, e.lineno + line, e))
+                print(
+                    '%s:%d: SyntaxError: %s' % (source_path, e.lineno + line, e)
+                )
     cycles = find_cycles(used_imports)
     if cycles:
         firstmods = set()
@@ -743,5 +805,6 @@
         any_errors = True
     return any_errors != 0
 
+
 if __name__ == '__main__':
     sys.exit(int(main(sys.argv)))
diff --git a/mercurial/cacheutil.py b/mercurial/cacheutil.py
--- a/mercurial/cacheutil.py
+++ b/mercurial/cacheutil.py
@@ -8,6 +8,7 @@
 
 from . import repoview
 
+
 def cachetocopy(srcrepo):
     """return the list of cache file valuable to copy during a clone"""
     # In local clones we're copying all nodes, not just served
diff --git a/mercurial/diffhelper.py b/mercurial/diffhelper.py
--- a/mercurial/diffhelper.py
+++ b/mercurial/diffhelper.py
@@ -14,6 +14,7 @@
     pycompat,
 )
 
+
 def addlines(fp, hunk, lena, lenb, a, b):
     """Read lines from fp into the hunk
 
@@ -47,6 +48,7 @@
                 b.append(s[1:])
                 a.append(s)
 
+
 def fixnewline(hunk, a, b):
     """Fix up the last lines of a and b when the patch has no newline at EOF"""
     l = hunk[-1]
@@ -62,6 +64,7 @@
         a[-1] = hline
     hunk[-1] = hline
 
+
 def testhunk(a, b, bstart):
     """Compare the lines in a with the lines in b
 
diff --git a/mercurial/dirstateguard.py b/mercurial/dirstateguard.py
--- a/mercurial/dirstateguard.py
+++ b/mercurial/dirstateguard.py
@@ -15,6 +15,7 @@
     util,
 )
 
+
 class dirstateguard(util.transactional):
     '''Restore dirstate at unexpected failure.
 
@@ -34,14 +35,16 @@
         self._active = False
         self._closed = False
         self._backupname = 'dirstate.backup.%s.%d' % (name, id(self))
-        self._narrowspecbackupname = ('narrowspec.backup.%s.%d' %
-                                      (name, id(self)))
+        self._narrowspecbackupname = 'narrowspec.backup.%s.%d' % (
+            name,
+            id(self),
+        )
         repo.dirstate.savebackup(repo.currenttransaction(), self._backupname)
         narrowspec.savewcbackup(repo, self._narrowspecbackupname)
         self._active = True
 
     def __del__(self):
-        if self._active: # still active
+        if self._active:  # still active
             # this may occur, even if this class is used correctly:
             # for example, releasing other resources like transaction
             # may raise exception before ``dirstateguard.release`` in
@@ -49,27 +52,33 @@
             self._abort()
 
     def close(self):
-        if not self._active: # already inactivated
-            msg = (_("can't close already inactivated backup: %s")
-                   % self._backupname)
+        if not self._active:  # already inactivated
+            msg = (
+                _("can't close already inactivated backup: %s")
+                % self._backupname
+            )
             raise error.Abort(msg)
 
-        self._repo.dirstate.clearbackup(self._repo.currenttransaction(),
-                                         self._backupname)
+        self._repo.dirstate.clearbackup(
+            self._repo.currenttransaction(), self._backupname
+        )
         narrowspec.clearwcbackup(self._repo, self._narrowspecbackupname)
         self._active = False
         self._closed = True
 
     def _abort(self):
         narrowspec.restorewcbackup(self._repo, self._narrowspecbackupname)
-        self._repo.dirstate.restorebackup(self._repo.currenttransaction(),
-                                           self._backupname)
+        self._repo.dirstate.restorebackup(
+            self._repo.currenttransaction(), self._backupname
+        )
         self._active = False
 
     def release(self):
         if not self._closed:
-            if not self._active: # already inactivated
-                msg = (_("can't release already inactivated backup: %s")
-                       % self._backupname)
+            if not self._active:  # already inactivated
+                msg = (
+                    _("can't release already inactivated backup: %s")
+                    % self._backupname
+                )
                 raise error.Abort(msg)
             self._abort()
diff --git a/mercurial/httpconnection.py b/mercurial/httpconnection.py
--- a/mercurial/httpconnection.py
+++ b/mercurial/httpconnection.py
@@ -43,8 +43,9 @@
         # requires authentication. Since we can't know until we try
         # once whether authentication will be required, just lie to
         # the user and maybe the push succeeds suddenly at 50%.
-        self._progress = ui.makeprogress(_('sending'), unit=_('kb'),
-                                         total=(self.length // 1024 * 2))
+        self._progress = ui.makeprogress(
+            _('sending'), unit=_('kb'), total=(self.length // 1024 * 2)
+        )
 
     def read(self, *args, **kwargs):
         ret = self._data.read(*args, **kwargs)
@@ -61,6 +62,7 @@
     def __exit__(self, exc_type, exc_val, exc_tb):
         self.close()
 
+
 # moved here from url.py to avoid a cycle
 def readauthforuri(ui, uri, user):
     uri = pycompat.bytesurl(uri)
@@ -109,10 +111,18 @@
             schemes, prefix = [p[0]], p[1]
         else:
             schemes = (auth.get('schemes') or 'https').split()
-        if ((prefix == '*' or hostpath.startswith(prefix)) and
-            (len(prefix) > bestlen or (len(prefix) == bestlen and
-                                       not bestuser and 'username' in auth))
-            and scheme in schemes):
+        if (
+            (prefix == '*' or hostpath.startswith(prefix))
+            and (
+                len(prefix) > bestlen
+                or (
+                    len(prefix) == bestlen
+                    and not bestuser
+                    and 'username' in auth
+                )
+            )
+            and scheme in schemes
+        ):
             bestlen = len(prefix)
             bestauth = group, auth
             bestuser = auth.get('username')
diff --git a/mercurial/minifileset.py b/mercurial/minifileset.py
--- a/mercurial/minifileset.py
+++ b/mercurial/minifileset.py
@@ -15,11 +15,13 @@
     pycompat,
 )
 
+
 def _sizep(x):
     # i18n: "size" is a keyword
     expr = filesetlang.getstring(x, _("size requires an expression"))
     return fileset.sizematcher(expr)
 
+
 def _compile(tree):
     if not tree:
         raise error.ParseError(_("missing argument"))
@@ -28,20 +30,23 @@
         return _compile(tree[1])
     elif op in {'symbol', 'string', 'kindpat'}:
         name = filesetlang.getpattern(tree, {'path'}, _('invalid file pattern'))
-        if name.startswith('**'): # file extension test, ex. "**.tar.gz"
+        if name.startswith('**'):  # file extension test, ex. "**.tar.gz"
             ext = name[2:]
             for c in pycompat.bytestr(ext):
                 if c in '*{}[]?/\\':
                     raise error.ParseError(_('reserved character: %s') % c)
             return lambda n, s: n.endswith(ext)
-        elif name.startswith('path:'): # directory or full path test
-            p = name[5:] # prefix
+        elif name.startswith('path:'):  # directory or full path test
+            p = name[5:]  # prefix
             pl = len(p)
-            f = lambda n, s: n.startswith(p) and (len(n) == pl
-                                                  or n[pl:pl + 1] == '/')
+            f = lambda n, s: n.startswith(p) and (
+                len(n) == pl or n[pl : pl + 1] == '/'
+            )
             return f
-        raise error.ParseError(_("unsupported file pattern: %s") % name,
-                               hint=_('paths must be prefixed with "path:"'))
+        raise error.ParseError(
+            _("unsupported file pattern: %s") % name,
+            hint=_('paths must be prefixed with "path:"'),
+        )
     elif op in {'or', 'patterns'}:
         funcs = [_compile(x) for x in tree[1:]]
         return lambda n, s: any(f(n, s) for f in funcs)
@@ -63,15 +68,18 @@
             return symbols[name]
 
         raise error.UnknownIdentifier(name, symbols.keys())
-    elif op == 'minus':     # equivalent to 'x and not y'
+    elif op == 'minus':  # equivalent to 'x and not y'
         func1 = _compile(tree[1])
         func2 = _compile(tree[2])
         return lambda n, s: func1(n, s) and not func2(n, s)
     elif op == 'list':
-        raise error.ParseError(_("can't use a list in this context"),
-                               hint=_('see \'hg help "filesets.x or y"\''))
+        raise error.ParseError(
+            _("can't use a list in this context"),
+            hint=_('see \'hg help "filesets.x or y"\''),
+        )
     raise error.ProgrammingError('illegal tree: %r' % (tree,))
 
+
 def compile(text):
     """generate a function (path, size) -> bool from filter specification.
 
diff --git a/mercurial/node.py b/mercurial/node.py
--- a/mercurial/node.py
+++ b/mercurial/node.py
@@ -20,6 +20,7 @@
     except binascii.Error as e:
         raise TypeError(e)
 
+
 nullrev = -1
 # In hex, this is '0000000000000000000000000000000000000000'
 nullid = b"\0" * 20
@@ -38,10 +39,11 @@
 
 # pseudo identifiers for working directory
 # (they are experimental, so don't add too many dependencies on them)
-wdirrev = 0x7fffffff
+wdirrev = 0x7FFFFFFF
 # In hex, this is 'ffffffffffffffffffffffffffffffffffffffff'
 wdirid = b"\xff" * 20
 wdirhex = hex(wdirid)
 
+
 def short(node):
     return hex(node[:6])
diff --git a/mercurial/policy.py b/mercurial/policy.py
--- a/mercurial/policy.py
+++ b/mercurial/policy.py
@@ -33,6 +33,7 @@
 
 try:
     from . import __modulepolicy__
+
     policy = __modulepolicy__.modulepolicy
 except ImportError:
     pass
@@ -51,6 +52,7 @@
 else:
     policy = os.environ.get(r'HGMODULEPOLICY', policy)
 
+
 def _importfrom(pkgname, modname):
     # from .<pkgname> import <modname> (where . is looked through this module)
     fakelocals = {}
@@ -63,6 +65,7 @@
     getattr(mod, r'__doc__', None)
     return fakelocals[modname]
 
+
 # keep in sync with "version" in C modules
 _cextversions = {
     (r'cext', r'base85'): 1,
@@ -80,13 +83,17 @@
     (r'cffi', r'parsers'): (r'pure', r'parsers'),
 }
 
+
 def _checkmod(pkgname, modname, mod):
     expected = _cextversions.get((pkgname, modname))
     actual = getattr(mod, r'version', None)
     if actual != expected:
-        raise ImportError(r'cannot import module %s.%s '
-                          r'(expected version: %d, actual: %r)'
-                          % (pkgname, modname, expected, actual))
+        raise ImportError(
+            r'cannot import module %s.%s '
+            r'(expected version: %d, actual: %r)'
+            % (pkgname, modname, expected, actual)
+        )
+
 
 def importmod(modname):
     """Import module according to policy and check API version"""
diff --git a/mercurial/pushkey.py b/mercurial/pushkey.py
--- a/mercurial/pushkey.py
+++ b/mercurial/pushkey.py
@@ -14,6 +14,7 @@
     phases,
 )
 
+
 def _nslist(repo):
     n = {}
     for k in _namespaces:
@@ -22,36 +23,45 @@
         n.pop('obsolete')
     return n
 
-_namespaces = {"namespaces": (lambda *x: False, _nslist),
-               "bookmarks": (bookmarks.pushbookmark, bookmarks.listbookmarks),
-               "phases": (phases.pushphase, phases.listphases),
-               "obsolete": (obsolete.pushmarker, obsolete.listmarkers),
-              }
+
+_namespaces = {
+    "namespaces": (lambda *x: False, _nslist),
+    "bookmarks": (bookmarks.pushbookmark, bookmarks.listbookmarks),
+    "phases": (phases.pushphase, phases.listphases),
+    "obsolete": (obsolete.pushmarker, obsolete.listmarkers),
+}
+
 
 def register(namespace, pushkey, listkeys):
     _namespaces[namespace] = (pushkey, listkeys)
 
+
 def _get(namespace):
     return _namespaces.get(namespace, (lambda *x: False, lambda *x: {}))
 
+
 def push(repo, namespace, key, old, new):
     '''should succeed iff value was old'''
     pk = _get(namespace)[0]
     return pk(repo, key, old, new)
 
+
 def list(repo, namespace):
     '''return a dict'''
     lk = _get(namespace)[1]
     return lk(repo)
 
+
 encode = encoding.fromlocal
 
 decode = encoding.tolocal
 
+
 def encodekeys(keys):
     """encode the content of a pushkey namespace for exchange over the wire"""
     return '\n'.join(['%s\t%s' % (encode(k), encode(v)) for k, v in keys])
 
+
 def decodekeys(data):
     """decode the content of a pushkey namespace from exchange over the wire"""
     result = {}
diff --git a/mercurial/rcutil.py b/mercurial/rcutil.py
--- a/mercurial/rcutil.py
+++ b/mercurial/rcutil.py
@@ -24,15 +24,18 @@
 systemrcpath = scmplatform.systemrcpath
 userrcpath = scmplatform.userrcpath
 
+
 def _expandrcpath(path):
     '''path could be a file or a directory. return a list of file paths'''
     p = util.expandpath(path)
     if os.path.isdir(p):
         join = os.path.join
-        return sorted(join(p, f) for f, k in util.listdir(p)
-                      if f.endswith('.rc'))
+        return sorted(
+            join(p, f) for f, k in util.listdir(p) if f.endswith('.rc')
+        )
     return [p]
 
+
 def envrcitems(env=None):
     '''Return [(section, name, value, source)] config items.
 
@@ -55,6 +58,7 @@
         result.append((section, configname, env[envname], '$%s' % envname))
     return result
 
+
 def defaultrcpath():
     '''return rc paths in default.d'''
     path = []
@@ -63,6 +67,7 @@
         path = _expandrcpath(defaultpath)
     return path
 
+
 def rccomponents():
     '''return an ordered [(type, obj)] about where to load configs.
 
@@ -92,6 +97,7 @@
         _rccomponents.extend(normpaths(userrcpath()))
     return _rccomponents
 
+
 def defaultpagerenv():
     '''return a dict of default environment variables and their values,
     intended to be set before starting a pager.
diff --git a/mercurial/rewriteutil.py b/mercurial/rewriteutil.py
--- a/mercurial/rewriteutil.py
+++ b/mercurial/rewriteutil.py
@@ -16,6 +16,7 @@
     revset,
 )
 
+
 def precheck(repo, revs, action='rewrite'):
     """check if revs can be rewritten
     action is used to control the error message.
@@ -40,6 +41,7 @@
     if newunstable:
         raise error.Abort(_("cannot %s changeset with children") % action)
 
+
 def disallowednewunstable(repo, revs):
     """Checks whether editing the revs will create new unstable changesets and
     are we allowed to create them.
diff --git a/mercurial/scmposix.py b/mercurial/scmposix.py
--- a/mercurial/scmposix.py
+++ b/mercurial/scmposix.py
@@ -18,17 +18,23 @@
 # 'less' as the default seems reasonable.
 fallbackpager = 'less'
 
+
 def _rcfiles(path):
     rcs = [os.path.join(path, 'hgrc')]
     rcdir = os.path.join(path, 'hgrc.d')
     try:
-        rcs.extend([os.path.join(rcdir, f)
-                    for f, kind in util.listdir(rcdir)
-                    if f.endswith(".rc")])
+        rcs.extend(
+            [
+                os.path.join(rcdir, f)
+                for f, kind in util.listdir(rcdir)
+                if f.endswith(".rc")
+            ]
+        )
     except OSError:
         pass
     return rcs
 
+
 def systemrcpath():
     path = []
     if pycompat.sysplatform == 'plan9':
@@ -43,6 +49,7 @@
     path.extend(_rcfiles('/' + root))
     return path
 
+
 def userrcpath():
     if pycompat.sysplatform == 'plan9':
         return [encoding.environ['home'] + '/lib/hgrc']
@@ -53,12 +60,16 @@
         if confighome is None or not os.path.isabs(confighome):
             confighome = os.path.expanduser('~/.config')
 
-        return [os.path.expanduser('~/.hgrc'),
-                os.path.join(confighome, 'hg', 'hgrc')]
+        return [
+            os.path.expanduser('~/.hgrc'),
+            os.path.join(confighome, 'hg', 'hgrc'),
+        ]
+
 
 def termsize(ui):
     try:
         import termios
+
         TIOCGWINSZ = termios.TIOCGWINSZ  # unavailable on IRIX (issue3449)
     except (AttributeError, ImportError):
         return 80, 24
diff --git a/mercurial/scmwindows.py b/mercurial/scmwindows.py
--- a/mercurial/scmwindows.py
+++ b/mercurial/scmwindows.py
@@ -11,6 +11,7 @@
 
 try:
     import _winreg as winreg
+
     winreg.CloseKey
 except ImportError:
     import winreg
@@ -18,6 +19,7 @@
 # MS-DOS 'more' is the only pager available by default on Windows.
 fallbackpager = 'more'
 
+
 def systemrcpath():
     '''return default os-specific hgrc search path'''
     rcpath = []
@@ -32,8 +34,9 @@
             if f.endswith('.rc'):
                 rcpath.append(os.path.join(progrcd, f))
     # else look for a system rcpath in the registry
-    value = util.lookupreg('SOFTWARE\\Mercurial', None,
-                           winreg.HKEY_LOCAL_MACHINE)
+    value = util.lookupreg(
+        'SOFTWARE\\Mercurial', None, winreg.HKEY_LOCAL_MACHINE
+    )
     if not isinstance(value, str) or not value:
         return rcpath
     value = util.localpath(value)
@@ -46,16 +49,17 @@
                     rcpath.append(os.path.join(p, f))
     return rcpath
 
+
 def userrcpath():
     '''return os-specific hgrc search path to the user dir'''
     home = os.path.expanduser('~')
-    path = [os.path.join(home, 'mercurial.ini'),
-            os.path.join(home, '.hgrc')]
+    path = [os.path.join(home, 'mercurial.ini'), os.path.join(home, '.hgrc')]
     userprofile = encoding.environ.get('USERPROFILE')
     if userprofile and userprofile != home:
         path.append(os.path.join(userprofile, 'mercurial.ini'))
         path.append(os.path.join(userprofile, '.hgrc'))
     return path
 
+
 def termsize(ui):
     return win32.termsize()
diff --git a/mercurial/stack.py b/mercurial/stack.py
--- a/mercurial/stack.py
+++ b/mercurial/stack.py
@@ -12,6 +12,7 @@
     scmutil,
 )
 
+
 def getstack(repo, rev=None):
     """return a sorted smartrev of the stack containing either rev if it is
     not None or the current working directory parent.
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,3 @@
+[tool.black]
+line-length = 80
+exclude = 'build/|wheelhouse/|dist/|packages/|\.hg/|\.mypy_cache/|\.venv/|mercurial/thirdparty/|hgext/fsmonitor/pywatchman/|contrib/python-zstandard/'