contrib: allow importing "symbols" from mercurial

contrib/import-checker.py

@@ -1,762 +1,763
 #!/usr/bin/env python
 
 from __future__ import absolute_import, print_function
 
 import ast
 import collections
 import os
 import re
 import sys
 
 # 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
     try:
         import BaseHTTPServer as basehttpserver
     except ImportError:
         basehttpserver = None
     import zlib
 
 # Whitelist of modules that symbols can be directly imported from.
 allowsymbolimports = (
     '__future__',
+    'mercurial',
     'mercurial.hgweb.common',
     'mercurial.hgweb.request',
     'mercurial.i18n',
     'mercurial.node',
     # for cffi modules to re-export pure functions
     'mercurial.pure.base85',
     'mercurial.pure.bdiff',
     'mercurial.pure.diffhelpers',
     'mercurial.pure.mpatch',
     'mercurial.pure.osutil',
     'mercurial.pure.parsers',
 )
 
 # Whitelist of symbols that can be directly imported.
 directsymbols = (
     'demandimport',
 )
 
 # Modules that must be aliased because they are commonly confused with
 # common variables and can create aliasing and readability issues.
 requirealias = {
     'ui': 'uimod',
 }
 
 def usingabsolute(root):
     """Whether absolute imports are being used."""
     if sys.version_info[0] >= 3:
         return True
 
     for node in ast.walk(root):
         if isinstance(node, ast.ImportFrom):
             if node.module == '__future__':
                 for n in node.names:
                     if n.name == 'absolute_import':
                         return True
 
     return False
 
 def walklocal(root):
     """Recursively yield all descendant nodes but not in a different scope"""
     todo = collections.deque(ast.iter_child_nodes(root))
     yield root, False
     while todo:
         node = todo.popleft()
         newscope = isinstance(node, ast.FunctionDef)
         if not newscope:
             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.
 
     >>> dotted_name_of_path('mercurial/error.py')
     'mercurial.error'
     >>> dotted_name_of_path('zlibmodule.so')
     'zlib'
     """
     parts = path.replace(os.sep, '/').split('/')
     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.
 
     `modulename` is an `dotted_name_of_path()`-ed source file path,
     which may have `.__init__` at the end of it, of the target source.
 
     `localmods` is a set of absolute `dotted_name_of_path()`-ed source file
     paths of locally defined (= Mercurial specific) modules.
 
     This function assumes that module names not existing in
     `localmods` are from the Python standard library.
 
     This function returns the function, which takes `name` argument,
     and returns `(absname, dottedpath, hassubmod)` tuple if `name`
     matches against locally defined module. Otherwise, it returns
     False.
 
     It is assumed that `name` doesn't have `.__init__`.
 
     `absname` is an absolute module name of specified `name`
     (e.g. "hgext.convert"). This can be used to compose prefix for sub
     modules or so.
 
     `dottedpath` is a `dotted_name_of_path()`-ed source file path
     (e.g. "hgext.convert.__init__") of `name`. This is used to look
     module up in `localmods` again.
 
     `hassubmod` is whether it may have sub modules under it (for
     convenient, even though this is also equivalent to "absname !=
     dottednpath")
 
     >>> localmods = {'foo.__init__', 'foo.foo1',
     ...              'foo.bar.__init__', 'foo.bar.bar1',
     ...              'baz.__init__', 'baz.baz1'}
     >>> fromlocal = fromlocalfunc('foo.xxx', localmods)
     >>> # relative
     >>> fromlocal('foo1')
     ('foo.foo1', 'foo.foo1', False)
     >>> fromlocal('bar')
     ('foo.bar', 'foo.bar.__init__', True)
     >>> fromlocal('bar.bar1')
     ('foo.bar.bar1', 'foo.bar.bar1', False)
     >>> # absolute
     >>> fromlocal('baz')
     ('baz', 'baz.__init__', True)
     >>> fromlocal('baz.baz1')
     ('baz.baz1', 'baz.baz1', False)
     >>> # unknown = maybe standard library
     >>> fromlocal('os')
     False
     >>> fromlocal(None, 1)
     ('foo', 'foo.__init__', True)
     >>> fromlocal('foo1', 1)
     ('foo.foo1', 'foo.foo1', False)
     >>> fromlocal2 = fromlocalfunc('foo.xxx.yyy', localmods)
     >>> fromlocal2(None, 2)
     ('foo', 'foo.__init__', True)
     >>> fromlocal2('bar2', 1)
     False
     >>> fromlocal2('bar', 2)
     ('foo.bar', 'foo.bar.__init__', True)
     """
     if not isinstance(modulename, str):
         modulename = modulename.decode('ascii')
     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:
             # If relative imports are used, level must not be absolute.
             assert level > 0
             candidates = ['.'.join(modulename.split('.')[:-level])]
         else:
             if not level:
                 # Check relative name first.
                 candidates = [prefix + name, name]
             else:
                 candidates = ['.'.join(modulename.split('.')[:-level]) +
                               '.' + name]
 
         for n in candidates:
             if n in localmods:
                 return (n, n, False)
             dottedpath = n + '.__init__'
             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.'):]
             newlocalmods.add('mercurial.cext.' + m)
             newlocalmods.add('mercurial.cffi._' + m)
     return newlocalmods
 
 def list_stdlib_modules():
     """List the modules present in the stdlib.
 
     >>> py3 = sys.version_info[0] >= 3
     >>> mods = set(list_stdlib_modules())
     >>> 'BaseHTTPServer' in mods or py3
     True
 
     os.path isn't really a module, so it's missing:
 
     >>> 'os.path' in mods
     False
 
     sys requires special treatment, because it's baked into the
     interpreter, but it should still appear:
 
     >>> 'sys' in mods
     True
 
     >>> 'collections' in mods
     True
 
     >>> 'cStringIO' in mods or py3
     True
 
     >>> 'cffi' in mods
     True
     """
     for m in sys.builtin_module_names:
         yield m
     # These modules only exist on windows, but we should always
     # consider them stdlib.
     for m in ['msvcrt', '_winreg']:
         yield m
     yield 'builtins' # python3 only
     for m in 'fcntl', 'grp', 'pwd', 'termios':  # Unix only
         yield m
     for m in 'cPickle', 'datetime': # in Python (not C) on PyPy
         yield m
     for m in ['cffi']:
         yield m
     stdlib_prefixes = {sys.prefix, sys.exec_prefix}
     # We need to supplement the list of prefixes for the search to work
     # when run from within a virtualenv.
     for mod in (basehttpserver, zlib):
         if mod is None:
             continue
         try:
             # Not all module objects have a __file__ attribute.
             filename = mod.__file__
         except AttributeError:
             continue
         dirname = os.path.dirname(filename)
         for prefix in stdlib_prefixes:
             if dirname.startswith(prefix):
                 # Then this directory is redundant.
                 break
         else:
             stdlib_prefixes.add(dirname)
     for libpath in sys.path:
         # We want to walk everything in sys.path that starts with
         # something in stdlib_prefixes.
         if 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')):
                     del dirs[i]
             for name in files:
                 if not name.endswith(('.py', '.so', '.pyc', '.pyo', '.pyd')):
                     continue
                 if name.startswith('__init__.py'):
                     full_path = top
                 else:
                     full_path = os.path.join(top, name)
                 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.
 
     Args:
       source: The python source to examine as a string.
       modulename: of specified python source (may have `__init__`)
       localmods: set of locally defined module names (may have `__init__`)
       ignore_nested: If true, import statements that do not start in
                      column zero will be ignored.
 
     Returns:
       A list of absolute module names imported by the given source.
 
     >>> f = 'foo/xxx.py'
     >>> modulename = 'foo.xxx'
     >>> localmods = {'foo.__init__': True,
     ...              'foo.foo1': True, 'foo.foo2': True,
     ...              'foo.bar.__init__': True, 'foo.bar.bar1': True,
     ...              'baz.__init__': True, 'baz.baz1': True }
     >>> # standard library (= not locally defined ones)
     >>> sorted(imported_modules(
     ...        'from stdlib1 import foo, bar; import stdlib2',
     ...        modulename, f, localmods))
     []
     >>> # relative importing
     >>> sorted(imported_modules(
     ...        'import foo1; from bar import bar1',
     ...        modulename, f, localmods))
     ['foo.bar.bar1', 'foo.foo1']
     >>> sorted(imported_modules(
     ...        'from bar.bar1 import name1, name2, name3',
     ...        modulename, f, localmods))
     ['foo.bar.bar1']
     >>> # absolute importing
     >>> sorted(imported_modules(
     ...        'from baz import baz1, name1',
     ...        modulename, f, localmods))
     ['baz.__init__', 'baz.baz1']
     >>> # mixed importing, even though it shouldn't be recommended
     >>> sorted(imported_modules(
     ...        'import stdlib, foo1, baz',
     ...        modulename, f, localmods))
     ['baz.__init__', 'foo.foo1']
     >>> # ignore_nested
     >>> sorted(imported_modules(
     ... '''import foo
     ... def wat():
     ...     import bar
     ... ''', modulename, f, localmods))
     ['foo.__init__', 'foo.bar.__init__']
     >>> sorted(imported_modules(
     ... '''import foo
     ... def wat():
     ...     import bar
     ... ''', modulename, f, localmods, ignore_nested=True))
     ['foo.__init__']
     """
     fromlocal = fromlocalfunc(modulename, localmods)
     for node in ast.walk(ast.parse(source, f)):
         if ignore_nested and getattr(node, 'col_offset', 0) > 0:
             continue
         if isinstance(node, ast.Import):
             for n in node.names:
                 found = fromlocal(n.name)
                 if not found:
                     # this should import standard library
                     continue
                 yield found[1]
         elif isinstance(node, ast.ImportFrom):
             found = fromlocal(node.module, node.level)
             if not found:
                 # this should import standard library
                 continue
 
             absname, dottedpath, hassubmod = found
             if not hassubmod:
                 # "dottedpath" is not a package; must be imported
                 yield dottedpath
                 # examination of "node.names" should be redundant
                 # e.g.: from mercurial.node import nullid, nullrev
                 continue
 
             modnotfound = False
             prefix = absname + '.'
             for n in node.names:
                 found = fromlocal(prefix + n.name)
                 if not found:
                     # this should be a function or a property of "node.module"
                     modnotfound = True
                     continue
                 yield found[1]
             if modnotfound:
                 # "dottedpath" is a package, but imported because of non-module
                 # lookup
                 yield dottedpath
 
 def verify_import_convention(module, source, localmods):
     """Verify imports match our established coding convention.
 
     We have 2 conventions: legacy and modern. The modern convention is in
     effect when using absolute imports.
 
     The legacy convention only looks for mixed imports. The modern convention
     is much more thorough.
     """
     root = ast.parse(source)
     absolute = usingabsolute(root)
 
     if absolute:
         return verify_modern_convention(module, root, localmods)
     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.
 
     The rules of the modern convention are:
 
     * Ordering is stdlib followed by local imports. Each group is lexically
       sorted.
     * Importing multiple modules via "import X, Y" is not allowed: use
       separate import statements.
     * Importing multiple modules via "from X import ..." is allowed if using
       parenthesis and one entry per line.
     * Only 1 relative import statement per import level ("from .", "from ..")
       is allowed.
     * Relative imports from higher levels must occur before lower levels. e.g.
       "from .." must be before "from .".
     * Imports from peer packages should use relative import (e.g. do not
       "import mercurial.foo" from a "mercurial.*" module).
     * Symbols can only be imported from specific modules (see
       `allowsymbolimports`). For other modules, first import the module then
       assign the symbol to a module-level variable. In addition, these imports
       must be performed before other local imports. This rule only
       applies to import statements outside of any blocks.
     * Relative imports from the standard library are not allowed.
     * Certain modules must be aliased to alternate names to avoid aliasing
       and readability problems. See `requirealias`.
     """
     if not isinstance(module, str):
         module = module.decode('ascii')
     topmodule = module.split('.')[0]
     fromlocal = fromlocalfunc(module, localmods)
 
     # Whether a local/non-stdlib import has been performed.
     seenlocal = None
     # Whether a local/non-stdlib, non-symbol import has been seen.
     seennonsymbollocal = False
     # The last name to be imported (for sorting).
     lastname = None
     laststdlib = None
     # Relative import levels encountered so far.
     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):
                 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))
 
             name = node.names[0].name
             asname = node.names[0].asname
 
             stdlib = name in stdlib_modules
 
             # 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)
 
             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)
 
             if not stdlib:
                 seenlocal = name
 
             # Import of sibling modules should use relative imports.
             topname = name.split('.')[0]
             if topname == topmodule:
                 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])
 
         elif isinstance(node, ast.ImportFrom):
             # Resolve the full imported module name.
             if node.level > 0:
                 fullname = '.'.join(module.split('.')[:-node.level])
                 if node.module:
                     fullname += '.%s' % node.module
             else:
                 assert node.module
                 fullname = node.module
 
                 topname = fullname.split('.')[0]
                 if topname == topmodule:
                     yield msg('import should be relative: %s', fullname)
 
             # __future__ is special since it needs to come first and use
             # symbol import.
             if fullname != '__future__':
                 if not fullname or fullname in stdlib_modules:
                     yield msg('relative import of stdlib module')
                 else:
                     seenlocal = fullname
 
             # Direct symbol import is only allowed from certain modules and
             # must occur before non-symbol imports.
             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))
             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)
 
                 if symbols and seennonsymbollocal:
                     yield msg('symbol import follows non-symbol import: %s',
                               fullname)
             if not symbols and fullname not in stdlib_modules:
                 seennonsymbollocal = True
 
             if not node.module:
                 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)
 
                 # 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)
 
                 seenlevels.add(node.level)
 
             # Entries in "from .X import ( ... )" lists must be lexically
             # sorted.
             lastentryname = None
 
             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)
 
                 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])
 
 def verify_stdlib_on_own_line(root):
     """Given some python source, verify that stdlib imports are done
     in separate statements from relative local module imports.
 
     >>> list(verify_stdlib_on_own_line(ast.parse('import sys, foo')))
     [('mixed imports\\n   stdlib:    sys\\n   relative:  foo', 1)]
     >>> list(verify_stdlib_on_own_line(ast.parse('import sys, os')))
     []
     >>> list(verify_stdlib_on_own_line(ast.parse('import foo, bar')))
     []
     """
     for node in ast.walk(root):
         if isinstance(node, ast.Import):
             from_stdlib = {False: [], True: []}
             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)
 
 class CircularImport(Exception):
     pass
 
 def checkmod(mod, imports):
     shortest = {}
     visit = [[mod]]
     while visit:
         path = visit.pop(0)
         for i in sorted(imports.get(path[-1], [])):
             if len(path) < shortest.get(i, 1000):
                 shortest[i] = len(path)
                 if i in path:
                     if i == path[0]:
                         raise CircularImport(path)
                     continue
                 visit.append(path + [i])
 
 def rotatecycle(cycle):
     """arrange a cycle so that the lexicographically first module listed first
 
     >>> rotatecycle(['foo', 'bar'])
     ['bar', 'foo', 'bar']
     """
     lowest = min(cycle)
     idx = cycle.index(lowest)
     return cycle[idx:] + cycle[:idx] + [lowest]
 
 def find_cycles(imports):
     """Find cycles in an already-loaded import graph.
 
     All module names recorded in `imports` should be absolute one.
 
     >>> from __future__ import print_function
     >>> imports = {'top.foo': ['top.bar', 'os.path', 'top.qux'],
     ...            'top.bar': ['top.baz', 'sys'],
     ...            'top.baz': ['top.foo'],
     ...            'top.qux': ['top.foo']}
     >>> print('\\n'.join(sorted(find_cycles(imports))))
     top.bar -> top.baz -> top.foo -> top.bar
     top.foo -> top.qux -> top.foo
     """
     cycles = set()
     for mod in sorted(imports.keys()):
         try:
             checkmod(mod, imports)
         except CircularImport as e:
             cycle = e.args[0]
             cycles.add(" -> ".join(rotatecycle(cycle)))
     return cycles
 
 def _cycle_sortkey(c):
     return len(c), c
 
 def embedded(f, modname, src):
     """Extract embedded python code
 
     >>> def _forcestr(thing):
     ...     if not isinstance(thing, str):
     ...         return thing.decode('ascii')
     ...     return thing
     >>> def test(fn, lines):
     ...     for s, m, f, l in embedded(fn, b"example", lines):
     ...         print("%s %s %d" % (_forcestr(m), _forcestr(f), l))
     ...         print(repr(_forcestr(s)))
     >>> lines = [
     ...   b'comment',
     ...   b'  >>> from __future__ import print_function',
     ...   b"  >>> ' multiline",
     ...   b"  ... string'",
     ...   b'  ',
     ...   b'comment',
     ...   b'  $ cat > foo.py <<EOF',
     ...   b'  > from __future__ import print_function',
     ...   b'  > EOF',
     ... ]
     >>> test(b"example.t", lines)
     example[2] doctest.py 2
     "from __future__ import print_function\\n' multiline\\nstring'\\n"
     example[7] foo.py 7
     'from __future__ import print_function\\n'
     """
     inlinepython = 0
     shpython = 0
     script = []
     prefix = 6
     t = ''
     n = 0
     for l in src:
         n += 1
         if not l.endswith(b'\n'):
             l += b'\n'
         if l.startswith(b'  >>> '): # python inlines
             if shpython:
                 print("%s:%d: Parse Error" % (f, n))
             if not inlinepython:
                 # We've just entered a Python block.
                 inlinepython = n
                 t = b'doctest.py'
             script.append(l[prefix:])
             continue
         if l.startswith(b'  ... '): # python inlines
             script.append(l[prefix:])
             continue
         cat = re.search(br"\$ \s*cat\s*>\s*(\S+\.py)\s*<<\s*EOF", l)
         if cat:
             if inlinepython:
                 yield b''.join(script), (b"%s[%d]" %
                        (modname, inlinepython)), t, inlinepython
                 script = []
                 inlinepython = 0
             shpython = n
             t = cat.group(1)
             continue
         if shpython and l.startswith(b'  > '): # sh continuation
             if l == b'  > EOF\n':
                 yield b''.join(script), (b"%s[%d]" %
                        (modname, shpython)), t, shpython
                 script = []
                 shpython = 0
             else:
                 script.append(l[4:])
             continue
         if inlinepython and l == b'  \n':
             yield b''.join(script), (b"%s[%d]" %
                    (modname, inlinepython)), t, inlinepython
             script = []
             inlinepython = 0
             continue
 
 def sources(f, modname):
     """Yields possibly multiple sources from a filepath
 
     input: filepath, modulename
     yields:  script(string), modulename, filepath, linenumber
 
     For embedded scripts, the modulename and filepath will be different
     from the function arguments. linenumber is an offset relative to
     the input file.
     """
     py = False
     if not f.endswith('.t'):
         with open(f, 'rb') as src:
             yield src.read(), modname, f, 0
             py = True
     if py or f.endswith('.t'):
         with open(f, 'rb') as src:
             for script, modname, t, line in embedded(f, modname, src):
                 yield script, modname, t, line
 
 def main(argv):
     if len(argv) < 2 or (argv[1] == '-' and len(argv) > 2):
         print('Usage: %s {-|file [file] [file] ...}')
         return 1
     if argv[1] == '-':
         argv = argv[:1]
         argv.extend(l.rstrip() for l in sys.stdin.readlines())
     localmodpaths = {}
     used_imports = {}
     any_errors = False
     for source_path in argv[1:]:
         modname = dotted_name_of_path(source_path)
         localmodpaths[modname] = source_path
     localmods = populateextmods(localmodpaths)
     for localmodname, source_path in sorted(localmodpaths.items()):
         if not isinstance(localmodname, bytes):
             # This is only safe because all hg's files are ascii
             localmodname = localmodname.encode('ascii')
         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):
                     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))
     cycles = find_cycles(used_imports)
     if cycles:
         firstmods = set()
         for c in sorted(cycles, key=_cycle_sortkey):
             first = c.split()[0]
             # As a rough cut, ignore any cycle that starts with the
             # same module as some other cycle. Otherwise we see lots
             # of cycles that are effectively duplicates.
             if first in firstmods:
                 continue
             print('Import cycle:', c)
             firstmods.add(first)
         any_errors = True
     return any_errors != 0
 
 if __name__ == '__main__':
     sys.exit(int(main(sys.argv)))