annotate contrib/import-checker.py @ 42093:edbcf5b239f9

config: read configs from directories in lexicographical order

Mercurial currently reads the .rc files specified in HGRCPATH (and the
system-default paths) in directory order, which is unspecified. My
team at work maintains a set of .rc files. So far there has been no
overlap between them, so we had not noticed this behavior. However, we
would now like to release some common .rc files and then have another
one per plaform with platform-specific overrides. It would be nice if
we can determine the load order by choosing names carefully. This
patch enables that by loading the .rc files in lexicographical order.

Before this patch, the added test case would consistently say "30" on
my file system (whatever I have -- some Linux FS).

Differential Revision: https://phab.mercurial-scm.org/D6193

#!/usr/bin/env python

from __future__ import absolute_import, print_function

import ast
import collections
import os
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

import testparseutil

# Whitelist of modules that symbols can be directly imported from.
allowsymbolimports = (
    '__future__',
    'bzrlib',
    'hgclient',
    'mercurial',
    'mercurial.hgweb.common',
    'mercurial.hgweb.request',
    'mercurial.i18n',
    'mercurial.node',
    # for revlog to re-export constant to extensions
    'mercurial.revlogutils.constants',
    # for cffi modules to re-export pure functions
    'mercurial.pure.base85',
    'mercurial.pure.bdiff',
    'mercurial.pure.mpatch',
    'mercurial.pure.osutil',
    'mercurial.pure.parsers',
    # third-party imports should be directly imported
    'mercurial.thirdparty',
    'mercurial.thirdparty.attr',
    'mercurial.thirdparty.zope',
    'mercurial.thirdparty.zope.interface',
)

# 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 '__builtin__'
    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
        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)
    sourceroot = os.path.abspath(os.path.dirname(os.path.dirname(__file__)))
    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)):
            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, unless that
      library is also a local module.
    * 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
                    and fullname not in localmods
                    and fullname + '.__init__' not in localmods):
                    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 1
    "from __future__ import print_function\\n' multiline\\nstring'\\n\\n"
    example[8] foo.py 7
    'from __future__ import print_function\\n'
    """
    errors = []
    for name, starts, ends, code in testparseutil.pyembedded(f, src, errors):
        if not name:
            # use 'doctest.py', in order to make already existing
            # doctest above pass instantly
            name = 'doctest.py'
        # "starts" is "line number" (1-origin), but embedded() is
        # expected to return "line offset" (0-origin). Therefore, this
        # yields "starts - 1".
        if not isinstance(modname, str):
            modname = modname.decode('utf8')
        yield code, "%s[%d]" % (modname, starts), name, starts - 1

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.encode('utf8'), 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)))