import-checker: establish modern import convention

We introduce a new convention for declaring imports and enforce it via
the import checker script.

The new convention is only active when absolute imports are used, which is
currently nowhere. Keying off "from __future__ import absolute_import" to
engage the new import convention seems like the easiest solution. It is
also beneficial for Mercurial to use this mode because it means less work
and ambiguity for the importer and potentially better performance due to
fewer stat() system calls because the importer won't look for modules in
relative paths unless explicitly asked.

Once all files are converted to use absolute import, we can refactor
this code to again only have a single import convention and we can
require use of absolute import in the style checker.

The rules for the new convention are documented in the docstring of the
added function. Tests have been added to test-module-imports.t. Some
tests are sensitive to newlines and source column position, which makes
docstring testing difficult and/or impossible.

contrib/import-checker.py

@@ -1,396 +1,570 b''
 import ast
 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.
 import BaseHTTPServer
 import zlib
 
+# Whitelist of modules that symbols can be directly imported from.
+allowsymbolimports = (
+    '__future__',
+    'mercurial.i18n',
+    'mercurial.node',
+)
+
+# 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 dotted_name_of_path(path, trimpure=False):
     """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('mercurial/pure/parsers.py', trimpure=True)
     'mercurial.parsers'
     >>> dotted_name_of_path('zlibmodule.so')
     'zlib'
     """
     parts = path.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]
     if trimpure:
         return '.'.join(p for p in parts if p != 'pure')
     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 dict (or set), of which key is an absolute
     `dotted_name_of_path()`-ed source file path of locally defined (=
     Mercurial specific) modules.
 
     This function assumes that module names not existing in
     `localmods` are ones of Python standard libarary.
 
     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__': True, 'foo.foo1': True,
     ...              'foo.bar.__init__': True, 'foo.bar.bar1': True,
     ...              'baz.__init__': True, 'baz.baz1': True }
     >>> 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)
     >>> fromlocal2 = fromlocalfunc('foo.xxx.yyy', localmods)
     >>> fromlocal2(None, 2)
     ('foo', 'foo.__init__', True)
     """
     prefix = '.'.join(modulename.split('.')[:-1])
     if prefix:
         prefix += '.'
     def fromlocal(name, level=0):
         # name is None when relative imports are used.
         if name is None:
             # If relative imports are used, level must not be absolute.
             assert level > 0
             candidates = ['.'.join(modulename.split('.')[:-level])]
         else:
             # Check relative name first.
             candidates = [prefix + name, 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 list_stdlib_modules():
     """List the modules present in the stdlib.
 
     >>> mods = set(list_stdlib_modules())
     >>> 'BaseHTTPServer' in mods
     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
     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
     # These get missed too
     for m in 'ctypes', 'email':
         yield m
     yield 'builtins' # python3 only
     for m in 'fcntl', 'grp', 'pwd', 'termios':  # Unix only
         yield m
     stdlib_prefixes = set([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):
         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. check-code suppressed because
         # the ast module used by this script implies the availability
         # of any().
         if not any(libpath.startswith(p) for p in stdlib_prefixes): # no-py24
             continue
         if 'site-packages' in libpath:
             continue
         for top, dirs, files in os.walk(libpath):
             for name in files:
                 if name == '__init__.py':
                     continue
                 if not (name.endswith('.py') or name.endswith('.so')
                         or name.endswith('.pyd')):
                     continue
                 full_path = os.path.join(top, name)
                 if 'site-packages' in full_path:
                     continue
                 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, 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: dict 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.
 
     >>> 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, localmods))
     []
     >>> # relative importing
     >>> sorted(imported_modules(
     ...        'import foo1; from bar import bar1',
     ...        modulename, localmods))
     ['foo.bar.__init__', 'foo.bar.bar1', 'foo.foo1']
     >>> sorted(imported_modules(
     ...        'from bar.bar1 import name1, name2, name3',
     ...        modulename, localmods))
     ['foo.bar.bar1']
     >>> # absolute importing
     >>> sorted(imported_modules(
     ...        'from baz import baz1, name1',
     ...        modulename, localmods))
     ['baz.__init__', 'baz.baz1']
     >>> # mixed importing, even though it shouldn't be recommended
     >>> sorted(imported_modules(
     ...        'import stdlib, foo1, baz',
     ...        modulename, localmods))
     ['baz.__init__', 'foo.foo1']
     >>> # ignore_nested
     >>> sorted(imported_modules(
     ... '''import foo
     ... def wat():
     ...     import bar
     ... ''', modulename, localmods))
     ['foo.__init__', 'foo.bar.__init__']
     >>> sorted(imported_modules(
     ... '''import foo
     ... def wat():
     ...     import bar
     ... ''', modulename, localmods, ignore_nested=True))
     ['foo.__init__']
     """
     fromlocal = fromlocalfunc(modulename, localmods)
     for node in ast.walk(ast.parse(source)):
         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
             yield dottedpath
             if not hassubmod:
                 # examination of "node.names" should be redundant
                 # e.g.: from mercurial.node import nullid, nullrev
                 continue
 
             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"
                     continue
                 yield found[1]
 
 def verify_import_convention(module, source):
-    """Verify imports match our established coding convention."""
+    """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)
 
-    return verify_stdlib_on_own_line(root)
+    if absolute:
+        return verify_modern_convention(module, root)
+    else:
+        return verify_stdlib_on_own_line(root)
+
+def verify_modern_convention(module, root):
+    """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 relative 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`.
+    """
+    topmodule = module.split('.')[0]
+
+    # Whether a local/non-stdlib import has been performed.
+    seenlocal = False
+    # Whether a relative, non-symbol import has been seen.
+    seennonsymbolrelative = False
+    # The last name to be imported (for sorting).
+    lastname = None
+    # Relative import levels encountered so far.
+    seenlevels = set()
+
+    for node in ast.walk(root):
+        if isinstance(node, ast.Import):
+            # Disallow "import foo, bar" and require separate imports
+            # for each module.
+            if len(node.names) > 1:
+                yield 'multiple imported names: %s' % ', '.join(
+                    n.name for n in node.names)
+
+            name = node.names[0].name
+            asname = node.names[0].asname
+
+            # Ignore sorting rules on imports inside blocks.
+            if node.col_offset == 0:
+                if lastname and name < lastname:
+                    yield 'imports not lexically sorted: %s < %s' % (
+                           name, lastname)
+
+                lastname = name
+
+            # stdlib imports should be before local imports.
+            stdlib = name in stdlib_modules
+            if stdlib and seenlocal and node.col_offset == 0:
+                yield 'stdlib import follows local import: %s' % name
+
+            if not stdlib:
+                seenlocal = True
+
+            # Import of sibling modules should use relative imports.
+            topname = name.split('.')[0]
+            if topname == topmodule:
+                yield 'import should be relative: %s' % name
+
+            if name in requirealias and asname != requirealias[name]:
+                yield '%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 '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 'relative import of stdlib module'
+                else:
+                    seenlocal = True
+
+            # Direct symbol import is only allowed from certain modules and
+            # must occur before non-symbol imports.
+            if node.module and node.col_offset == 0:
+                if fullname not in allowsymbolimports:
+                    yield 'direct symbol import from %s' % fullname
+
+                if seennonsymbolrelative:
+                    yield ('symbol import follows non-symbol import: %s' %
+                           fullname)
+
+            if not node.module:
+                assert node.level
+                seennonsymbolrelative = True
+
+                # Only allow 1 group per level.
+                if node.level in seenlevels and node.col_offset == 0:
+                    yield '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 '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 '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 '%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.
 
     Observing this limitation is important as it works around an
     annoying lib2to3 bug in relative import rewrites:
     http://bugs.python.org/issue19510.
 
     >>> list(verify_stdlib_on_own_line(ast.parse('import sys, foo')))
     ['mixed imports\\n   stdlib:    sys\\n   relative:  foo']
     >>> 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]))))
 
 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.
 
     >>> 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.iterkeys()):
         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 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())
     localmods = {}
     used_imports = {}
     any_errors = False
     for source_path in argv[1:]:
         modname = dotted_name_of_path(source_path, trimpure=True)
         localmods[modname] = source_path
     for modname, source_path in sorted(localmods.iteritems()):
         f = open(source_path)
         src = f.read()
         used_imports[modname] = sorted(
             imported_modules(src, modname, localmods, ignore_nested=True))
         for error in verify_import_convention(modname, src):
             any_errors = True
             print source_path, error
         f.close()
     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 not any_errors
 
 if __name__ == '__main__':
     sys.exit(int(main(sys.argv)))

tests/test-module-imports.t

@@ -1,37 +1,131 b''
 #require test-repo
 
   $ import_checker="$TESTDIR"/../contrib/import-checker.py
 
 Run the doctests from the import checker, and make sure
 it's working correctly.
   $ TERM=dumb
   $ export TERM
   $ python -m doctest $import_checker
 
+Run additional tests for the import checker
+
+  $ mkdir testpackage
+
+  $ cat > testpackage/multiple.py << EOF
+  > from __future__ import absolute_import
+  > import os, sys
+  > EOF
+
+  $ cat > testpackage/unsorted.py << EOF
+  > from __future__ import absolute_import
+  > import sys
+  > import os
+  > EOF
+
+  $ cat > testpackage/stdafterlocal.py << EOF
+  > from __future__ import absolute_import
+  > from . import unsorted
+  > import os
+  > EOF
+
+  $ cat > testpackage/requirerelative.py << EOF
+  > from __future__ import absolute_import
+  > import testpackage.unsorted
+  > EOF
+
+  $ cat > testpackage/importalias.py << EOF
+  > from __future__ import absolute_import
+  > import ui
+  > EOF
+
+  $ cat > testpackage/relativestdlib.py << EOF
+  > from __future__ import absolute_import
+  > from .. import os
+  > EOF
+
+  $ cat > testpackage/symbolimport.py << EOF
+  > from __future__ import absolute_import
+  > from .unsorted import foo
+  > EOF
+
+  $ cat > testpackage/latesymbolimport.py << EOF
+  > from __future__ import absolute_import
+  > from . import unsorted
+  > from mercurial.node import hex
+  > EOF
+
+  $ cat > testpackage/multiplegroups.py << EOF
+  > from __future__ import absolute_import
+  > from . import unsorted
+  > from . import more
+  > EOF
+
+  $ mkdir testpackage/subpackage
+  $ cat > testpackage/subpackage/levelpriority.py << EOF
+  > from __future__ import absolute_import
+  > from . import foo
+  > from .. import parent
+  > EOF
+
+  $ cat > testpackage/sortedentries.py << EOF
+  > from __future__ import absolute_import
+  > from . import (
+  >     foo,
+  >     bar,
+  > )
+  > EOF
+
+  $ cat > testpackage/importfromalias.py << EOF
+  > from __future__ import absolute_import
+  > from . import ui
+  > EOF
+
+  $ cat > testpackage/importfromrelative.py << EOF
+  > from __future__ import absolute_import
+  > from testpackage.unsorted import foo
+  > EOF
+
+  $ python "$import_checker" testpackage/*.py testpackage/subpackage/*.py
+  testpackage/importalias.py ui module must be "as" aliased to uimod
+  testpackage/importfromalias.py ui from testpackage must be "as" aliased to uimod
+  testpackage/importfromrelative.py import should be relative: testpackage.unsorted
+  testpackage/importfromrelative.py direct symbol import from testpackage.unsorted
+  testpackage/latesymbolimport.py symbol import follows non-symbol import: mercurial.node
+  testpackage/multiple.py multiple imported names: os, sys
+  testpackage/multiplegroups.py multiple "from . import" statements
+  testpackage/relativestdlib.py relative import of stdlib module
+  testpackage/requirerelative.py import should be relative: testpackage.unsorted
+  testpackage/sortedentries.py imports from testpackage not lexically sorted: bar < foo
+  testpackage/stdafterlocal.py stdlib import follows local import: os
+  testpackage/subpackage/levelpriority.py higher-level import should come first: testpackage
+  testpackage/symbolimport.py direct symbol import from testpackage.unsorted
+  testpackage/unsorted.py imports not lexically sorted: os < sys
+
   $ cd "$TESTDIR"/..
 
 There are a handful of cases here that require renaming a module so it
 doesn't overlap with a stdlib module name. There are also some cycles
 here that we should still endeavor to fix, and some cycles will be
 hidden by deduplication algorithm in the cycle detector, so fixing
 these may expose other cycles.
 
   $ hg locate 'mercurial/**.py' 'hgext/**.py' | sed 's-\\-/-g' | python "$import_checker" -
   mercurial/dispatch.py mixed imports
      stdlib:    commands
      relative:  error, extensions, fancyopts, hg, hook, util
   mercurial/fileset.py mixed imports
      stdlib:    parser
      relative:  error, merge, util
   mercurial/revset.py mixed imports
      stdlib:    parser
      relative:  error, hbisect, phases, util
   mercurial/templater.py mixed imports
      stdlib:    parser
      relative:  config, error, templatefilters, templatekw, util
   mercurial/ui.py mixed imports
      stdlib:    formatter
      relative:  config, error, progress, scmutil, util
   Import cycle: mercurial.cmdutil -> mercurial.context -> mercurial.subrepo -> mercurial.cmdutil
   Import cycle: hgext.largefiles.basestore -> hgext.largefiles.localstore -> hgext.largefiles.basestore
   Import cycle: mercurial.commands -> mercurial.commandserver -> mercurial.dispatch -> mercurial.commands