import-checker: refactor sys.path prefix check (issue4129)

This patch refactors the logic in contrib/import-checker.py responsible for
checking the beginnings of the paths in sys.path.  In particular, it adds a
variable that defines the set of allowed prefixes.

The primary purpose of this change is to make it easier to add more allowed
prefixes.  This will be useful in resolving issue4129, which involves making
the function list_stdlib_modules() work when run from a virtualenv.

contrib/import-checker.py

@@ -1,221 +1,224
 import ast
 import os
 import sys
 
 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'
     """
     parts = path.split('/')
     parts[-1] = parts[-1][:-3] # remove .py
     return '.'.join(parts)
 
 
 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
+    stdlib_prefixes = set([sys.prefix, sys.exec_prefix])
     for libpath in sys.path:
-        # We want to walk everything in sys.path that starts with
-        # either sys.prefix or sys.exec_prefix.
-        if not (libpath.startswith(sys.prefix)
-                or libpath.startswith(sys.exec_prefix)):
+        # We want to walk everything in sys.path that starts with something
+        # in stdlib_prefixes.
+        for prefix in stdlib_prefixes:
+            if libpath.startswith(prefix):
+                break
+        else:
             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')):
                     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, 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.
       ignore_nested: If true, import statements that do not start in
                      column zero will be ignored.
 
     Returns:
       A list of module names imported by the given source.
 
     >>> sorted(imported_modules(
     ...         'import foo ; from baz import bar; import foo.qux'))
     ['baz.bar', 'foo', 'foo.qux']
     >>> sorted(imported_modules(
     ... '''import foo
     ... def wat():
     ...     import bar
     ... ''', ignore_nested=True))
     ['foo']
     """
     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:
                 yield n.name
         elif isinstance(node, ast.ImportFrom):
             prefix = node.module + '.'
             for n in node.names:
                 yield prefix + n.name
 
 def verify_stdlib_on_own_line(source):
     """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('import sys, foo'))
     ['mixed stdlib and relative imports:\\n   foo, sys']
     >>> list(verify_stdlib_on_own_line('import sys, os'))
     []
     >>> list(verify_stdlib_on_own_line('import foo, bar'))
     []
     """
     for node in ast.walk(ast.parse(source)):
         if isinstance(node, ast.Import):
             from_stdlib = {}
             for n in node.names:
                 from_stdlib[n.name] = n.name in stdlib_modules
             num_std = len([x for x in from_stdlib.values() if x])
             if num_std not in (len(from_stdlib.values()), 0):
                 yield ('mixed stdlib and relative imports:\n   %s' %
                        ', '.join(sorted(from_stdlib.iterkeys())))
 
 class CircularImport(Exception):
     pass
 
 
 def cyclekey(names):
     return tuple(sorted(set(names)))
 
 def check_one_mod(mod, imports, path=None, ignore=None):
     if path is None:
         path = []
     if ignore is None:
         ignore = []
     path = path + [mod]
     for i in sorted(imports.get(mod, [])):
         if i not in stdlib_modules:
             i = mod.rsplit('.', 1)[0] + '.' + i
         if i in path:
             firstspot = path.index(i)
             cycle = path[firstspot:] + [i]
             if cyclekey(cycle) not in ignore:
                 raise CircularImport(cycle)
             continue
         check_one_mod(i, imports, path=path, ignore=ignore)
 
 def rotatecycle(cycle):
     """arrange a cycle so that the lexicographically first module listed first
 
     >>> rotatecycle(['foo', 'bar', 'foo'])
     ['bar', 'foo', 'bar']
     """
     lowest = min(cycle)
     idx = cycle.index(lowest)
     return cycle[idx:] + cycle[1:idx] + [lowest]
 
 def find_cycles(imports):
     """Find cycles in an already-loaded import graph.
 
     >>> imports = {'top.foo': ['bar', 'os.path', 'qux'],
     ...            'top.bar': ['baz', 'sys'],
     ...            'top.baz': ['foo'],
     ...            'top.qux': ['foo']}
     >>> print '\\n'.join(sorted(find_cycles(imports)))
     top.bar -> top.baz -> top.foo -> top.bar -> top.bar
     top.foo -> top.qux -> top.foo -> top.foo
     """
     cycles = {}
     for mod in sorted(imports.iterkeys()):
         try:
             check_one_mod(mod, imports, ignore=cycles)
         except CircularImport, e:
             cycle = e.args[0]
             cycles[cyclekey(cycle)] = ' -> '.join(rotatecycle(cycle))
     return cycles.values()
 
 def _cycle_sortkey(c):
     return len(c), c
 
 def main(argv):
     if len(argv) < 2:
         print 'Usage: %s file [file] [file] ...'
         return 1
     used_imports = {}
     any_errors = False
     for source_path in argv[1:]:
         f = open(source_path)
         modname = dotted_name_of_path(source_path)
         src = f.read()
         used_imports[modname] = sorted(
             imported_modules(src, ignore_nested=True))
         for error in verify_stdlib_on_own_line(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)))