Last week we discussed adding this module to the standard library.

Here's a draft.  I have no immediate use for it, but I'd like this to
be available for experimentation.  I may withdraw it or change it
radically up to and including the release of Python 2.3b1.

Lib/pkgutil.py

@@ -0,0 +1,78 @@
+"""Utilities to support packages."""
+
+import os
+import sys
+
+def extend_path(path, name):
+    """Extend a package's path.
+
+    Intended use is to place the following code in a package's __init__.py:
+
+        from pkgutil import extend_path
+        __path__ = extend_path(__path__, __name__)
+
+    This will add to the package's __path__ all subdirectories of
+    directories on sys.path named after the package.  This is useful
+    if one wants to distribute different parts of a single logical
+    package as multiple directories.
+
+    It also looks for *.pkg files beginning where * matches the name
+    argument.  This feature is similar to *.pth files (see site.py),
+    except that it doesn't special-case lines starting with 'import'.
+    A *.pkg file is trusted at face value: apart from checking for
+    duplicates, all entries found in a *.pkg file are added to the
+    path, regardless of whether they are exist the filesystem.  (This
+    is a feature.)
+
+    If the input path is not a list (as is the case for frozen
+    packages) it is returned unchanged.  The input path is not
+    modified; an extended copy is returned.  Items are only appended
+    to the copy at the end.
+
+    It is assumed that sys.path is a sequence.  Items of sys.path that
+    are not (unicode or 8-bit) strings referring to existing
+    directories are ignored.  Unicode items of sys.path that cause
+    errors when used as filenames may cause this function to raise an
+    exception (in line with os.path.isdir() behavior).
+    """
+
+    if not isinstance(path, list):
+        # This could happen e.g. when this is called from inside a
+        # frozen package.  Return the path unchanged in that case.
+        return path
+
+    pname = os.path.join(*name.split('.')) # Reconstitute as relative path
+    # Just in case os.extsep != '.'
+    sname = os.extsep.join(name.split('.'))
+    sname_pkg = sname + os.extsep + "pkg"
+    init_py = "__init__" + os.extsep + "py"
+
+    path = path[:] # Start with a copy of the existing path
+
+    for dir in sys.path:
+        if not isinstance(dir, (str, unicode)) or not os.path.isdir(dir):
+            continue
+        subdir = os.path.join(dir, pname)
+        # XXX This may still add duplicate entries to path on
+        # case-insensitive filesystems
+        initfile = os.path.join(subdir, init_py)
+        if subdir not in path and os.path.isfile(initfile):
+            path.append(subdir)
+        # XXX Is this the right thing for subpackages like zope.app?
+        # It looks for a file named "zope.app.pkg"
+        pkgfile = os.path.join(dir, sname_pkg)
+        if os.path.isfile(pkgfile):
+            try:
+                f = open(pkgfile)
+            except IOError, msg:
+                sys.stderr.write("Can't open %s: %s\n" %
+                                 (pkgfile, msg))
+            else:
+                for line in f:
+                    line = line.rstrip('\n')
+                    if not line or line.startswith('#'):
+                        continue
+                    path.append(line) # Don't check for existence!
+                f.close()
+
+    return path