knee.py for Python 2.6, 2.7, and 3.2

knee.py

"""
An Python re-implementation of hierarchical module import.

Function names and arguments have been chosen to mimic the C code in
`Python/import.c` whenever possible.

This code is intended to be read, not executed.  However, it does work
-- all you need to do to enable it is "import knee".

(The name is a pun on the klunkier predecessor of this module, "ni".)

"""

import __builtin__
import imp
import sys

from types import ModuleType
from warnings import warn

def get_parent(globals, level):
    """
    parent, name = get_parent(globals, level)

    Return the package that an import is being performed in.  If globals comes
    from the module foo.bar.bat (not itself a package), this returns the
    sys.modules entry for foo.bar.  If globals is from a package's __init__.py,
    the package's entry in sys.modules is returned.

    If globals doesn't come from a package or a module in a package, or a
    corresponding entry is not found in sys.modules, None is returned.
    """
    orig_level = level

    if not level or not isinstance(globals, dict):
        return None, ''

    pkgname = globals.get('__package__', None)

    if pkgname is not None:
        # __package__ is set, so use it
        if not hasattr(pkgname, 'rindex'):
            raise ValueError('__package__ set to non-string')
        if len(pkgname) == 0:
            if level > 0:
                raise ValueError('Attempted relative import in non-package')
            return None, ''
        name = pkgname
    else:
        # __package__ not set, so figure it out and set it
        if '__name__' not in globals:
            return None, ''
        modname = globals['__name__']

        if '__path__' in globals:
            # __path__ is set, so modname is already the package name
            globals['__package__'] = name = modname
        else:
            # Normal module, so work out the package name if any
            lastdot = modname.rfind('.')
            if lastdot < 0 and level > 0:
                raise ValueError("Attempted relative import in non-package")
            if lastdot < 0:
                globals['__package__'] = None
                return None, ''
            globals['__package__'] = name = modname[:lastdot]

    dot = len(name)
    for x in xrange(level, 1, -1):
        try:
            dot = name.rindex('.', 0, dot)
        except ValueError:
            raise ValueError("attempted relative import beyond top-level "
                             "package")
    name = name[:dot]

    try:
        parent = sys.modules[name]
    except:
        if orig_level < 1:
            warn("Parent module '%.200s' not found while handling absolute "
                 "import" % name)
            parent = None
        else:
            raise SystemError("Parent module '%.200s' not loaded, cannot "
                              "perform relative import" % name)

    # We expect, but can't guarantee, if parent != None, that:
    # - parent.__name__ == name
    # - parent.__dict__ is globals
    # If this is violated...  Who cares?
    return parent, name

def load_next(mod, altmod, name, buf):
    """
    mod, name, buf = load_next(mod, altmod, name, buf)

    altmod is either None or same as mod
    """

    if len(name) == 0:
        # completely empty module name should only happen in
        # 'from . import' (or '__import__("")')
        return mod, None, buf

    dot = name.find('.')
    if dot == 0:
        raise ValueError('Empty module name')

    if dot < 0:
        subname = name
        next = None
    else:
        subname = name[:dot]
        next = name[dot+1:]

    if buf != '':
        buf += '.'
    buf += subname

    result = import_submodule(mod, subname, buf)
    if result is None and mod != altmod:
        result = import_submodule(altmod, subname, subname)
        if result is not None:
            buf = subname

    if result is None:
        raise ImportError("No module named %.200s" % name)

    return result, next, buf

def import_submodule(mod, subname, fullname):
    """m = import_submodule(mod, subname, fullname)"""
    # Require:
    # if mod == None: subname == fullname
    # else: mod.__name__ + "." + subname == fullname

    if fullname in sys.modules:
        m = sys.modules[fullname]
    else:
        if mod is None:
            path = None
        elif hasattr(mod, '__path__'):
            path = mod.__path__
        else:
            return None

        try:
            fp, filename, stuff  = imp.find_module(subname, path)
        except ImportError:
            return None

        try:
            m = imp.load_module(fullname, fp, filename, stuff)
        finally:
            if fp: fp.close()

        add_submodule(mod, m, fullname, subname)

    return m

def add_submodule(mod, submod, fullname, subname):
    """mod.{subname} = submod"""
    if mod is None:
        return #Nothing to do here.

    if submod is None:
        submod = sys.modules[fullname]

    setattr(mod, subname, submod)

    return

def ensure_fromlist(mod, fromlist, buf, recursive):
    """Handle 'from module import a, b, c' imports."""
    if not hasattr(mod, '__path__'):
        return
    for item in fromlist:
        if not hasattr(item, 'rindex'):
            raise TypeError("Item in ``from list'' not a string")
        if item == '*':
            if recursive:
                continue # avoid endless recursion
            try:
                all = mod.__all__
            except AttributeError:
                pass
            else:
                ret = ensure_fromlist(mod, all, buf, 1)
                if not ret:
                    return 0
        elif not hasattr(mod, item):
            import_submodule(mod, item, buf + '.' + item)

def import_module_level(name, globals=None, locals=None, fromlist=None, level=-1):
    """Replacement for __import__()"""
    parent, buf = get_parent(globals, level)

    head, name, buf = load_next(parent, None if level < 0 else parent, name, buf)

    tail = head
    while name:
        tail, name, buf = load_next(tail, tail, name, buf)

    # If tail is None, both get_parent and load_next found
    # an empty module name: someone called __import__("") or
    # doctored faulty bytecode
    if tail is None:
        raise ValueError('Empty module name')

    if not fromlist:
        return head

    ensure_fromlist(tail, fromlist, buf, 0)
    return tail

modules_reloading = {}

def reload_module(m):
    """Replacement for reload()."""
    if not isinstance(m, ModuleType):
        raise TypeError("reload() argument must be module")

    name = m.__name__

    if name not in sys.modules:
        raise ImportError("reload(): module %.200s not in sys.modules" % name)

    global modules_reloading
    try:
        return modules_reloading[name]
    except:
        modules_reloading[name] = m

    dot = name.rfind('.')
    if dot < 0:
        subname = name
        path = None
    else:
        try:
            parent = sys.modules[name[:dot]]
        except KeyError:
            modules_reloading.clear()
            raise ImportError("reload(): parent %.200s not in sys.modules" % name[:dot])
        subname = name[dot+1:]
        path = getattr(parent, "__path__", None)

    try:
        fp, filename, stuff  = imp.find_module(subname, path)
    finally:
        modules_reloading.clear()

    try:
        newm = imp.load_module(name, fp, filename, stuff)
    except:
         # load_module probably removed name from modules because of
         # the error.  Put back the original module object.
        sys.modules[name] = m
        raise
    finally:
        if fp: fp.close()

    modules_reloading.clear()
    return newm

# Save the original hooks
original_import = __builtin__.__import__
try:
    original_reload = __builtin__.reload
except AttributeError:
    original_reload = imp.reload    # Python 3

__builtin__.__import__ = import_module_level
__builtin__.reload = reload_module

See also importlib -- "An implementation of import" -- which is provided in Python 3.1 and later.