1 '''Demand load modules when used, not when imported.'''

     2 

     3 __author__ = '''Copyright 2006 Vadim Gelfer <vadim.gelfer@gmail.com>.

     4 This software may be used and distributed according to the terms

     5 of the GNU General Public License, incorporated herein by reference.'''

     6 

     7 # this is based on matt's original demandload module.  it is a

     8 # complete rewrite.  some time, we may need to support syntax of

     9 # "import foo as bar".

    10 

    11 class _importer(object):

    12     '''import a module.  it is not imported until needed, and is

    13     imported at most once per scope.'''

    14 

    15     def __init__(self, scope, modname, fromlist):

    16         '''scope is context (globals() or locals()) in which import

    17         should be made.  modname is name of module to import.

    18         fromlist is list of modules for "from foo import ..."

    19         emulation.'''

    20 

    21         self.scope = scope

    22         self.modname = modname

    23         self.fromlist = fromlist

    24         self.mod = None

    25 

    26     def module(self):

    27         '''import the module if needed, and return.'''

    28         if self.mod is None:

    29             self.mod = __import__(self.modname, self.scope, self.scope,

    30                                   self.fromlist)

    31             del self.modname, self.fromlist

    32         return self.mod

    33 

    34 class _replacer(object):

    35     '''placeholder for a demand loaded module. demandload puts this in

    36     a target scope.  when an attribute of this object is looked up,

    37     this object is replaced in the target scope with the actual

    38     module.

    39 

    40     we use __getattribute__ to avoid namespace clashes between

    41     placeholder object and real module.'''

    42 

    43     def __init__(self, importer, target):

    44         self.importer = importer

    45         self.target = target

    46         # consider case where we do this:

    47         #   demandload(globals(), 'foo.bar foo.quux')

    48         # foo will already exist in target scope when we get to

    49         # foo.quux.  so we remember that we will need to demandload

    50         # quux into foo's scope when we really load it.

    51         self.later = []

    52 

    53     def module(self):

    54         return object.__getattribute__(self, 'importer').module()

    55 

    56     def __getattribute__(self, key):

    57         '''look up an attribute in a module and return it. replace the

    58         name of the module in the caller\'s dict with the actual

    59         module.'''

    60 

    61         module = object.__getattribute__(self, 'module')()

    62         target = object.__getattribute__(self, 'target')

    63         importer = object.__getattribute__(self, 'importer')

    64         later = object.__getattribute__(self, 'later')

    65 

    66         if later:

    67             demandload(module.__dict__, ' '.join(later))

    68 

    69         importer.scope[target] = module

    70 

    71         return getattr(module, key)

    72 

    73 class _replacer_from(_replacer):

    74     '''placeholder for a demand loaded module.  used for "from foo

    75     import ..." emulation. semantics of this are different than

    76     regular import, so different implementation needed.'''

    77 

    78     def module(self):

    79         importer = object.__getattribute__(self, 'importer')

    80         target = object.__getattribute__(self, 'target')

    81 

    82         return getattr(importer.module(), target)

    83 

    84     def __call__(self, *args, **kwargs):

    85         target = object.__getattribute__(self, 'module')()

    86         return target(*args, **kwargs)

    87 

    88 def demandload(scope, modules):

    89     '''import modules into scope when each is first used.

    90 

    91     scope should be the value of globals() in the module calling this

    92     function, or locals() in the calling function.

    93 

    94     modules is a string listing module names, separated by white

    95     space.  names are handled like this:

    96 

    97     foo            import foo

    98     foo bar        import foo, bar

    99     foo@bar        import foo as bar

   100     foo.bar        import foo.bar

   101     foo:bar        from foo import bar

   102     foo:bar,quux   from foo import bar, quux

   103     foo.bar:quux   from foo.bar import quux'''

   104 

   105     for mod in modules.split():

   106         col = mod.find(':')

   107         if col >= 0:

   108             fromlist = mod[col+1:].split(',')

   109             mod = mod[:col]

   110         else:

   111             fromlist = []

   112         as_ = None

   113         if '@' in mod:

   114             mod, as_ = mod.split("@")

   115         importer = _importer(scope, mod, fromlist)

   116         if fromlist:

   117             for name in fromlist:

   118                 scope[name] = _replacer_from(importer, name)

   119         else:

   120             dot = mod.find('.')

   121             if dot >= 0:

   122                 basemod = mod[:dot]

   123                 val = scope.get(basemod)

   124                 # if base module has already been demandload()ed,

   125                 # remember to load this submodule into its namespace

   126                 # when needed.

   127                 if isinstance(val, _replacer):

   128                     later = object.__getattribute__(val, 'later')

   129                     later.append(mod[dot+1:])

   130                     continue

   131             else:

   132                 basemod = mod

   133             if not as_:

   134                 as_ = basemod

   135             scope[as_] = _replacer(importer, as_)