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      1 import sys
      2 
      3 
      4 class VendorImporter:
      5     """
      6     A PEP 302 meta path importer for finding optionally-vendored
      7     or otherwise naturally-installed packages from root_name.
      8     """
      9 
     10     def __init__(self, root_name, vendored_names=(), vendor_pkg=None):
     11         self.root_name = root_name
     12         self.vendored_names = set(vendored_names)
     13         self.vendor_pkg = vendor_pkg or root_name.replace('extern', '_vendor')
     14 
     15     @property
     16     def search_path(self):
     17         """
     18         Search first the vendor package then as a natural package.
     19         """
     20         yield self.vendor_pkg + '.'
     21         yield ''
     22 
     23     def find_module(self, fullname, path=None):
     24         """
     25         Return self when fullname starts with root_name and the
     26         target module is one vendored through this importer.
     27         """
     28         root, base, target = fullname.partition(self.root_name + '.')
     29         if root:
     30             return
     31         if not any(map(target.startswith, self.vendored_names)):
     32             return
     33         return self
     34 
     35     def load_module(self, fullname):
     36         """
     37         Iterate over the search path to locate and load fullname.
     38         """
     39         root, base, target = fullname.partition(self.root_name + '.')
     40         for prefix in self.search_path:
     41             try:
     42                 extant = prefix + target
     43                 __import__(extant)
     44                 mod = sys.modules[extant]
     45                 sys.modules[fullname] = mod
     46                 # mysterious hack:
     47                 # Remove the reference to the extant package/module
     48                 # on later Python versions to cause relative imports
     49                 # in the vendor package to resolve the same modules
     50                 # as those going through this importer.
     51                 if sys.version_info > (3, 3):
     52                     del sys.modules[extant]
     53                 return mod
     54             except ImportError:
     55                 pass
     56         else:
     57             raise ImportError(
     58                 "The '{target}' package is required; "
     59                 "normally this is bundled with this package so if you get "
     60                 "this warning, consult the packager of your "
     61                 "distribution.".format(**locals())
     62             )
     63 
     64     def install(self):
     65         """
     66         Install this importer into sys.meta_path if not already present.
     67         """
     68         if self not in sys.meta_path:
     69             sys.meta_path.append(self)
     70 
     71 
     72 names = 'packaging', 'pyparsing', 'six', 'appdirs'
     73 VendorImporter(__name__, names).install()
     74