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import sys
class VendorImporter:
"""
A PEP 302 meta path importer for finding optionally-vendored
or otherwise naturally-installed packages from root_name.
"""
def __init__(self, root_name, vendored_names=(), vendor_pkg=None):
self.root_name = root_name
self.vendored_names = set(vendored_names)
self.vendor_pkg = vendor_pkg or root_name.replace('extern', '_vendor')
@property
def search_path(self):
"""
Search first the vendor package then as a natural package.
"""
yield self.vendor_pkg + '.'
yield ''
def find_module(self, fullname, path=None):
"""
Return self when fullname starts with root_name and the
target module is one vendored through this importer.
"""
root, base, target = fullname.partition(self.root_name + '.')
if root:
return
if not any(map(target.startswith, self.vendored_names)):
return
return self
def load_module(self, fullname):
"""
Iterate over the search path to locate and load fullname.
"""
root, base, target = fullname.partition(self.root_name + '.')
for prefix in self.search_path:
try:
__import__(prefix + target)
mod = sys.modules[prefix + target]
sys.modules[fullname] = mod
if sys.version_info > (3, 3):
del sys.modules[prefix + target]
return mod
except ImportError:
pass
else:
raise ImportError(
"The '{target}' package is required; "
"normally this is bundled with this package so if you get "
"this warning, consult the packager of your "
"distribution.".format(**locals())
)
def install(self):
"""
Install this importer into sys.meta_path if not already present.
"""
if self not in sys.meta_path:
sys.meta_path.append(self)
names = 'packaging',
VendorImporter(__name__, names).install()
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