def open(cls, pkg, is_from_file=True):
"""
Return an |OpcPackage| instance loaded with the contents of
*pkg_file*.
"""
if is_from_file:
pkg_reader = PackageReader.from_file(pkg)
else:
pkg_reader = PackageReader.from_str(pkg)
package = cls()
Unmarshaller.unmarshal(pkg_reader, package, PartFactory)
return package
