How to instantiate in a lazy way?
Slaunger at gmail.com
Mon Dec 1 15:01:48 CET 2008
> class PayloadOnDemand(object):
> Behaves as a PayloadInstant object, but instantiation is faster
> as only the position of the payload in the file is stored
> initially in the object.
> Only when acessing the initially non-existing data attribute
> are the data actually read and the attribure created and bound to
> the instance.
> This will actually be a little slower than in PayloadInstant as
> the correct file position
> has to be seeked out first.
> On later calls the object has as efficient attribute access as
> def read_from_file(cls, f, size):
> pos = f.tell()
> f.seek(pos + size) #Skip to end of payload
> return cls(pos)
Extend with ref to file instead:
return cls(f, pos)
> # I probably need some __getattr__ or __getattribute__ magic
> # there...??
To answer my own rethorical question I guess I should do something
def __getattr__(self, attr_name):
Only called if attr_name is not in the __dict__ for the
if attr_name == 'data':
self.__dict__[attr_name] = read_data(self.f,
> def __init__(self, a_file_position):
> self.file_position = a_file_position
and then I need to also store a reference to the file in the
def __init__(self, a_file, a_file_position):
self.f = a_file
self.file_position = a_file_position
Have I understood correctly how to to it the on demand way?
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