Emulate large file in python, with file-like object interface and without memory overload

gistfile1.py

file_size=5*2**30 #5GB

class FakeFile:
    def __init__(self):
        self.remain_size=file_size
    def read(self, size):
        if self.remain_size == 0:
            return None
        elif self.remain_size >= size:
            self.remain_size-=size
            return 'x'*size
        elif self.remain_size < size:
            tail_size = self.remain_size
            self.remain_size = 0
            return 'x'*tail_size
    def tell(self):
        return 0