A very simple class to conveniently emulate file operations in memory using python.

MockFile.py

class MockFile:
    def __init__(self):
        self.data = []
        self.position = 0

    def seek(self, offset, type=0):
        if type == SEEK_END:
            self.position = len(self.data) - offset
        else:
            self.position = offset

    def write(self, data):
        for d in data:
            if len(self.data) > self.position:
                self.data[self.position] = d
            elif len(self.data) == self.position:
                self.data += [d]
            self.position += 1

    def read(self, n):
        data = self.data[self.position: self.position+n]
        if len(data) < n:
            raise Exception
        self.position += n
        return bytes(data)

    def tell(self):
        return self.position

    def get_bytes(self):
        return bytes(self.data)