_pickle.UnpicklingError: could not find MARK
<p>I got exceptions like <strong>UnicodeDecodeError</strong> raised when pickling (a list of) objects of EventFrame with a member <em>participants</em> that was an empty set.</p>

<pre><code>class EventFrame:
    """Frame for an event"""
    def __init__(self, id=0):
        ...
        self.participants = set()
        ...
</code></pre>

<p>When it wasn't empty, there were no problems, so I first set <em>participants</em> to something and then pickled it. But during runtime it may happen that <em>participants</em> is emptied again.</p>

<p>So I tried to manually delete the object in this case. After that I dumped it again using pickle.</p>

<pre><code>if len(frame.participants) == 0:
    frame_list.remove(frame)
</code></pre>

<p>That doesn't seem to be a good choice, because this <strong>UnpicklingError</strong> was raised:</p>

<pre><code>....
frame_list.append (pickle.load(f))
_pickle.UnpicklingError: could not find MARK
</code></pre>

<p>I don't know what it means and I couldn't find anything useful about it.</p>

<p><strong>Note</strong> that this error is raised on loading the pickle file.</p>

<p>Here is the way I'm picklng and unpickling:</p>

<pre><code>f = open("myfile", "r+b")
frame_list = []
while 1:
    try:
        frame_list.append (pickle.load(f))
        frame_list = sum(frame_list, [])
    except EOFError:
        break
f.close()
</code></pre>

<p>and dumping:</p>

<pre><code>f = open("myfile", "r+b")
pickle.dump(frame_list, f)
f.close()   
</code></pre>