Are min() and max() thread-safe?

Steven D'Aprano steven at
Thu Sep 17 06:33:05 CEST 2009

I have two threads, one running min() and the other running max() over 
the same list. I'm getting some mysterious results which I'm having 
trouble debugging. Are min() and max() thread-safe, or am I doing 
something fundamentally silly by having them walk over the same list 

My code is as follows. Is there anything obviously wrong with it?

import threading, time

class MMThread(threading.Thread):
    def __init__(self, data, func, target, where):
        super(MMThread, self).__init__()
        self._data = data
        self._func = func
        self._target = target
        self._where = where
    def run(self):
        self._target[self._where] = self._func(self._data)

def minmax(seq):
    result = [None, None]
    t1 = MMThread(seq, min, result, 0)
    t2 = MMThread(seq, max, result, 1)
    # Block until all threads are done.
    while any([t1.isAlive(), t2.isAlive()]):
    assert None not in result
    return tuple(result)


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