coveragepy: Executed lines reported as missing in threads

Originally reported by k3it (Bitbucket: k3it, GitHub: k3it)

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I apologize for a cross post from http://stackoverflow.com/q/43991865/1653558 but it looks like this is a better place for the report

I’m getting some low coverage numbers on script with threads activated with a run() method. It seems that the coverage module is unable to keep track of all hits. Here is the code that demonstrates the problem. Running it should produce 100% coverage, but I’m getting 71.

#!python

from threading import Thread

class MyNestedThread(Thread):
        def run(self):
                print("hello from the nested thread!")

class MyThread(Thread):
        def run(self):
                print("Hello from thread")
                t = MyNestedThread()
                t.start()
                return

if __name__ == '__main__':
        for i in range(3):
                t = MyThread()
                t.start()

#!python


Hello from thread
hello from the nested thread!
Hello from thread
Hello from thread
hello from the nested thread!
hello from the nested thread!

Name     Stmts   Miss  Cover
----------------------------
foo.py      14      4    71%

re-running ‘coverage run foo.py’ sometimes gives a different %, and even 100% on occasion.

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• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/582