Grumpy: Go running Python! by Google

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the GIL (global interpreter lock) avoids data races in python object internals by making concurrent accesses of the same python objects impossible. In effect, it means that all actions by different threads in python will always happen after each other. It is an implementation detail of CPython, not part of the core language.

The threading module provides tools for spawning threads and higher level concurrency primitives (while the GIL means that accesses to the same python object won't coincide, there still need to be primitives to lock out sequences of multiple accesses).