Add materials for What Is the Python Global Interpreter Lock (GIL)?

python-gil/README.md

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+# What Is the Python Global Interpreter Lock (GIL)?
+
+This folder provides the code examples for the Real Python tutorial [What Is the Python Global Interpreter Lock (GIL)?](https://realpython.com/python-gil/).
+
+The tutorial shows some of its examples at the REPL. Those are provided here as
+runnable scripts that print the values the tutorial discusses:
+
+| Script | Tutorial section |
+| --- | --- |
+| `reference_counting.py` | What Problem Did the GIL Solve for Python? |
+| `single_threaded.py` | The Impact on Multi-Threaded Python Programs |
+| `multi_threaded.py` | The Impact on Multi-Threaded Python Programs |
+| `switch_interval.py` | Why Wasn't It Removed in Python 3? |
+| `multiprocess.py` | How to Deal With Python's GIL |
+
+## Running the Examples
+
+The examples only use the Python standard library, so there's nothing to
+install. Create and activate a [virtual environment](https://realpython.com/python-virtual-environments-a-primer/),
+then run any of the scripts:
+
+```sh
+$ python -m venv venv/
+$ source venv/bin/activate
+(venv) $ python single_threaded.py
+```
+
+The timings printed by `single_threaded.py`, `multi_threaded.py`, and
+`multiprocess.py` depend on your machine, so they won't match the numbers in
+the tutorial exactly. What matters is how they compare to each other.

python-gil/multi_threaded.py

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+import time
+from threading import Thread
+
+COUNT = 50000000
+
+
+def countdown(n):
+    while n > 0:
+        n -= 1
+
+
+t1 = Thread(target=countdown, args=(COUNT // 2,))
+t2 = Thread(target=countdown, args=(COUNT // 2,))
+
+start = time.time()
+t1.start()
+t2.start()
+t1.join()
+t2.join()
+end = time.time()
+
+print("Time taken in seconds -", end - start)

python-gil/multiprocess.py

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+from multiprocessing import Pool
+import time
+
+COUNT = 50000000
+
+
+def countdown(n):
+    while n > 0:
+        n -= 1
+
+
+if __name__ == "__main__":
+    pool = Pool(processes=2)
+    start = time.time()
+    r1 = pool.apply_async(countdown, [COUNT // 2])
+    r2 = pool.apply_async(countdown, [COUNT // 2])
+    pool.close()
+    pool.join()
+    end = time.time()
+    print("Time taken in seconds -", end - start)

python-gil/reference_counting.py

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+import sys
+
+a = []
+b = a
+print(sys.getrefcount(a))

python-gil/single_threaded.py

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+import time
+
+COUNT = 50000000
+
+
+def countdown(n):
+    while n > 0:
+        n -= 1
+
+
+start = time.time()
+countdown(COUNT)
+end = time.time()
+
+print("Time taken in seconds -", end - start)

python-gil/switch_interval.py

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+import sys
+
+# The switch interval is set to 5 milliseconds:
+print(sys.getswitchinterval())