Trying out free-threaded Python on macOS

python/trying-free-threaded-python.md

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+# Trying out free-threaded Python on macOS
+
+Inspired by [py-free-threading.github.io](https://py-free-threading.github.io/) I decided to try out a beta of Python 3.13 with the new free-threaded mode enabled, which removes the GIL.
+
+## Installation
+
+I chose to use the macOS installer to get a pre-built binary. I downloaded and ran the macOS installer from [www.python.org/downloads/release/python-3130b3/](https://www.python.org/downloads/release/python-3130b3/), and then when I got to this screen:
+
+![Installation dialog with a Customize button at the bottom](https://github.com/user-attachments/assets/7e57d8f1-6a4b-4551-babd-127317dff5cd)
+
+I selected the "Customize" option and checked this additional box:
+
+![Customize screen - I have checked the Free-threaded Python (experimental) option](https://github.com/user-attachments/assets/5aa8d4dd-5c70-493e-a183-2f0799079830)
+
+Once it had finished installing I didn't run the script to add it to my path (I didn't want to intefere with my many other Python versions).
+
+This gave me two new Python binaries - one with free-threading enabled and one without:
+
+- `/Library/Frameworks/Python.framework/Versions/3.13/bin/python3.13`
+- `/Library/Frameworks/PythonT.framework/Versions/3.13/bin/python3.13t`
+
+Note the `PythonT.framework` in the second path.
+
+It also seemed to setup the following aliases in `/usr/local/bin`:
+
+- `/usr/local/bin/python3.13`
+- `/usr/local/bin/python3.13t`
+
+These are symlinks:
+```bash
+ls -lah /usr/local/bin | grep python3.13
+```
+```
+lrwxr-xr-x   1 root  wheel    73B Jul 12 16:26 python3.13 -> ../../../Library/Frameworks/Python.framework/Versions/3.13/bin/python3.13
+lrwxr-xr-x   1 root  wheel    80B Jul 12 16:26 python3.13-config -> ../../../Library/Frameworks/Python.framework/Versions/3.13/bin/python3.13-config
+lrwxr-xr-x   1 root  wheel    81B Jul 12 16:26 python3.13-intel64 -> ../../../Library/Frameworks/Python.framework/Versions/3.13/bin/python3.13-intel64
+lrwxr-xr-x   1 root  wheel    75B Jul 12 16:26 python3.13t -> ../../../Library/Frameworks/PythonT.framework/Versions/3.13/bin/python3.13t
+lrwxr-xr-x   1 root  wheel    82B Jul 12 16:26 python3.13t-config -> ../../../Library/Frameworks/PythonT.framework/Versions/3.13/bin/python3.13t-config
+```
+
+Starting those Python processes shows which one has free-threading in the interpreter header:
+```
+% /usr/local/bin/python3.13
+Python 3.13.0b3 (v3.13.0b3:7b413952e8, Jun 27 2024, 09:57:31) [Clang 15.0.0 (clang-1500.3.9.4)] on darwin
+Type "help", "copyright", "credits" or "license" for more information.
+>>>
+% /usr/local/bin/python3.13t
+Python 3.13.0b3 experimental free-threading build (v3.13.0b3:7b413952e8, Jun 27 2024, 10:04:51) [Clang 15.0.0 (clang-1500.3.9.4)] on darwin
+Type "help", "copyright", "credits" or "license" for more information.
+>>> 
+```
+
+## Testing it out
+
+I asked Claude 3.5 Sonnet to write me [a quick test script](https://claude.site/artifacts/92aec50b-43c7-463a-b8a1-5d5ea0600708), then iterated on and simplified the result myself until I got to this:
+
+```python
+import argparse
+import time
+from concurrent.futures import ThreadPoolExecutor
+
+
+def cpu_bound_task(n):
+    """A CPU-bound task that computes the sum of squares up to n."""
+    return sum(i * i for i in range(n))
+
+
+def main():
+    parser = argparse.ArgumentParser(description="Run a CPU-bound task with threads")
+    parser.add_argument("--threads", type=int, default=4, help="Number of threads")
+    parser.add_argument("--tasks", type=int, default=10, help="Number of tasks")
+    parser.add_argument(
+        "--size", type=int, default=5000000, help="Task size (n for sum of squares)"
+    )
+    args = parser.parse_args()
+
+    print(f"Running {args.tasks} tasks of size {args.size} with {args.threads} threads")
+
+    start_time = time.time()
+    with ThreadPoolExecutor(max_workers=args.threads) as executor:
+        list(executor.map(cpu_bound_task, [args.tasks] * args.size))
+    end_time = time.time()
+    duration = end_time - start_time
+
+    print(f"Time with threads: {duration:.2f} seconds")
+
+
+if __name__ == "__main__":
+    main()
+```
+I saved this as `gildemo.py` and tried running it with the new Python binaries the one with free-threading enabled and the one without.
+
+Here's what I saw in Activity Monitor while running the scripts:
+
+No free-threading (with the GIL) - reported 99% CPU usage:
+
+![Activity Monitor window showing a Python process using 99.2% CPU with 5 threads. Terminal visible above running a script: "Running 10 tasks of size 10000000 with 4 threads"](https://github.com/user-attachments/assets/686e2dcd-daff-4cfe-ba1e-c4aa38b0a10a)
+
+Free-threading (no GIL) - reported 258.8% CPU usage:
+
+![Activity Monitor window showing a PythonT process using 258.8% CPU with 5 threads. Terminal visible above running a script: "Running 10 tasks of size 10000000 with 4 threads".](https://github.com/user-attachments/assets/e7b8d291-a8a1-4e98-a73f-880403e14e8c)
+
+And here are some results from running the script. First with the GIL in place:
+```
+% python3.13 gildemo.py --size 1000000 
+Running 10 tasks of size 1000000 with 4 threads
+Time with threads: 17.14 seconds
+```
+And then without the GIL:
+```
+% python3.13t gildemo.py --size 1000000 
+Running 10 tasks of size 1000000 with 4 threads
+Time with threads: 8.19 seconds
+```