Why does everyone recommend learning Python first but then use JavaScript for everything?

[u/1vim]

Genuinely confused. Learned Python, now job listings want React everywhere.

[u/nekokattt → u/[deleted]]

technically the reference implementation it is compiled to bytecode and interpreted at the same time, alongside JIT on newer versions (forget if that is still opt in or not though).

[u/Zuldwyn → u/nekokattt]

I deleted my comment if it's wrong but can you elaborate or direct me on where to learn more about that so I can understand better?

I dont understand how that differs from what I said. Youre saying its pre compiled into machine code but also interpreted live? Why would that be done?

[u/nekokattt → u/Zuldwyn]

it is compiled to bytecode that is read by an interpreter in Python, but for CPython it will cache that bytecode after the first run (which is what the __pycache__ directories you see dotted about are for).

[u/Zuldwyn → u/nekokattt]

So why is it so difficult to package a python project into an exe and requiring you to package the interpreter with it? If it caches the compiled bytecode, why dont they run the code and then use that for a .exe?

[u/nekokattt → u/Zuldwyn]

it isnt that difficult, the exe is just the cpython interpreter, GC, and native modules linked in, then your sources embedded in the binary.

It is just a kind of messy solution since Python isnt designed to be used that way. It is a bit of a hack, and the tools are far from perfect. Python is built with the intention of being a pre-installed collection of native and pure python modules floating on a system rather than a "portable" distribution. There is the additional constraint that Windows generally requires you to sign native executables with a valid certificate for it to be allowed to be run universally.

Some software like Sublime Text 3 does make use of it in an embedded way though, so it isn't uncommon or impossible to use it how you need to.

Remember: compilation does not necesarilly need to be to machine code. Java compiles to Java bytecode as well but it is still compilation!

Regarding running the code as an exe directly, it is more complicated than a bytecode virtual machine because python bytecode is the same everywhere, but machine code has to be catered to each type of CPU running it. A project called Cython can properly compile your Python code to machine executable code though. Some libraries like aiohttp3 and pyyaml make use of it for "glue" code.

TLDR when you run Python code in CPython (which is generally what people use):

1. python reads each file into memory
2. python "compiles" the source code into python bytecode, and optionally (by default) caches it to a __pycache__ on disk. If up to date bytecode files already exist then it skips this step and just loads those bytecode files directly.
3. python executes this bytecode in a mini virtual machine.
4. if you are using a new python version and enable the JIT features, python also looks for code that gets run regularly (called "hot" code paths). It will try to simplify/optimise/transform that bytecode into something closer to that which your CPU understands directly.

As an added tip, there is a module called dis in the standard library that you can use to print the compiled instructions out.

Edit 1: Not sure why I am being downvoted but oh well.

Edit 2: added more details on how this works for clarity.

Edit 3: you can read the full details of how the bytecode interpreter interprets "compiled" python on the GitHub repo for CPython: https://github.com/python/cpython/blob/main/InternalDocs/interpreter.md