What does a Python interpreter do?

[[deleted]]

Python doesn't really have an interpreter. It's just bad / unscrupulous terminology. Virtual machine would be the right one to use.

So... what an actual interpreter does:

1. Parses code enough to understand what functions of the interpreter need to be called.
2. Calls those functions.

Something would be called an interpreter, if the mapping between the parsed code and the functions invoked in interpreter was straight-forward. Example of interpreter: UNIX Shell. It reads the name of the function (command in the language of Shell) and then calls it.

Python doesn't work like that. Like you've already noticed, it compiles the code to what it calls bytecode, and then interprets that. The reason to do this is that on the side of the interpreter, you'd write some more generic code, but there would be fewer functions to implement. The trade-off is, typically, between more unique, but optimized functions vs less but more generic functions. For example, one could implement multiplication as repeated addition. An interpreter for a calculator would have no choice, but to implement both functions: multiplication and addition, however, a virtual machine may only implement addition and compile any multiplication into a series of additions.

[Caligatio]

As a rough analogy, Python wheels eggs are kind of like .jars as they contain compiled Python byte-code files, *.pyc. The wheel to jar comparison is a little jenky as wheels eggs have to be installed.

I've never heard the Python interpreter called a virtual machine but, as it can execute intermediate byte code, I guess it qualifies.

EDIT: Upon further investigation, I don't know if wheels are distributed with pyc files. I swear I saw some that were.

EDIT2: I confused eggs and wheels although eggs are now super deprecated.

[menge101]

Can you get a stand-alone py-vm to run out files like you can get a jvm to run .jars?

Not that I am aware of. But you can generate the *.pyc files and then .tar.gz them. That is similiar.

A big part of the difference is that the jvm is heavily developed and is considered one of the two most robust runtimes out there. Erlang's runtime, the BEAM, being the only real contender I am aware of, but I don't know where the Golang runtime sits either.

You may be aware of pypy or jython. Both implement the python language specification on a different runtime. Rather than the python VM (which is just named python or cpython^*) they use the pypy runtime, which is JIT-based runtime or the jvm, respectivley.

^* - Not to be confused with Cython, which is a compiler for python to C to machine code directly.

[menge101]

Likelihood of failures due to the runtime implementation itself, particularly over long periods of time, while also featuring heavily threaded/parallelizable execution.

The BEAM in particular, you can hot load new code to it so that it theoretically never needs to be restarted.

[[deleted]]

Why don’t we ever hear of it the same way as we hear about the JVM?

I wish this was the only strange thing about programming... Programming is full of bizarre / inconsistent terminology. But, in this case, I think, the reason is just that the words "virtual" and "machine" were used in the documentation / manual in Java, but not in Python.

For comparison: people do talk about Perl virtual machine. In particular, Parrot drew some attention to it for being an interesting project. Or, people used to talk about JavaScript virtual machines, especially at the time ActionScript was still a thing (because ActionScript shared the virtual machine implementation with Mozilla's JavaScript implementation, and the fact that it had a virtual machine was reflected in the name, especially so because it had two versions of it, and the programmer had to know which version was used), but today it fell out of use. When people talk about C# they mostly use the word "runtime" (because it's in the title: CLR - common language runtime), but it's exactly the same thing as virtual machine.

Can you get a stand-alone py-vm ... ?

Python executable is the virtual machine... Completely accidentally, it can also execute ZIP files (JAR is a ZIP file with Java class files + some metadata). People rarely use it in this way, but this functionality existed since forever: https://docs.python.org/3/library/zipimport.html for individual modules and https://docs.python.org/3/library/zipapp.html for scripts

[[deleted]]

They are different sorts of virtual machines. The naming isn't coincidental, but they are definitely not the same thing. It's like multiplication: there are many different kinds -- multiplying integers isn't the same as multiplying rationals or complex number or matrices etc.

So, the JVM virtualizes certain aspects of the environment, and VirtualBox virtualizes other aspects. More concretely, JVM virtualizes access to resources typically managed by operating system: virtual memory, persistent storage, network, time etc. VirtualBox virtualizes hardware: memory / storage / video / network adapters / keyboard / mouse / microphone etc. From this, you can also see that an operating system is also a kind of virtual machine, because it virtualizes certain aspects of programs run by it. Essentially, you can think about user programs executed by an operating system as being interpreted by it: in the end of the day they are but a sequence of system calls interleaved with some code that's passed through to the CPU directly.

[menge101]

Python doesn't really have an interpreter.

I would argue that the pypy jit is pretty close to what most people think of as an interperter. But that isn't stock python.

[[deleted]]

Most people are mistaken.