Initial documentation for python users (#1378)

* Fix instructions for building rust modules in python examples

When I ran `rustc +nightly ...` the compiler just looked for a source file
called `+nightly`. I changed these instructions to use rustup + rustc instead.

* Initial documentation for python users

Added documentation for using the Wasmtime loader in python, and explained the
first two examples in the repo. Changed the import example to demonstrate
working with module linear memory.

* Fix include in python guide

* Wording

* Clarify memory usage

* Flow through the example better

* More word choice

* Make rustup a prereq

* Fix source code paths in python guide

* Fix rustup example in python guide

Co-Authored-By: Samrat Man Singh <samratmansingh@gmail.com>

* Replace command examples with preformat blocks

* Revert "Fix instructions for building rust modules in python examples"

This reverts commit 1738888a2d.

* Left a block quote in a preformat example

Co-authored-by: Samrat Man Singh <samratmansingh@gmail.com>

crates/misc/py/examples/import/demo.rs

@@ -2,10 +2,11 @@ extern "C" {
     fn callback(s: *const u8, s_len: u32) -> u32;
 }
 
+static MSG: &str = "Hello, world!";
+
 #[no_mangle]
 pub extern "C" fn test() {
-    let msg = "Hello, world!";
     unsafe {
-        callback(msg.as_ptr(), msg.len() as u32);
+        callback(MSG.as_ptr(), MSG.len() as u32);
     }
 }

crates/misc/py/examples/import/env.py

@@ -1,10 +1,8 @@
-def callback(msg_p: 'i32', msg_len: 'i32') -> 'i32':
+import demo
-    print('callback:', msg_p, msg_len)
-
-#    global memory
-#    mv = memoryview(memory)
 
-#    msg = bytes(mv[msg_p:(msg_p + msg_len)]).decode('utf-8')
+def callback(msg_p: 'i32', msg_len: 'i32') -> 'i32':
-#    print(msg)
+    mv = memoryview(demo.memory)
+    msg = bytes(mv[msg_p:(msg_p + msg_len)]).decode('utf-8')
 
+    print(msg)
     return 42

docs/lang-python.md

@@ -1,3 +1,120 @@
 # Using WebAssembly from Python
 
-... more coming soon
+Wasmtime can be used as a python module loader, which allows almost any
+Webassembly module to be used as a python module. This guide will go over adding
+Wasmtime to your project, and some provided examples of what can be done with
+Webassembly modules.
+
+## Prerequisites
+
+To follow this guide, you'll need
+
+ - Python 3.6 or newer
+ - The [Webassembly binary toolkit](https://github.com/WebAssembly/wabt/releases)
+ - The rust toolchain installer [rustup](https://rustup.rs/)
+
+## Getting started and simple example
+
+First, copy this example Webassembly text module into your project. It exports a
+function for calculating the greatest common denominator of two numbers.
+
+```wat
+{{#include ../examples/gcd.wat}}
+```
+
+Before we can do anything with this module, we need to convert it to the
+Webassembly binary format. We can do this with the command line tools provided
+by the Webassembly binary toolkit
+
+```bash
+wat2wasm gcd.wat
+```
+
+This will create the binary form of the gcd module `gcd.wasm`, we'll use this
+module in the following steps.
+
+Next, install the Wasmtime module loader, which is provided as a [python package](https://pypi.org/project/wasmtime/)
+on PyPi. It can be installed as a dependency through Pip or related tools such
+as Pipenv.
+
+```bash
+pip install wasmtime
+```
+
+Or
+
+```bash
+pipenv install wasmtime
+```
+
+After you have Wasmtime installed and you've imported `wasmtime`, you can import
+Webassembly modules in your project like any other python module.
+
+```python
+{{#include ../crates/misc/py/examples/gcd/run.py}}
+```
+
+This script should output
+
+```bash
+gcd(27, 6) = 3
+```
+
+If this is the output you see, congrats! You've successfully ran your first
+Webassembly code in python!
+
+## Host interaction and memory
+
+In the first example, we called a function exported by a Webassembly
+module. Depeding on what you need to accomplish, Webassembly modules can also
+call functions from other modules and python itself. This is done through the
+module imports mechanism, which allows other modules and the host environment to
+provide functions, globals, and memory spaces. The following example will show
+you how to use module imports and work with module linear memory.
+
+> Note: At the moment, the Wasmtime python module can only import functions and
+> memories.
+
+To show how we can use functions from the host, take a look at this rust code
+
+```rust
+{{#include ../crates/misc/py/examples/import/demo.rs}}
+```
+
+We have a `test` function which calls `callback`. Since it's wrapped in `extern "C"`,
+this function will be dynamically linked. The Wasmtime module does this linking
+automatically by importing any needed modules at runtime. If we compile this
+example without any extra linker options, the result module will import
+`callback` from a module called `env`, so we need to provide an implementation of
+`callback` inside an `env.py` module.
+
+```python
+{{#include ../crates/misc/py/examples/import/env.py}}
+```
+
+The module provides `callback` with a pointer to a string message. We use this
+to index into the demo module's memory, extract the message bytes and print them
+as a string. Every Webassembly module exports its main linear memory as "memory"
+by default, so it's accessible as `demo.memory` in python. We wrap the memory
+into a `memoryview` so we can safely access the values inside.
+
+Before we move on, note the type annotations on `callback`. These are necessary
+for representing your function as something callable in Webassembly, since
+Webassembly functions only operate on 32 and 64 bit floats and integers. When
+defining functions for use by Webassembly modules, make sure the parameters and
+return value are annotated appropriately as any of `'i32'`, `'i64'`, `'f32'`, or
+`'f64'`.
+
+Before we can use `demo.rs` we need to compile it
+
+```bash
+rustup run nightly rustc --target=wasm32-unknown-unknown --crate-type=cdylib demo.rs
+```
+
+We can then use it like this
+
+```python
+{{#include ../crates/misc/py/examples/import/run.py}}
+```
+
+The script should print `Hello, world!` and exit.