gh-119786: added InternalDocs/generators.md

InternalDocs/README.md

@@ -25,7 +25,7 @@ Runtime Objects
 
 - [Code Objects](code_objects.md)
 
-- [Generators (coming soon)](generators.md)
+- [Generators](generators.md)
 
 - [Frames](frames.md)
 

InternalDocs/generators.md

@@ -1,8 +1,57 @@
+
+Generators and Coroutines
+=========================
+
 Generators
-========== 
+----------
+
+The implementation of generators in CPython consists of the builtin object type
+`PyGenObject` and bytecode instructions that operate on instances of this type.
+
+A generator object executes in its own [`frame`](frames.md), like a function.
+The difference is that a function returns only once, while a generator
+"returns" to the caller every time it emits a new item with a
+[`yield` expression](https://docs.python.org/dev/reference/expressions.html#yield-expressions).
+This is implemented by the
+[`YIELD_VALUE`](https://docs.python.org/dev/library/dis.html#opcode-YIELD_VALUE)
+bytecode, which is similar to
+[`RETURN_VALUE`](https://docs.python.org/dev/library/dis.html#opcode-RETURN_VALUE)
+in the sense that it puts a value on the stack and returns execution to the
+calling frame, but it also needs to perform additional work to leave the generator
+frame in a state that allows it to be resumed.  In particular, it updates the frame's
+instruction pointer and stores the interpreter's exception state on the generator
+object. When the generator it resumed, this exception state is copied back to the
+interpreter state.
+
+The `frame` of a generator is embedded in the generator object struct (see
+`_PyGenObject_HEAD` in [`pycore_genobject.h`](../Include/internal/pycore_genobject.h)).
+This means that we can get the frame from the generator or the generator
+from the frame (see `_PyGen_GetGeneratorFromFrame` in the same file).
+Other fields of the generator struct include metadata (such as the name of
+the generator function) and runtime state information (such as whether its
+frame is executing, suspended, cleared, etc.).
+
+Chained Generators
+------------------
+
+A `yield from` expression creates a generator that efficiently yields the
+sequence created by another generator. This is implemented with the
+[`SEND` instruction](https://docs.python.org/dev/library/dis.html#opcode-SEND),
+which pushes the value of its arg to the stack of the generator's frame, sets
+the exception state on this frame, and resumes execution of the chained generator. 
+
+Coroutines
+----------
 
-Coming soon.
+Coroutines are generators that use the value returned from a `yield` expression,
+i.e., the argument that was passed to the `.send()` call that resumed it after
+it yielded. This makes it possible for data to flow in both directions: from
+the generator to the caller via the argument of the `yield` expression, and
+from the caller to the generator via the send argument to the `send()` call.
+A `yield from` expression passes the `send` argument to the chained generator,
+so this data flow works along the chain (see `gen_send_ex2()` in
+[`genobject.c`](../Objects/genobject.c)).
 
-<!--
-- Generators, async functions, async generators, and ``yield from`` (next, send, throw, close; and await; and how this code breaks the interpreter abstraction)
--->
+Recall that a generator's `__next__` function simply calls `self.send(None)`,
+so all this works the same in generators and coroutines, but only coroutines
+use the value of the argument to `send`.