bpo-33216: Clarify the documentation for CALL_FUNCTION_* (#8338)

Clarify the documentation for the CALL_FUNCTION_* bytecodes.  They changed in 3.5 in subtle ways and the documentation has never been correct, much less clear.

2 files changed, 72 insertions, 22 deletions

diff --git a/Doc/library/dis.rst b/Doc/library/dis.rst
index 186aab40f6..5168638ec4 100644
--- a/Doc/library/dis.rst
+++ b/Doc/library/dis.rst
@@ -960,21 +960,25 @@ the more significant byte last.
 
 .. opcode:: RAISE_VARARGS (argc)
 
-   Raises an exception. *argc* indicates the number of parameters to the raise
+   Raises an exception. *argc* indicates the number of arguments to the raise
    statement, ranging from 0 to 3.  The handler will find the traceback as TOS2,
    the parameter as TOS1, and the exception as TOS.
 
 
 .. opcode:: CALL_FUNCTION (argc)
 
-   Calls a function.  The low byte of *argc* indicates the number of positional
-   parameters, the high byte the number of keyword parameters. On the stack, the
-   opcode finds the keyword parameters first.  For each keyword argument, the
-   value is on top of the key.  Below the keyword parameters, the positional
-   parameters are on the stack, with the right-most parameter on top.  Below the
-   parameters, the function object to call is on the stack.  Pops all function
-   arguments, and the function itself off the stack, and pushes the return
-   value.
+   Calls a callable object.  The low byte of *argc* indicates the number of
+   positional arguments, the high byte the number of keyword arguments.
+   The stack contains keyword arguments on top (if any), then the positional
+   arguments below that (if any), then the callable object to call below that.
+   Each keyword argument is represented with two values on the stack:
+   the argument's name, and its value, with the argument's value above the
+   name on the stack.
+   The positional arguments are pushed in the order that they are passed in
+   to the callable object, with the right-most positional argument on top.
+   ``CALL_FUNCTION`` pops all arguments and the callable object off the stack,
+   calls the callable object with those arguments, and pushes the return value
+   returned by the callable object.
 
 
 .. opcode:: MAKE_FUNCTION (argc)
@@ -982,12 +986,12 @@ the more significant byte last.
    Pushes a new function object on the stack.  From bottom to top, the consumed
    stack must consist of
 
-   * ``argc & 0xFF`` default argument objects in positional order
+   * ``argc & 0xFF`` default argument objects in positional order, for positional parameters
    * ``(argc >> 8) & 0xFF`` pairs of name and default argument, with the name
      just below the object on the stack, for keyword-only parameters
    * ``(argc >> 16) & 0x7FFF`` parameter annotation objects
    * a tuple listing the parameter names for the annotations (only if there are
-     ony annotation objects)
+     any annotation objects)
    * the code associated with the function (at TOS1)
    * the :term:`qualified name` of the function (at TOS)
 
@@ -1020,24 +1024,68 @@ the more significant byte last.
 
 .. opcode:: CALL_FUNCTION_VAR (argc)
 
-   Calls a function. *argc* is interpreted as in :opcode:`CALL_FUNCTION`. The
-   top element on the stack contains the variable argument list, followed by
-   keyword and positional arguments.
+   Calls a callable object, similarly to :opcode:`CALL_FUNCTION`.
+   *argc* represents the number of keyword and positional
+   arguments, identically to :opcode:`CALL_FUNCTION`.
+   The top of the stack contains keyword arguments (if any), stored
+   identically to :opcode:`CALL_FUNCTION`.
+   Below that
+   is an iterable object containing additional positional arguments.
+   Below that are positional arguments (if any)
+   and a callable object, identically to :opcode:`CALL_FUNCTION`.
+   Before the callable object is called, the iterable object
+   is "unpacked" and its contents are appended to the positional
+   arguments passed in.
+   The iterable object is ignored when computing
+   the value of ``argc``.
+
+   .. versionchanged:: 3.5
+      In versions 3.0 to 3.4, the iterable object was above
+      the keyword arguments; in 3.5 the iterable object was moved
+      below the keyword arguments.
+
 
 
 .. opcode:: CALL_FUNCTION_KW (argc)
 
-   Calls a function. *argc* is interpreted as in :opcode:`CALL_FUNCTION`. The
-   top element on the stack contains the keyword arguments dictionary, followed
-   by explicit keyword and positional arguments.
+   Calls a callable object, similarly to :opcode:`CALL_FUNCTION`.
+   *argc* represents the number of keyword and positional
+   arguments, identically to :opcode:`CALL_FUNCTION`.
+   The top of the stack contains a mapping object containing additional keyword
+   arguments.
+   Below this are keyword arguments (if any), positional arguments (if any),
+   and a callable object, identically to :opcode:`CALL_FUNCTION`.
+   Before the callable is called, the mapping object at the top of the stack is
+   "unpacked" and its contents are appended to the keyword arguments passed in.
+   The mapping object at the top of the stack is ignored when computing
+   the value of ``argc``.
 
 
 .. opcode:: CALL_FUNCTION_VAR_KW (argc)
 
-   Calls a function. *argc* is interpreted as in :opcode:`CALL_FUNCTION`.  The
-   top element on the stack contains the keyword arguments dictionary, followed
-   by the variable-arguments tuple, followed by explicit keyword and positional
-   arguments.
+   Calls a callable object, similarly to :opcode:`CALL_FUNCTION_VAR` and
+   :opcode:`CALL_FUNCTION_KW`.
+   *argc* represents the number of keyword and positional
+   arguments, identically to :opcode:`CALL_FUNCTION`.
+   The top of the stack contains a mapping object, as per
+   :opcode:`CALL_FUNCTION_KW`.
+   Below that are keyword arguments (if any), stored
+   identically to :opcode:`CALL_FUNCTION`.
+   Below that
+   is an iterable object containing additional positional arguments.
+   Below that are positional arguments (if any)
+   and a callable object, identically to :opcode:`CALL_FUNCTION`.
+   Before the callable is called, the mapping object and iterable object
+   are each "unpacked" and their contents passed in as keyword and
+   positional arguments respectively,
+   identically to :opcode:`CALL_FUNCTION_VAR` and :opcode:`CALL_FUNCTION_KW`.
+   The mapping object and iterable object are both ignored when computing
+   the value of ``argc``.
+
+   .. versionchanged:: 3.5
+      In versions 3.0 to 3.4, the iterable object was above
+      the keyword arguments; in 3.5 the iterable object was moved
+      below the keyword arguments.
 
 
 .. opcode:: HAVE_ARGUMENT
@@ -1071,7 +1119,7 @@ instructions:
 
 .. data:: hasconst
 
-   Sequence of bytecodes that have a constant parameter.
+   Sequence of bytecodes that access a constant.
 
 
 .. data:: hasfree
diff --git a/Misc/NEWS.d/next/Documentation/2018-07-19-00-02-23.bpo-33216.YrBgBe.rst b/Misc/NEWS.d/next/Documentation/2018-07-19-00-02-23.bpo-33216.YrBgBe.rst
new file mode 100644
index 0000000000..5668915840
--- /dev/null
+++ b/Misc/NEWS.d/next/Documentation/2018-07-19-00-02-23.bpo-33216.YrBgBe.rst
@@ -0,0 +1,2 @@
+Clarify the documentation for CALL_FUNCTION_VAR, CALL_FUNCTION_KW, and
+CALL_FUNCTION_VAR_KW.