Argument Clinic How-To
**********************

autor:
   Larry Hastings


Streszczenie
^^^^^^^^^^^^

Argument Clinic is a preprocessor for CPython C files. Its purpose is
to automate all the boilerplate involved with writing argument parsing
code for "builtins". This document shows you how to convert your first
C function to work with Argument Clinic, and then introduces some
advanced topics on Argument Clinic usage.

Currently Argument Clinic is considered internal-only for CPython.
Its use is not supported for files outside CPython, and no guarantees
are made regarding backwards compatibility for future versions.  In
other words: if you maintain an external C extension for CPython,
you're welcome to experiment with Argument Clinic in your own code.
But the version of Argument Clinic that ships with the next version of
CPython *could* be totally incompatible and break all your code.


The Goals Of Argument Clinic
============================

Argument Clinic's primary goal is to take over responsibility for all
argument parsing code inside CPython.  This means that, when you
convert a function to work with Argument Clinic, that function should
no longer do any of its own argument parsing—the code generated by
Argument Clinic should be a "black box" to you, where CPython calls in
at the top, and your code gets called at the bottom, with "PyObject
*args" (and maybe "PyObject *kwargs") magically converted into the C
variables and types you need.

In order for Argument Clinic to accomplish its primary goal, it must
be easy to use.  Currently, working with CPython's argument parsing
library is a chore, requiring maintaining redundant information in a
surprising number of places. When you use Argument Clinic, you don't
have to repeat yourself.

Obviously, no one would want to use Argument Clinic unless it's
solving their problem—and without creating new problems of its own. So
it's paramount that Argument Clinic generate correct code. It'd be
nice if the code was faster, too, but at the very least it should not
introduce a major speed regression.  (Eventually Argument Clinic
*should* make a major speedup possible—we could rewrite its code
generator to produce tailor-made argument parsing code, rather than
calling the general-purpose CPython argument parsing library.  That
would make for the fastest argument parsing possible!)

Additionally, Argument Clinic must be flexible enough to work with any
approach to argument parsing.  Python has some functions with some
very strange parsing behaviors; Argument Clinic's goal is to support
all of them.

Finally, the original motivation for Argument Clinic was to provide
introspection "signatures" for CPython builtins. It used to be, the
introspection query functions would throw an exception if you passed
in a builtin.  With Argument Clinic, that's a thing of the past!

One idea you should keep in mind, as you work with Argument Clinic:
the more information you give it, the better job it'll be able to do.
Argument Clinic is admittedly relatively simple right now.  But as it
evolves it will get more sophisticated, and it should be able to do
many interesting and smart things with all the information you give
it.


Basic Concepts And Usage
========================

Argument Clinic ships with CPython; you'll find it in
"Tools/clinic/clinic.py". If you run that script, specifying a C file
as an argument:

   $ python3 Tools/clinic/clinic.py foo.c

Argument Clinic will scan over the file looking for lines that look
exactly like this:

   /*[clinic input]

When it finds one, it reads everything up to a line that looks exactly
like this:

   [clinic start generated code]*/

Everything in between these two lines is input for Argument Clinic.
All of these lines, including the beginning and ending comment lines,
are collectively called an Argument Clinic "block".

When Argument Clinic parses one of these blocks, it generates output.
This output is rewritten into the C file immediately after the block,
followed by a comment containing a checksum. The Argument Clinic block
now looks like this:

   /*[clinic input]
   ... clinic input goes here ...
   [clinic start generated code]*/
   ... clinic output goes here ...
   /*[clinic end generated code: checksum=...]*/

If you run Argument Clinic on the same file a second time, Argument
Clinic will discard the old output and write out the new output with a
fresh checksum line.  However, if the input hasn't changed, the output
won't change either.

You should never modify the output portion of an Argument Clinic
block.  Instead, change the input until it produces the output you
want.  (That's the purpose of the checksum—to detect if someone
changed the output, as these edits would be lost the next time
Argument Clinic writes out fresh output.)

For the sake of clarity, here's the terminology we'll use with
Argument Clinic:

* The first line of the comment ("/*[clinic input]") is the *start
  line*.

* The last line of the initial comment ("[clinic start generated
  code]*/") is the *end line*.

* The last line ("/*[clinic end generated code: checksum=...]*/") is
  the *checksum line*.

* In between the start line and the end line is the *input*.

* In between the end line and the checksum line is the *output*.

* All the text collectively, from the start line to the checksum line
  inclusively, is the *block*.  (A block that hasn't been successfully
  processed by Argument Clinic yet doesn't have output or a checksum
  line, but it's still considered a block.)


Converting Your First Function
==============================

The best way to get a sense of how Argument Clinic works is to convert
a function to work with it.  Here, then, are the bare minimum steps
you'd need to follow to convert a function to work with Argument
Clinic.  Note that for code you plan to check in to CPython, you
really should take the conversion farther, using some of the advanced
concepts you'll see later on in the document (like "return converters"
and "self converters"). But we'll keep it simple for this walkthrough
so you can learn.

Let's dive in!

* Make sure you're working with a freshly updated checkout of the
  CPython trunk.

* Find a Python builtin that calls either "PyArg_ParseTuple()" or
  "PyArg_ParseTupleAndKeywords()", and hasn't been converted to work
  with Argument Clinic yet. For my example I'm using
  "_pickle.Pickler.dump()".

* If the call to the "PyArg_Parse" function uses any of the following
  format units:

     O&
     O!
     es
     es#
     et
     et#

  or if it has multiple calls to "PyArg_ParseTuple()", you should
  choose a different function.  Argument Clinic *does* support all of
  these scenarios.  But these are advanced topics—let's do something
  simpler for your first function.

  Also, if the function has multiple calls to "PyArg_ParseTuple()" or
  "PyArg_ParseTupleAndKeywords()" where it supports different types
  for the same argument, or if the function uses something besides
  PyArg_Parse functions to parse its arguments, it probably isn't
  suitable for conversion to Argument Clinic.  Argument Clinic doesn't
  support generic functions or polymorphic parameters.

* Add the following boilerplate above the function, creating our
  block:

     /*[clinic input]
     [clinic start generated code]*/

* Cut the docstring and paste it in between the "[clinic]" lines,
  removing all the junk that makes it a properly quoted C string. When
  you're done you should have just the text, based at the left margin,
  with no line wider than 80 characters. (Argument Clinic will
  preserve indents inside the docstring.)

  If the old docstring had a first line that looked like a function
  signature, throw that line away.  (The docstring doesn't need it
  anymore—when you use "help()" on your builtin in the future, the
  first line will be built automatically based on the function's
  signature.)

  Sample:

     /*[clinic input]
     Write a pickled representation of obj to the open file.
     [clinic start generated code]*/

* If your docstring doesn't have a "summary" line, Argument Clinic
  will complain.  So let's make sure it has one.  The "summary" line
  should be a paragraph consisting of a single 80-column line at the
  beginning of the docstring.

  (Our example docstring consists solely of a summary line, so the
  sample code doesn't have to change for this step.)

* Above the docstring, enter the name of the function, followed by a
  blank line.  This should be the Python name of the function, and
  should be the full dotted path to the function—it should start with
  the name of the module, include any sub-modules, and if the function
  is a method on a class it should include the class name too.

  Sample:

     /*[clinic input]
     _pickle.Pickler.dump

     Write a pickled representation of obj to the open file.
     [clinic start generated code]*/

* If this is the first time that module or class has been used with
  Argument Clinic in this C file, you must declare the module and/or
  class.  Proper Argument Clinic hygiene prefers declaring these in a
  separate block somewhere near the top of the C file, in the same way
  that include files and statics go at the top.  (In our sample code
  we'll just show the two blocks next to each other.)

  The name of the class and module should be the same as the one seen
  by Python.  Check the name defined in the "PyModuleDef" or
  "PyTypeObject" as appropriate.

  When you declare a class, you must also specify two aspects of its
  type in C: the type declaration you'd use for a pointer to an
  instance of this class, and a pointer to the "PyTypeObject" for this
  class.

  Sample:

     /*[clinic input]
     module _pickle
     class _pickle.Pickler "PicklerObject *" "&Pickler_Type"
     [clinic start generated code]*/

     /*[clinic input]
     _pickle.Pickler.dump

     Write a pickled representation of obj to the open file.
     [clinic start generated code]*/

* Declare each of the parameters to the function.  Each parameter
  should get its own line.  All the parameter lines should be indented
  from the function name and the docstring.

  The general form of these parameter lines is as follows:

     name_of_parameter: converter

  If the parameter has a default value, add that after the converter:

     name_of_parameter: converter = default_value

  Argument Clinic's support for "default values" is quite
  sophisticated; please see the section below on default values for
  more information.

  Add a blank line below the parameters.

  What's a "converter"?  It establishes both the type of the variable
  used in C, and the method to convert the Python value into a C value
  at runtime. For now you're going to use what's called a "legacy
  converter"—a convenience syntax intended to make porting old code
  into Argument Clinic easier.

  For each parameter, copy the "format unit" for that parameter from
  the "PyArg_Parse()" format argument and specify *that* as its
  converter, as a quoted string.  ("format unit" is the formal name
  for the one-to-three character substring of the "format" parameter
  that tells the argument parsing function what the type of the
  variable is and how to convert it.  For more on format units please
  see Pobieranie kolejnych rzeczy podanych na wejściu i konstruowanie
  wartości..)

  For multicharacter format units like "z#", use the entire two-or-
  three character string.

  Sample:

      /*[clinic input]
      module _pickle
      class _pickle.Pickler "PicklerObject *" "&Pickler_Type"
      [clinic start generated code]*/

      /*[clinic input]
      _pickle.Pickler.dump

         obj: 'O'

     Write a pickled representation of obj to the open file.
     [clinic start generated code]*/

* If your function has "|" in the format string, meaning some
  parameters have default values, you can ignore it.  Argument Clinic
  infers which parameters are optional based on whether or not they
  have default values.

  If your function has "$" in the format string, meaning it takes
  keyword-only arguments, specify "*" on a line by itself before the
  first keyword-only argument, indented the same as the parameter
  lines.

  ("_pickle.Pickler.dump" has neither, so our sample is unchanged.)

* If the existing C function calls "PyArg_ParseTuple()" (as opposed to
  "PyArg_ParseTupleAndKeywords()"), then all its arguments are
  positional-only.

  To mark all parameters as positional-only in Argument Clinic, add a
  "/" on a line by itself after the last parameter, indented the same
  as the parameter lines.

  Currently this is all-or-nothing; either all parameters are
  positional-only, or none of them are.  (In the future Argument
  Clinic may relax this restriction.)

  Sample:

     /*[clinic input]
     module _pickle
     class _pickle.Pickler "PicklerObject *" "&Pickler_Type"
     [clinic start generated code]*/

     /*[clinic input]
     _pickle.Pickler.dump

         obj: 'O'
         /

     Write a pickled representation of obj to the open file.
     [clinic start generated code]*/

* It's helpful to write a per-parameter docstring for each parameter.
  But per-parameter docstrings are optional; you can skip this step if
  you prefer.

  Here's how to add a per-parameter docstring.  The first line of the
  per-parameter docstring must be indented further than the parameter
  definition.  The left margin of this first line establishes the left
  margin for the whole per-parameter docstring; all the text you write
  will be outdented by this amount.  You can write as much text as you
  like, across multiple lines if you wish.

  Sample:

     /*[clinic input]
     module _pickle
     class _pickle.Pickler "PicklerObject *" "&Pickler_Type"
     [clinic start generated code]*/

     /*[clinic input]
     _pickle.Pickler.dump

         obj: 'O'
             The object to be pickled.
         /

     Write a pickled representation of obj to the open file.
     [clinic start generated code]*/

* Save and close the file, then run "Tools/clinic/clinic.py" on it.
  With luck everything worked---your block now has output, and a
  ".c.h" file has been generated! Reopen the file in your text editor
  to see:

     /*[clinic input]
     _pickle.Pickler.dump

         obj: 'O'
             The object to be pickled.
         /

     Write a pickled representation of obj to the open file.
     [clinic start generated code]*/

     static PyObject *
     _pickle_Pickler_dump(PicklerObject *self, PyObject *obj)
     /*[clinic end generated code: output=87ecad1261e02ac7 input=552eb1c0f52260d9]*/

  Obviously, if Argument Clinic didn't produce any output, it's
  because it found an error in your input.  Keep fixing your errors
  and retrying until Argument Clinic processes your file without
  complaint.

  For readability, most of the glue code has been generated to a
  ".c.h" file.  You'll need to include that in your original ".c"
  file, typically right after the clinic module block:

     #include "clinic/_pickle.c.h"

* Double-check that the argument-parsing code Argument Clinic
  generated looks basically the same as the existing code.

  First, ensure both places use the same argument-parsing function.
  The existing code must call either "PyArg_ParseTuple()" or
  "PyArg_ParseTupleAndKeywords()"; ensure that the code generated by
  Argument Clinic calls the *exact* same function.

  Second, the format string passed in to "PyArg_ParseTuple()" or
  "PyArg_ParseTupleAndKeywords()" should be *exactly* the same as the
  hand-written one in the existing function, up to the colon or semi-
  colon.

  (Argument Clinic always generates its format strings with a ":"
  followed by the name of the function.  If the existing code's format
  string ends with ";", to provide usage help, this change is
  harmless—don't worry about it.)

  Third, for parameters whose format units require two arguments (like
  a length variable, or an encoding string, or a pointer to a
  conversion function), ensure that the second argument is *exactly*
  the same between the two invocations.

  Fourth, inside the output portion of the block you'll find a
  preprocessor macro defining the appropriate static "PyMethodDef"
  structure for this builtin:

     #define __PICKLE_PICKLER_DUMP_METHODDEF    \
     {"dump", (PyCFunction)__pickle_Pickler_dump, METH_O, __pickle_Pickler_dump__doc__},

  This static structure should be *exactly* the same as the existing
  static "PyMethodDef" structure for this builtin.

  If any of these items differ in *any way*, adjust your Argument
  Clinic function specification and rerun "Tools/clinic/clinic.py"
  until they *are* the same.

* Notice that the last line of its output is the declaration of your
  "impl" function.  This is where the builtin's implementation goes.
  Delete the existing prototype of the function you're modifying, but
  leave the opening curly brace.  Now delete its argument parsing code
  and the declarations of all the variables it dumps the arguments
  into. Notice how the Python arguments are now arguments to this impl
  function; if the implementation used different names for these
  variables, fix it.

  Let's reiterate, just because it's kind of weird.  Your code should
  now look like this:

     static return_type
     your_function_impl(...)
     /*[clinic end generated code: checksum=...]*/
     {
     ...

  Argument Clinic generated the checksum line and the function
  prototype just above it.  You should write the opening (and closing)
  curly braces for the function, and the implementation inside.

  Sample:

     /*[clinic input]
     module _pickle
     class _pickle.Pickler "PicklerObject *" "&Pickler_Type"
     [clinic start generated code]*/
     /*[clinic end generated code: checksum=da39a3ee5e6b4b0d3255bfef95601890afd80709]*/

     /*[clinic input]
     _pickle.Pickler.dump

         obj: 'O'
             The object to be pickled.
         /

     Write a pickled representation of obj to the open file.
     [clinic start generated code]*/

     PyDoc_STRVAR(__pickle_Pickler_dump__doc__,
     "Write a pickled representation of obj to the open file.\n"
     "\n"
     ...
     static PyObject *
     _pickle_Pickler_dump_impl(PicklerObject *self, PyObject *obj)
     /*[clinic end generated code: checksum=3bd30745bf206a48f8b576a1da3d90f55a0a4187]*/
     {
         /* Check whether the Pickler was initialized correctly (issue3664).
            Developers often forget to call __init__() in their subclasses, which
            would trigger a segfault without this check. */
         if (self->write == NULL) {
             PyErr_Format(PicklingError,
                          "Pickler.__init__() was not called by %s.__init__()",
                          Py_TYPE(self)->tp_name);
             return NULL;
         }

         if (_Pickler_ClearBuffer(self) < 0)
             return NULL;

         ...

* Remember the macro with the "PyMethodDef" structure for this
  function?  Find the existing "PyMethodDef" structure for this
  function and replace it with a reference to the macro.  (If the
  builtin is at module scope, this will probably be very near the end
  of the file; if the builtin is a class method, this will probably be
  below but relatively near to the implementation.)

  Note that the body of the macro contains a trailing comma.  So when
  you replace the existing static "PyMethodDef" structure with the
  macro, *don't* add a comma to the end.

  Sample:

     static struct PyMethodDef Pickler_methods[] = {
         __PICKLE_PICKLER_DUMP_METHODDEF
         __PICKLE_PICKLER_CLEAR_MEMO_METHODDEF
         {NULL, NULL}                /* sentinel */
     };

* Compile, then run the relevant portions of the regression-test
  suite. This change should not introduce any new compile-time
  warnings or errors, and there should be no externally-visible change
  to Python's behavior.

  Well, except for one difference: "inspect.signature()" run on your
  function should now provide a valid signature!

  Congratulations, you've ported your first function to work with
  Argument Clinic!


Advanced Topics
===============

Now that you've had some experience working with Argument Clinic, it's
time for some advanced topics.


Symbolic default values
-----------------------

The default value you provide for a parameter can't be any arbitrary
expression.  Currently the following are explicitly supported:

* Numeric constants (integer and float)

* String constants

* "True", "False", and "None"

* Simple symbolic constants like "sys.maxsize", which must start with
  the name of the module

In case you're curious, this is implemented in  "from_builtin()" in
"Lib/inspect.py".

(In the future, this may need to get even more elaborate, to allow
full expressions like "CONSTANT - 1".)


Renaming the C functions and variables generated by Argument Clinic
-------------------------------------------------------------------

Argument Clinic automatically names the functions it generates for
you. Occasionally this may cause a problem, if the generated name
collides with the name of an existing C function.  There's an easy
solution: override the names used for the C functions.  Just add the
keyword ""as"" to your function declaration line, followed by the
function name you wish to use. Argument Clinic will use that function
name for the base (generated) function, then add ""_impl"" to the end
and use that for the name of the impl function.

For example, if we wanted to rename the C function names generated for
"pickle.Pickler.dump", it'd look like this:

   /*[clinic input]
   pickle.Pickler.dump as pickler_dumper

   ...

The base function would now be named "pickler_dumper()", and the impl
function would now be named "pickler_dumper_impl()".

Similarly, you may have a problem where you want to give a parameter a
specific Python name, but that name may be inconvenient in C.
Argument Clinic allows you to give a parameter different names in
Python and in C, using the same ""as"" syntax:

   /*[clinic input]
   pickle.Pickler.dump

       obj: object
       file as file_obj: object
       protocol: object = NULL
       *
       fix_imports: bool = True

Here, the name used in Python (in the signature and the "keywords"
array) would be "file", but the C variable would be named "file_obj".

You can use this to rename the "self" parameter too!


Converting functions using PyArg_UnpackTuple
--------------------------------------------

To convert a function parsing its arguments with
"PyArg_UnpackTuple()", simply write out all the arguments, specifying
each as an "object".  You may specify the "type" argument to cast the
type as appropriate.  All arguments should be marked positional-only
(add a "/" on a line by itself after the last argument).

Currently the generated code will use "PyArg_ParseTuple()", but this
will change soon.


Optional Groups
---------------

Some legacy functions have a tricky approach to parsing their
arguments: they count the number of positional arguments, then use a
"switch" statement to call one of several different
"PyArg_ParseTuple()" calls depending on how many positional arguments
there are.  (These functions cannot accept keyword-only arguments.)
This approach was used to simulate optional arguments back before
"PyArg_ParseTupleAndKeywords()" was created.

While functions using this approach can often be converted to use
"PyArg_ParseTupleAndKeywords()", optional arguments, and default
values, it's not always possible.  Some of these legacy functions have
behaviors "PyArg_ParseTupleAndKeywords()" doesn't directly support.
The most obvious example is the builtin function "range()", which has
an optional argument on the *left* side of its required argument!
Another example is "curses.window.addch()", which has a group of two
arguments that must always be specified together.  (The arguments are
called "x" and "y"; if you call the function passing in "x", you must
also pass in "y"—and if you don't pass in "x" you may not pass in "y"
either.)

In any case, the goal of Argument Clinic is to support argument
parsing for all existing CPython builtins without changing their
semantics. Therefore Argument Clinic supports this alternate approach
to parsing, using what are called *optional groups*. Optional groups
are groups of arguments that must all be passed in together. They can
be to the left or the right of the required arguments.  They can
*only* be used with positional-only parameters.

Informacja:

  Optional groups are *only* intended for use when converting
  functions that make multiple calls to "PyArg_ParseTuple()"!
  Functions that use *any* other approach for parsing arguments should
  *almost never* be converted to Argument Clinic using optional
  groups.  Functions using optional groups currently cannot have
  accurate signatures in Python, because Python just doesn't
  understand the concept.  Please avoid using optional groups wherever
  possible.

To specify an optional group, add a "[" on a line by itself before the
parameters you wish to group together, and a "]" on a line by itself
after these parameters.  As an example, here's how
"curses.window.addch" uses optional groups to make the first two
parameters and the last parameter optional:

   /*[clinic input]

   curses.window.addch

       [
       x: int
         X-coordinate.
       y: int
         Y-coordinate.
       ]

       ch: object
         Character to add.

       [
       attr: long
         Attributes for the character.
       ]
       /

   ...

Uwagi:

* For every optional group, one additional parameter will be passed
  into the impl function representing the group.  The parameter will
  be an int named "group_{direction}_{number}", where "{direction}" is
  either "right" or "left" depending on whether the group is before or
  after the required parameters, and "{number}" is a monotonically
  increasing number (starting at 1) indicating how far away the group
  is from the required parameters.  When the impl is called, this
  parameter will be set to zero if this group was unused, and set to
  non-zero if this group was used. (By used or unused, I mean whether
  or not the parameters received arguments in this invocation.)

* If there are no required arguments, the optional groups will behave
  as if they're to the right of the required arguments.

* In the case of ambiguity, the argument parsing code favors
  parameters on the left (before the required parameters).

* Optional groups can only contain positional-only parameters.

* Optional groups are *only* intended for legacy code.  Please do not
  use optional groups for new code.


Using real Argument Clinic converters, instead of "legacy converters"
---------------------------------------------------------------------

To save time, and to minimize how much you need to learn to achieve
your first port to Argument Clinic, the walkthrough above tells you to
use "legacy converters".  "Legacy converters" are a convenience,
designed explicitly to make porting existing code to Argument Clinic
easier.  And to be clear, their use is acceptable when porting code
for Python 3.4.

However, in the long term we probably want all our blocks to use
Argument Clinic's real syntax for converters.  Why?  A couple reasons:

* The proper converters are far easier to read and clearer in their
  intent.

* There are some format units that are unsupported as "legacy
  converters", because they require arguments, and the legacy
  converter syntax doesn't support specifying arguments.

* In the future we may have a new argument parsing library that isn't
  restricted to what "PyArg_ParseTuple()" supports; this flexibility
  won't be available to parameters using legacy converters.

Therefore, if you don't mind a little extra effort, please use the
normal converters instead of legacy converters.

In a nutshell, the syntax for Argument Clinic (non-legacy) converters
looks like a Python function call.  However, if there are no explicit
arguments to the function (all functions take their default values),
you may omit the parentheses.  Thus "bool" and "bool()" are exactly
the same converters.

All arguments to Argument Clinic converters are keyword-only. All
Argument Clinic converters accept the following arguments:

   "c_default"
      The default value for this parameter when defined in C.
      Specifically, this will be the initializer for the variable
      declared in the "parse function".  See the section on default
      values for how to use this. Specified as a string.

   "annotation"
      The annotation value for this parameter.  Not currently
      supported, because **PEP 8** mandates that the Python library
      may not use annotations.

In addition, some converters accept additional arguments.  Here is a
list of these arguments, along with their meanings:

   "accept"
      A set of Python types (and possibly pseudo-types); this
      restricts the allowable Python argument to values of these
      types. (This is not a general-purpose facility; as a rule it
      only supports specific lists of types as shown in the legacy
      converter table.)

      To accept "None", add "NoneType" to this set.

   "bitwise"
      Only supported for unsigned integers.  The native integer value
      of this Python argument will be written to the parameter without
      any range checking, even for negative values.

   "converter"
      Only supported by the "object" converter.  Specifies the name of
      a C "converter function" to use to convert this object to a
      native type.

   "encoding"
      Only supported for strings.  Specifies the encoding to use when
      converting this string from a Python str (Unicode) value into a
      C "char *" value.

   "subclass_of"
      Only supported for the "object" converter.  Requires that the
      Python value be a subclass of a Python type, as expressed in C.

   "type"
      Only supported for the "object" and "self" converters.
      Specifies the C type that will be used to declare the variable.
      Default value is ""PyObject *"".

   "zeroes"
      Only supported for strings.  If true, embedded NUL bytes
      ("'\\0'") are permitted inside the value.  The length of the
      string will be passed in to the impl function, just after the
      string parameter, as a parameter named
      "<parameter_name>_length".

Please note, not every possible combination of arguments will work.
Usually these arguments are implemented by specific "PyArg_ParseTuple"
*format units*, with specific behavior.  For example, currently you
cannot call "unsigned_short" without also specifying "bitwise=True".
Although it's perfectly reasonable to think this would work, these
semantics don't map to any existing format unit.  So Argument Clinic
doesn't support it.  (Or, at least, not yet.)

Below is a table showing the mapping of legacy converters into real
Argument Clinic converters.  On the left is the legacy converter, on
the right is the text you'd replace it with.

+-----------+-----------------------------------------------------------------------------------+
| "'B'"     | "unsigned_char(bitwise=True)"                                                     |
+-----------+-----------------------------------------------------------------------------------+
| "'b'"     | "unsigned_char"                                                                   |
+-----------+-----------------------------------------------------------------------------------+
| "'c'"     | "char"                                                                            |
+-----------+-----------------------------------------------------------------------------------+
| "'C'"     | "int(accept={str})"                                                               |
+-----------+-----------------------------------------------------------------------------------+
| "'d'"     | "double"                                                                          |
+-----------+-----------------------------------------------------------------------------------+
| "'D'"     | "Py_complex"                                                                      |
+-----------+-----------------------------------------------------------------------------------+
| "'es'"    | "str(encoding='name_of_encoding')"                                                |
+-----------+-----------------------------------------------------------------------------------+
| "'es#'"   | "str(encoding='name_of_encoding', zeroes=True)"                                   |
+-----------+-----------------------------------------------------------------------------------+
| "'et'"    | "str(encoding='name_of_encoding', accept={bytes, bytearray, str})"                |
+-----------+-----------------------------------------------------------------------------------+
| "'et#'"   | "str(encoding='name_of_encoding', accept={bytes, bytearray, str}, zeroes=True)"   |
+-----------+-----------------------------------------------------------------------------------+
| "'f'"     | "float"                                                                           |
+-----------+-----------------------------------------------------------------------------------+
| "'h'"     | "short"                                                                           |
+-----------+-----------------------------------------------------------------------------------+
| "'H'"     | "unsigned_short(bitwise=True)"                                                    |
+-----------+-----------------------------------------------------------------------------------+
| "'i'"     | "int"                                                                             |
+-----------+-----------------------------------------------------------------------------------+
| "'I'"     | "unsigned_int(bitwise=True)"                                                      |
+-----------+-----------------------------------------------------------------------------------+
| "'k'"     | "unsigned_long(bitwise=True)"                                                     |
+-----------+-----------------------------------------------------------------------------------+
| "'K'"     | "unsigned_long_long(bitwise=True)"                                                |
+-----------+-----------------------------------------------------------------------------------+
| "'I'"     | "long"                                                                            |
+-----------+-----------------------------------------------------------------------------------+
| "'L'"     | "long long"                                                                       |
+-----------+-----------------------------------------------------------------------------------+
| "'n'"     | "Py_ssize_t"                                                                      |
+-----------+-----------------------------------------------------------------------------------+
| "'O'"     | "object"                                                                          |
+-----------+-----------------------------------------------------------------------------------+
| "'O!'"    | "object(subclass_of='&PySomething_Type')"                                         |
+-----------+-----------------------------------------------------------------------------------+
| "'O&'"    | "object(converter='name_of_c_function')"                                          |
+-----------+-----------------------------------------------------------------------------------+
| "'p'"     | "bool"                                                                            |
+-----------+-----------------------------------------------------------------------------------+
| "'S'"     | "PyBytesObject"                                                                   |
+-----------+-----------------------------------------------------------------------------------+
| "'s'"     | "str"                                                                             |
+-----------+-----------------------------------------------------------------------------------+
| "'s#'"    | "str(zeroes=True)"                                                                |
+-----------+-----------------------------------------------------------------------------------+
| "'s*'"    | "Py_buffer(accept={buffer, str})"                                                 |
+-----------+-----------------------------------------------------------------------------------+
| "'U'"     | "unicode"                                                                         |
+-----------+-----------------------------------------------------------------------------------+
| "'u'"     | "Py_UNICODE"                                                                      |
+-----------+-----------------------------------------------------------------------------------+
| "'u#'"    | "Py_UNICODE(zeroes=True)"                                                         |
+-----------+-----------------------------------------------------------------------------------+
| "'w*'"    | "Py_buffer(accept={rwbuffer})"                                                    |
+-----------+-----------------------------------------------------------------------------------+
| "'Y'"     | "PyByteArrayObject"                                                               |
+-----------+-----------------------------------------------------------------------------------+
| "'y'"     | "str(accept={bytes})"                                                             |
+-----------+-----------------------------------------------------------------------------------+
| "'y#'"    | "str(accept={robuffer}, zeroes=True)"                                             |
+-----------+-----------------------------------------------------------------------------------+
| "'y*'"    | "Py_buffer"                                                                       |
+-----------+-----------------------------------------------------------------------------------+
| "'Z'"     | "Py_UNICODE(accept={str, NoneType})"                                              |
+-----------+-----------------------------------------------------------------------------------+
| "'Z#'"    | "Py_UNICODE(accept={str, NoneType}, zeroes=True)"                                 |
+-----------+-----------------------------------------------------------------------------------+
| "'z'"     | "str(accept={str, NoneType})"                                                     |
+-----------+-----------------------------------------------------------------------------------+
| "'z#'"    | "str(accept={str, NoneType}, zeroes=True)"                                        |
+-----------+-----------------------------------------------------------------------------------+
| "'z*'"    | "Py_buffer(accept={buffer, str, NoneType})"                                       |
+-----------+-----------------------------------------------------------------------------------+

As an example, here's our sample "pickle.Pickler.dump" using the
proper converter:

   /*[clinic input]
   pickle.Pickler.dump

       obj: object
           The object to be pickled.
       /

   Write a pickled representation of obj to the open file.
   [clinic start generated code]*/

One advantage of real converters is that they're more flexible than
legacy converters.  For example, the "unsigned_int" converter (and all
the "unsigned_" converters) can be specified without "bitwise=True".
Their default behavior performs range checking on the value, and they
won't accept negative numbers.  You just can't do that with a legacy
converter!

Argument Clinic will show you all the converters it has available.
For each converter it'll show you all the parameters it accepts, along
with the default value for each parameter. Just run
"Tools/clinic/clinic.py --converters" to see the full list.


Py_buffer
---------

When using the "Py_buffer" converter (or the "'s*'", "'w*'", "'*y'",
or "'z*'" legacy converters), you *must* not call "PyBuffer_Release()"
on the provided buffer. Argument Clinic generates code that does it
for you (in the parsing function).


Advanced converters
-------------------

Remember those format units you skipped for your first time because
they were advanced?  Here's how to handle those too.

The trick is, all those format units take arguments—either conversion
functions, or types, or strings specifying an encoding. (But "legacy
converters" don't support arguments.  That's why we skipped them for
your first function.)  The argument you specified to the format unit
is now an argument to the converter; this argument is either
"converter" (for "O&"), "subclass_of" (for "O!"), or "encoding" (for
all the format units that start with "e").

When using "subclass_of", you may also want to use the other custom
argument for "object()": "type", which lets you set the type actually
used for the parameter.  For example, if you want to ensure that the
object is a subclass of "PyUnicode_Type", you probably want to use the
converter "object(type='PyUnicodeObject *',
subclass_of='&PyUnicode_Type')".

One possible problem with using Argument Clinic: it takes away some
possible flexibility for the format units starting with "e".  When
writing a "PyArg_Parse" call by hand, you could theoretically decide
at runtime what encoding string to pass in to "PyArg_ParseTuple()".
But now this string must be hard-coded at Argument-Clinic-
preprocessing-time.  This limitation is deliberate; it made supporting
this format unit much easier, and may allow for future optimizations.
This restriction doesn't seem unreasonable; CPython itself always
passes in static hard-coded encoding strings for parameters whose
format units start with "e".


Parameter default values
------------------------

Default values for parameters can be any of a number of values. At
their simplest, they can be string, int, or float literals:

   foo: str = "abc"
   bar: int = 123
   bat: float = 45.6

They can also use any of Python's built-in constants:

   yep:  bool = True
   nope: bool = False
   nada: object = None

There's also special support for a default value of "NULL", and for
simple expressions, documented in the following sections.


The "NULL" default value
------------------------

For string and object parameters, you can set them to "None" to
indicate that there's no default.  However, that means the C variable
will be initialized to "Py_None".  For convenience's sakes, there's a
special value called "NULL" for just this reason: from Python's
perspective it behaves like a default value of "None", but the C
variable is initialized with "NULL".


Expressions specified as default values
---------------------------------------

The default value for a parameter can be more than just a literal
value. It can be an entire expression, using math operators and
looking up attributes on objects.  However, this support isn't exactly
simple, because of some non-obvious semantics.

Consider the following example:

   foo: Py_ssize_t = sys.maxsize - 1

"sys.maxsize" can have different values on different platforms.
Therefore Argument Clinic can't simply evaluate that expression
locally and hard-code it in C.  So it stores the default in such a way
that it will get evaluated at runtime, when the user asks for the
function's signature.

What namespace is available when the expression is evaluated?  It's
evaluated in the context of the module the builtin came from.  So, if
your module has an attribute called ""max_widgets"", you may simply
use it:

   foo: Py_ssize_t = max_widgets

If the symbol isn't found in the current module, it fails over to
looking in "sys.modules".  That's how it can find "sys.maxsize" for
example.  (Since you don't know in advance what modules the user will
load into their interpreter, it's best to restrict yourself to modules
that are preloaded by Python itself.)

Evaluating default values only at runtime means Argument Clinic can't
compute the correct equivalent C default value.  So you need to tell
it explicitly. When you use an expression, you must also specify the
equivalent expression in C, using the "c_default" parameter to the
converter:

   foo: Py_ssize_t(c_default="PY_SSIZE_T_MAX - 1") = sys.maxsize - 1

Another complication: Argument Clinic can't know in advance whether or
not the expression you supply is valid.  It parses it to make sure it
looks legal, but it can't *actually* know.  You must be very careful
when using expressions to specify values that are guaranteed to be
valid at runtime!

Finally, because expressions must be representable as static C values,
there are many restrictions on legal expressions.  Here's a list of
Python features you're not permitted to use:

* Function calls.

* Inline if statements ("3 if foo else 5").

* Automatic sequence unpacking ("*[1, 2, 3]").

* List/set/dict comprehensions and generator expressions.

* Tuple/list/set/dict literals.


Using a return converter
------------------------

By default the impl function Argument Clinic generates for you returns
"PyObject *". But your C function often computes some C type, then
converts it into the "PyObject *" at the last moment.  Argument Clinic
handles converting your inputs from Python types into native C
types—why not have it convert your return value from a native C type
into a Python type too?

That's what a "return converter" does.  It changes your impl function
to return some C type, then adds code to the generated (non-impl)
function to handle converting that value into the appropriate
"PyObject *".

The syntax for return converters is similar to that of parameter
converters. You specify the return converter like it was a return
annotation on the function itself.  Return converters behave much the
same as parameter converters; they take arguments, the arguments are
all keyword-only, and if you're not changing any of the default
arguments you can omit the parentheses.

(If you use both ""as"" *and* a return converter for your function,
the ""as"" should come before the return converter.)

There's one additional complication when using return converters: how
do you indicate an error has occurred?  Normally, a function returns a
valid (non-"NULL") pointer for success, and "NULL" for failure.  But
if you use an integer return converter, all integers are valid.  How
can Argument Clinic detect an error?  Its solution: each return
converter implicitly looks for a special value that indicates an
error.  If you return that value, and an error has been set
("PyErr_Occurred()" returns a true value), then the generated code
will propagate the error.  Otherwise it will encode the value you
return like normal.

Currently Argument Clinic supports only a few return converters:

   bool
   int
   unsigned int
   long
   unsigned int
   size_t
   Py_ssize_t
   float
   double
   DecodeFSDefault

None of these take parameters.  For the first three, return -1 to
indicate error.  For "DecodeFSDefault", the return type is "const char
*"; return a "NULL" pointer to indicate an error.

(There's also an experimental "NoneType" converter, which lets you
return "Py_None" on success or "NULL" on failure, without having to
increment the reference count on "Py_None".  I'm not sure it adds
enough clarity to be worth using.)

To see all the return converters Argument Clinic supports, along with
their parameters (if any), just run "Tools/clinic/clinic.py
--converters" for the full list.


Cloning existing functions
--------------------------

If you have a number of functions that look similar, you may be able
to use Clinic's "clone" feature.  When you clone an existing function,
you reuse:

* its parameters, including

  * their names,

  * their converters, with all parameters,

  * their default values,

  * their per-parameter docstrings,

  * their *kind* (whether they're positional only, positional or
    keyword, or keyword only), and

* its return converter.

The only thing not copied from the original function is its docstring;
the syntax allows you to specify a new docstring.

Here's the syntax for cloning a function:

   /*[clinic input]
   module.class.new_function [as c_basename] = module.class.existing_function

   Docstring for new_function goes here.
   [clinic start generated code]*/

(The functions can be in different modules or classes.  I wrote
"module.class" in the sample just to illustrate that you must use the
full path to *both* functions.)

Sorry, there's no syntax for partially-cloning a function, or cloning
a function then modifying it.  Cloning is an all-or nothing
proposition.

Also, the function you are cloning from must have been previously
defined in the current file.


Calling Python code
-------------------

The rest of the advanced topics require you to write Python code which
lives inside your C file and modifies Argument Clinic's runtime state.
This is simple: you simply define a Python block.

A Python block uses different delimiter lines than an Argument Clinic
function block.  It looks like this:

   /*[python input]
   # python code goes here
   [python start generated code]*/

All the code inside the Python block is executed at the time it's
parsed.  All text written to stdout inside the block is redirected
into the "output" after the block.

As an example, here's a Python block that adds a static integer
variable to the C code:

   /*[python input]
   print('static int __ignored_unused_variable__ = 0;')
   [python start generated code]*/
   static int __ignored_unused_variable__ = 0;
   /*[python checksum:...]*/


Using a "self converter"
------------------------

Argument Clinic automatically adds a "self" parameter for you using a
default converter.  It automatically sets the "type" of this parameter
to the "pointer to an instance" you specified when you declared the
type.  However, you can override Argument Clinic's converter and
specify one yourself. Just add your own "self" parameter as the first
parameter in a block, and ensure that its converter is an instance of
"self_converter" or a subclass thereof.

What's the point?  This lets you override the type of "self", or give
it a different default name.

How do you specify the custom type you want to cast "self" to? If you
only have one or two functions with the same type for "self", you can
directly use Argument Clinic's existing "self" converter, passing in
the type you want to use as the "type" parameter:

   /*[clinic input]

   _pickle.Pickler.dump

     self: self(type="PicklerObject *")
     obj: object
     /

   Write a pickled representation of the given object to the open file.
   [clinic start generated code]*/

On the other hand, if you have a lot of functions that will use the
same type for "self", it's best to create your own converter,
subclassing "self_converter" but overwriting the "type" member:

   /*[python input]
   class PicklerObject_converter(self_converter):
       type = "PicklerObject *"
   [python start generated code]*/

   /*[clinic input]

   _pickle.Pickler.dump

     self: PicklerObject
     obj: object
     /

   Write a pickled representation of the given object to the open file.
   [clinic start generated code]*/


Writing a custom converter
--------------------------

As we hinted at in the previous section... you can write your own
converters! A converter is simply a Python class that inherits from
"CConverter". The main purpose of a custom converter is if you have a
parameter using the "O&" format unit—parsing this parameter means
calling a "PyArg_ParseTuple()" "converter function".

Your converter class should be named "*something*_converter". If the
name follows this convention, then your converter class will be
automatically registered with Argument Clinic; its name will be the
name of your class with the "_converter" suffix stripped off.  (This
is accomplished with a metaclass.)

You shouldn't subclass "CConverter.__init__".  Instead, you should
write a "converter_init()" function.  "converter_init()" always
accepts a "self" parameter; after that, all additional parameters
*must* be keyword-only.  Any arguments passed in to the converter in
Argument Clinic will be passed along to your "converter_init()".

There are some additional members of "CConverter" you may wish to
specify in your subclass.  Here's the current list:

"type"
   The C type to use for this variable. "type" should be a Python
   string specifying the type, e.g. "int". If this is a pointer type,
   the type string should end with "' *'".

"default"
   The Python default value for this parameter, as a Python value. Or
   the magic value "unspecified" if there is no default.

"py_default"
   "default" as it should appear in Python code, as a string. Or
   "None" if there is no default.

"c_default"
   "default" as it should appear in C code, as a string. Or "None" if
   there is no default.

"c_ignored_default"
   The default value used to initialize the C variable when there is
   no default, but not specifying a default may result in an
   "uninitialized variable" warning.  This can easily happen when
   using option groups—although properly-written code will never
   actually use this value, the variable does get passed in to the
   impl, and the C compiler will complain about the "use" of the
   uninitialized value.  This value should always be a non-empty
   string.

"converter"
   The name of the C converter function, as a string.

"impl_by_reference"
   A boolean value.  If true, Argument Clinic will add a "&" in front
   of the name of the variable when passing it into the impl function.

"parse_by_reference"
   A boolean value.  If true, Argument Clinic will add a "&" in front
   of the name of the variable when passing it into
   "PyArg_ParseTuple()".

Here's the simplest example of a custom converter, from
"Modules/zlibmodule.c":

   /*[python input]

   class ssize_t_converter(CConverter):
       type = 'Py_ssize_t'
       converter = 'ssize_t_converter'

   [python start generated code]*/
   /*[python end generated code: output=da39a3ee5e6b4b0d input=35521e4e733823c7]*/

This block adds a converter to Argument Clinic named "ssize_t".
Parameters declared as "ssize_t" will be declared as type
"Py_ssize_t", and will be parsed by the "'O&'" format unit, which will
call the "ssize_t_converter" converter function.  "ssize_t" variables
automatically support default values.

More sophisticated custom converters can insert custom C code to
handle initialization and cleanup. You can see more examples of custom
converters in the CPython source tree; grep the C files for the string
"CConverter".


Writing a custom return converter
---------------------------------

Writing a custom return converter is much like writing a custom
converter.  Except it's somewhat simpler, because return converters
are themselves much simpler.

Return converters must subclass "CReturnConverter". There are no
examples yet of custom return converters, because they are not widely
used yet.  If you wish to write your own return converter, please read
"Tools/clinic/clinic.py", specifically the implementation of
"CReturnConverter" and all its subclasses.


METH_O and METH_NOARGS
----------------------

To convert a function using "METH_O", make sure the function's single
argument is using the "object" converter, and mark the arguments as
positional-only:

   /*[clinic input]
   meth_o_sample

        argument: object
        /
   [clinic start generated code]*/

To convert a function using "METH_NOARGS", just don't specify any
arguments.

You can still use a self converter, a return converter, and specify a
"type" argument to the object converter for "METH_O".


tp_new and tp_init functions
----------------------------

You can convert "tp_new" and "tp_init" functions.  Just name them
"__new__" or "__init__" as appropriate.  Notes:

* The function name generated for "__new__" doesn't end in "__new__"
  like it would by default.  It's just the name of the class,
  converted into a valid C identifier.

* No "PyMethodDef" "#define" is generated for these functions.

* "__init__" functions return "int", not "PyObject *".

* Use the docstring as the class docstring.

* Although "__new__" and "__init__" functions must always accept both
  the "args" and "kwargs" objects, when converting you may specify any
  signature for these functions that you like. (If your function
  doesn't support keywords, the parsing function generated will throw
  an exception if it receives any.)


Changing and redirecting Clinic's output
----------------------------------------

It can be inconvenient to have Clinic's output interspersed with your
conventional hand-edited C code.  Luckily, Clinic is configurable: you
can buffer up its output for printing later (or earlier!), or write
its output to a separate file.  You can also add a prefix or suffix to
every line of Clinic's generated output.

While changing Clinic's output in this manner can be a boon to
readability, it may result in Clinic code using types before they are
defined, or your code attempting to use Clinic-generated code before
it is defined. These problems can be easily solved by rearranging the
declarations in your file, or moving where Clinic's generated code
goes.  (This is why the default behavior of Clinic is to output
everything into the current block; while many people consider this
hampers readability, it will never require rearranging your code to
fix definition-before-use problems.)

Let's start with defining some terminology:

*field*
   A field, in this context, is a subsection of Clinic's output. For
   example, the "#define" for the "PyMethodDef" structure is a field,
   called "methoddef_define".  Clinic has seven different fields it
   can output per function definition:

      docstring_prototype
      docstring_definition
      methoddef_define
      impl_prototype
      parser_prototype
      parser_definition
      impl_definition

   All the names are of the form ""<a>_<b>"", where ""<a>"" is the
   semantic object represented (the parsing function, the impl
   function, the docstring, or the methoddef structure) and ""<b>""
   represents what kind of statement the field is.  Field names that
   end in ""_prototype"" represent forward declarations of that thing,
   without the actual body/data of the thing; field names that end in
   ""_definition"" represent the actual definition of the thing, with
   the body/data of the thing.  (""methoddef"" is special, it's the
   only one that ends with ""_define"", representing that it's a
   preprocessor #define.)

*destination*
   A destination is a place Clinic can write output to.  There are
   five built-in destinations:

   "block"
      The default destination: printed in the output section of the
      current Clinic block.

   "buffer"
      A text buffer where you can save text for later.  Text sent here
      is appended to the end of any existing text.  It's an error to
      have any text left in the buffer when Clinic finishes processing
      a file.

   "file"
      A separate "clinic file" that will be created automatically by
      Clinic. The filename chosen for the file is
      "{basename}.clinic{extension}", where "basename" and "extension"
      were assigned the output from "os.path.splitext()" run on the
      current file.  (Example: the "file" destination for "_pickle.c"
      would be written to "_pickle.clinic.c".)

      **Important: When using a** "file" **destination, you** *must
      check in* **the generated file!**

   "two-pass"
      A buffer like "buffer".  However, a two-pass buffer can only be
      dumped once, and it prints out all text sent to it during all
      processing, even from Clinic blocks *after* the dumping point.

   "suppress"
      The text is suppressed—thrown away.

Clinic defines five new directives that let you reconfigure its
output.

The first new directive is "dump":

   dump <destination>

This dumps the current contents of the named destination into the
output of the current block, and empties it.  This only works with
"buffer" and "two-pass" destinations.

The second new directive is "output".  The most basic form of "output"
is like this:

   output <field> <destination>

This tells Clinic to output *field* to *destination*.  "output" also
supports a special meta-destination, called "everything", which tells
Clinic to output *all* fields to that *destination*.

"output" has a number of other functions:

   output push
   output pop
   output preset <preset>

"output push" and "output pop" allow you to push and pop
configurations on an internal configuration stack, so that you can
temporarily modify the output configuration, then easily restore the
previous configuration.  Simply push before your change to save the
current configuration, then pop when you wish to restore the previous
configuration.

"output preset" sets Clinic's output to one of several built-in preset
configurations, as follows:

   "block"
      Clinic's original starting configuration.  Writes everything
      immediately after the input block.

      Suppress the "parser_prototype" and "docstring_prototype", write
      everything else to "block".

   "file"
      Designed to write everything to the "clinic file" that it can.
      You then "#include" this file near the top of your file. You may
      need to rearrange your file to make this work, though usually
      this just means creating forward declarations for various
      "typedef" and "PyTypeObject" definitions.

      Suppress the "parser_prototype" and "docstring_prototype", write
      the "impl_definition" to "block", and write everything else to
      "file".

      The default filename is ""{dirname}/clinic/{basename}.h"".

   "buffer"
      Save up most of the output from Clinic, to be written into your
      file near the end.  For Python files implementing modules or
      builtin types, it's recommended that you dump the buffer just
      above the static structures for your module or builtin type;
      these are normally very near the end.  Using "buffer" may
      require even more editing than "file", if your file has static
      "PyMethodDef" arrays defined in the middle of the file.

      Suppress the "parser_prototype", "impl_prototype", and
      "docstring_prototype", write the "impl_definition" to "block",
      and write everything else to "file".

   "two-pass"
      Similar to the "buffer" preset, but writes forward declarations
      to the "two-pass" buffer, and definitions to the "buffer". This
      is similar to the "buffer" preset, but may require less editing
      than "buffer".  Dump the "two-pass" buffer near the top of your
      file, and dump the "buffer" near the end just like you would
      when using the "buffer" preset.

      Suppresses the "impl_prototype", write the "impl_definition" to
      "block", write "docstring_prototype", "methoddef_define", and
      "parser_prototype" to "two-pass", write everything else to
      "buffer".

   "partial-buffer"
      Similar to the "buffer" preset, but writes more things to
      "block", only writing the really big chunks of generated code to
      "buffer". This avoids the definition-before-use problem of
      "buffer" completely, at the small cost of having slightly more
      stuff in the block's output. Dump the "buffer" near the end,
      just like you would when using the "buffer" preset.

      Suppresses the "impl_prototype", write the
      "docstring_definition" and "parser_definition" to "buffer",
      write everything else to "block".

The third new directive is "destination":

   destination <name> <command> [...]

This performs an operation on the destination named "name".

There are two defined subcommands: "new" and "clear".

The "new" subcommand works like this:

   destination <name> new <type>

This creates a new destination with name "<name>" and type "<type>".

There are five destination types:

   "suppress"
      Throws the text away.

   "block"
      Writes the text to the current block.  This is what Clinic
      originally did.

   "buffer"
      A simple text buffer, like the "buffer" builtin destination
      above.

   "file"
      A text file.  The file destination takes an extra argument, a
      template to use for building the filename, like so:

         destination <name> new <type> <file_template>

      The template can use three strings internally that will be
      replaced by bits of the filename:

         {path}
            The full path to the file, including directory and full
            filename.

         {dirname}
            The name of the directory the file is in.

         {basename}
            Just the name of the file, not including the directory.

         {basename_root}
            Basename with the extension clipped off (everything up to
            but not including the last '.').

         {basename_extension}
            The last '.' and everything after it.  If the basename
            does not contain a period, this will be the empty string.

      If there are no periods in the filename, {basename} and
      {filename} are the same, and {extension} is empty.
      "{basename}{extension}" is always exactly the same as
      "{filename}"."

   "two-pass"
      A two-pass buffer, like the "two-pass" builtin destination
      above.

The "clear" subcommand works like this:

   destination <name> clear

It removes all the accumulated text up to this point in the
destination. (I don't know what you'd need this for, but I thought
maybe it'd be useful while someone's experimenting.)

The fourth new directive is "set":

   set line_prefix "string"
   set line_suffix "string"

"set" lets you set two internal variables in Clinic. "line_prefix" is
a string that will be prepended to every line of Clinic's output;
"line_suffix" is a string that will be appended to every line of
Clinic's output.

Both of these support two format strings:

   "{block comment start}"
      Turns into the string "/*", the start-comment text sequence for
      C files.

   "{block comment end}"
      Turns into the string "*/", the end-comment text sequence for C
      files.

The final new directive is one you shouldn't need to use directly,
called "preserve":

   preserve

This tells Clinic that the current contents of the output should be
kept, unmodified. This is used internally by Clinic when dumping
output into "file" files; wrapping it in a Clinic block lets Clinic
use its existing checksum functionality to ensure the file was not
modified by hand before it gets overwritten.


The #ifdef trick
----------------

If you're converting a function that isn't available on all platforms,
there's a trick you can use to make life a little easier.  The
existing code probably looks like this:

   #ifdef HAVE_FUNCTIONNAME
   static module_functionname(...)
   {
   ...
   }
   #endif /* HAVE_FUNCTIONNAME */

And then in the "PyMethodDef" structure at the bottom the existing
code will have:

   #ifdef HAVE_FUNCTIONNAME
   {'functionname', ... },
   #endif /* HAVE_FUNCTIONNAME */

In this scenario, you should enclose the body of your impl function
inside the "#ifdef", like so:

   #ifdef HAVE_FUNCTIONNAME
   /*[clinic input]
   module.functionname
   ...
   [clinic start generated code]*/
   static module_functionname(...)
   {
   ...
   }
   #endif /* HAVE_FUNCTIONNAME */

Then, remove those three lines from the "PyMethodDef" structure,
replacing them with the macro Argument Clinic generated:

   MODULE_FUNCTIONNAME_METHODDEF

(You can find the real name for this macro inside the generated code.
Or you can calculate it yourself: it's the name of your function as
defined on the first line of your block, but with periods changed to
underscores, uppercased, and ""_METHODDEF"" added to the end.)

Perhaps you're wondering: what if "HAVE_FUNCTIONNAME" isn't defined?
The "MODULE_FUNCTIONNAME_METHODDEF" macro won't be defined either!

Here's where Argument Clinic gets very clever.  It actually detects
that the Argument Clinic block might be deactivated by the "#ifdef".
When that happens, it generates a little extra code that looks like
this:

   #ifndef MODULE_FUNCTIONNAME_METHODDEF
       #define MODULE_FUNCTIONNAME_METHODDEF
   #endif /* !defined(MODULE_FUNCTIONNAME_METHODDEF) */

That means the macro always works.  If the function is defined, this
turns into the correct structure, including the trailing comma.  If
the function is undefined, this turns into nothing.

However, this causes one ticklish problem: where should Argument
Clinic put this extra code when using the "block" output preset?  It
can't go in the output block, because that could be deactivated by the
"#ifdef".  (That's the whole point!)

In this situation, Argument Clinic writes the extra code to the
"buffer" destination. This may mean that you get a complaint from
Argument Clinic:

   Warning in file "Modules/posixmodule.c" on line 12357:
   Destination buffer 'buffer' not empty at end of file, emptying.

When this happens, just open your file, find the "dump buffer" block
that Argument Clinic added to your file (it'll be at the very bottom),
then move it above the "PyMethodDef" structure where that macro is
used.


Using Argument Clinic in Python files
-------------------------------------

It's actually possible to use Argument Clinic to preprocess Python
files. There's no point to using Argument Clinic blocks, of course, as
the output wouldn't make any sense to the Python interpreter.  But
using Argument Clinic to run Python blocks lets you use Python as a
Python preprocessor!

Since Python comments are different from C comments, Argument Clinic
blocks embedded in Python files look slightly different.  They look
like this:

   #/*[python input]
   #print("def foo(): pass")
   #[python start generated code]*/
   def foo(): pass
   #/*[python checksum:...]*/
