python-clinic / clinic-pep.rst

PEP: --- Title: The Argument Clinic DSL Version: $Revision$ Last-Modified: $Date$ Author: Larry Hastings <larry@hastings.org> Discussions-To: Python-Dev <python-dev@python.org> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 22-Feb-2013

Abstract

This document proposes "Argument Clinic", a DSL designed to facilitate argument processing for built-in functions in the implementation of CPython.

Rationale and Goals

The primary implementation of Python, "CPython", is written in a mixture of Python and C. One of the implementation details of CPython are what are called "built-in" functions--functions available to Python programs but written in C. When a Python program calls a built-in function and passes in arguments, those arguments must be translated from Python values into C values. This process is called "parsing arguments".

As of CPython 3.4, arguments to functions are primarily parsed with one of two functions: the original PyArg_ParseTuple(), [1] and the more modern PyArg_ParseTupleAndKeywords(). [2] The former function only handles positional parameters; the latter also accomodates keyword and keyword-only parameters, and is preferred for new code.

PyArg_ParseTuple() was a reasonable approach when it was first concieved. The programmer specified the translation for the arguments in a "format string": [3] each parameter matched to a "format unit", a one-or-two character sequence telling PyArg_ParseTuple() what Python types to accept and how to translate them into the appropriate C value for that parameter. There were only a dozen or so of these "format units", and each one was distinct and easy to understand.

Over the years the PyArg_Parse interface has been extended in numerous ways. The modern API is quite complex, to the point that it is somewhat painful to use. Consider:

• There are now forty different "format units"; a few are even three characters long. This overload of symbology makes it difficult to understand what the format string says without constantly cross-indexing it with the documentation.

• There are also six meta-format units that may be buried in the format string. (They are: "()|\$:;".)

• The more format units are added, the less likely it is the implementor can pick an easy-to-use mnemonic for the format unit, because the character of choice is probably already in use. In other words, the more format units we have, the more obtuse the format units become.

• Several format units are nearly identical to others, having only subtle differences. This makes understanding the exact semantics of the format string even harder.

• The docstring is specified as a static C string, which is mildly bothersome to read and edit.

• When adding a new parameter to a function using PyArg_ParseTupleAndKeywords(), it's necessary to touch six different places in the code: [4]

• Declaring the variable to store the argument.
• Passing in the pointer to the argument to PyArg_ParseTupleAndKeywords().
• Adding the name of the argument to the "keywords" array passed in to PyArg_ParseTupleAndKeywords().
• Adding the format unit to the correct spot in the format string.
• Adding the parameter to the prototype in the docstring.
• Documenting the parameter in the docstring.
• There is currently no mechanism for builtin functions to provide their "signature" information (see inspect.getfullargspec and inspect.Signature). Adding this information using a mechanism similar to the existing PyArg_Parse functions would require repeating ourselves yet again.

The goal of Argument Clinic is to replace this API with a mechanism inheriting none of these downsides:

• You need specify each parameter only once.
• For each parameter, you specify its type in C; Argument Clinic handles the translation from Python value into C value for you.
• Argument Clinic also allows for fine-tuning of argument processing behavior with highly-readable "flags", both per-parameter and applying across the whole function.
• Docstrings are written in plain text.
• From this, Argument Clinic generates for you all the mundane, repetitious code and data structures CPython needs internally. Once you've specified the interface, the next step is simply to write your implementation using native C types. Every detail of argument parsing is handled for you.

Future goals of Argument Clinic include:

• providing signature information for builtins, and
• speed improvements to the generated code.

DSL Syntax Summary

The Argument Clinic DSL is specified as a comment embedded in a C file, as follows. The "Example" column on the right shows you sample input to the Argument Clinic DSL, and the "Section" column on the left specifies what each line represents in turn.

+-----------------------+-----------------------------------------------------+
| Section               | Example                                             |
+-----------------------+-----------------------------------------------------+
| Clinic DSL start      | /*[clinic]                                          |
| Function declaration  | module.function_name -> return_annotation           |
| Function flags        | flag flag2 flag3=value                              |
| Parameter declaration |       type name = default                           |
| Parameter flags       |       flag flag2 flag3=value                        |
| Parameter docstring   |           Lorem ipsum dolor sit amet, consectetur   |
|                       |           adipisicing elit, sed do eiusmod tempor   |
| Function docstring    | Lorem ipsum dolor sit amet, consectetur adipisicing |
|                       | elit, sed do eiusmod tempor incididunt ut labore et |
| Clinic DSL end        | [clinic]*/                                          |
| Clinic output         | ...                                                 |
| Clinic output end     | /*[clinic end output:<checksum>]*/                  |
+-----------------------+-----------------------------------------------------+


General Behavior Of the Argument Clinic DSL

All lines support # as a line comment delimiter except docstrings. Blank lines are always ignored.

Like Python itself, leading whitespace is significant in the Argument Clinic DSL. The first line of the "function" section is the declaration; all subsequent lines at the same indent are function flags. Once you indent, the first line is a parameter declaration; subsequent lines at that indent are parameter flags. Indent one more time for the lines of the parameter docstring. Finally, outdent back to the same level as the function declaration for the function docstring.

Function Declaration

The return annotation is optional.

Parameter Declaration

The "type" is a C type. If it's a pointer type, you must specify a single space between the type and the "*", and zero spaces between the "*" and the name. (e.g. "PyObject *foo", not "PyObject* foo")

The "name" must be a legal C identifier.

The "default" is a Python value. Default values are optional; if not specified you must omit the equals sign too. Parameters which don't have a default are implicitly required. The default value is dynamically assigned, "live" in the generated C code, and although it's specified as a Python value, it's translated into a native C value in the generated C code.

It's explicitly permitted to end the parameter declaration line with a semicolon, though the semicolon is optional. This is intended to allow directly cutting and pasting in declarations from C code. However, the preferred style is without the semicolon.

Flags

"Flags" are like "make -D" arguments. They're unordered. Flags lines are parsed much like the shell (specifically, using shlex.split()). You can have as many flag lines as you like. Specifying a flag twice is currently an error.

Supported flags for functions:

basename
The basename to use for the generated C functions. By default this is the name of the function from the DSL, only with periods replaced by underscores.
positional-only
This function only supports positional parameters, not keyword parameters. See Functions With Positional-Only Parameters below.

Supported flags for parameters:

bitwise
If the Python integer passed in is signed, copy the bits directly even if it is negative. Only valid for unsigned integer types.
converter
Backwards-compatibility support for parameter "converter" functions. The value should be the name of the converter function in C. Only valid when the type of the parameter is void *.
default
The Python value to use in place of the parameter's actual default in Python contexts. Specifically, when specified, this value will be used for the parameter's default in the docstring, and in the Signature. (TBD: If the string is a valid Python expression, renderable into a Python value using eval(), then the result of eval() on it will be used as the default in the Signature.) Ignored if there is no default.
encoding
Encoding to use when encoding a Unicode string to a char *. Only valid when the type of the parameter is char *.
group=
This parameter is part of a group of options that must either all be specified or none specified. Parameters in the same "group" must be contiguous. The value of the group flag is the name used for the group. Only valid for functions marked "positional-only"; see Functions With Positional-Only Parameters below.
immutable
Only accept immutable values.
keyword-only
This parameter (and all subsequent parameters) is keyword-only. Keyword-only parameters must also be optional parameters. Not valid for positional-only functions.
length
This is an iterable type, and we also want its length. The DSL will generate a second Py_ssize_t variable; its name will be this parameter's name appended with "_length".
nullable
None is a legal argument for this parameter. If None is supplied on the Python side, the equivalent C argument will be NULL. Only valid for pointer types.
required
Normally any parameter that has a default value is automatically optional. A parameter that has "required" set will be considered required (non-optional) even if it has a default value. The generated documentation will also not show any default value.
types

Space-separated list of acceptable Python types for this object. There are also four special-case types which represent Python protocols:

• buffer
• mapping
• number
• sequence
zeroes
This parameter is a string type, and its value should be allowed to have embedded zeroes. Not valid for all varieties of string parameters.

Python Code

Argument Clinic also permits embedding Python code inside C files, which is executed in-place when Argument Clinic processes the file. Embedded code looks like this:

::

/*[python]

# this is python code! print("/" + "* Hello world! *" + "/")

[python]*/

Output

Argument Clinic writes its output in-line in the C file, immediately after the section of Clinic code. For "python" sections, the output is everything printed using builtins.print. For "clinic" sections, the output is valid C code, including:

• a #define providing the correct methoddef structure for the function
• a prototype for the "impl" function--this is what you'll write to implement this function
• a function that handles all argument processing, which calls your "impl" function
• the definition line of the "impl" function
• and a comment indicating the end of output.

The intention is that you will write the body of your impl function immediately after the output--as in, you write a left-curly-brace immediately after the end-of-output comment and write the implementation of the builtin in the body there. (It's a bit strange at first--but oddly convenient.)

Argument Clinic will define the parameters of the impl function for you. The function will take the "self" parameter passed in originally, all the parameters you define, and possibly some extra generated parameters ("length" and/or "group" parameters, see next section).

Argument Clinic also writes a checksum for the output section. This is a valuable safety feature: if you modify the output by hand, Clinic will notice that the checksum doesn't match, and will refuse to overwrite the file. (You can force Clinic to overwrite with the "-f" command-line argument; Clinic will also ignore the checksums when using the "-o" command-line argument.)

Functions With Positional-Only Parameters

A significant fraction of Python builtins implemented in C use the older positional-only API for processing arguments (PyArg_ParseTuple()). In some instances, these builtins parse their arguments differently based on how many arguments were passed in. This can provide some bewildering flexibility: there may be groups of optional parameters, which must either all be specified or none specified. And occasionally these groups are on the left! (For example: curses.window.addch().)

Argument Clinic supports these legacy use-cases with a special set of flags. First, set the flag "positional-only" on the entire function. Then, for every group of parameters that is collectively optional, add a "group=" flag with a unique string to all the parameters in that group. Note that these groups are permitted on the right or left of any required parameters! However, all groups (including the group of required parameters) must be contiguous.

The impl function generated by Clinic will add an extra parameter for every group, "int <group>_group". This argument will be nonzero if the group was specified on this call, and zero if it was not.

Note that when operating in this mode, you cannot specify default arguments. You can simulate defaults by putting parameters in individual groups and detecting whether or not they were specified--but generally speaking it's better to simply not use "positional-only" where it isn't absolutely necessary. (TBD: It might be possible to relax this restriction. But adding default arguments into the mix of groups would seemingly make calculating which groups are active a good deal harder.)

Also, note that it's possible--even easy--to specify a set of groups to a function such that there are several valid mappings from the number of arguments to a valid set of groups. If this happens, Clinic will exit with an error message. This should not be a problem, as positional-only operation is only intended for legacy use cases, and all the legacy functions using this quirky behavior should have unambiguous mappings.

Current Status

As of this writing, there is a working prototype implementation of Argument Clinic available online. [5] The prototype implements the syntax above, and generates code using the existing PyArg_Parse APIs. It supports translating to all current format units except "w*".

Extending Argument Clinic

The prototype also currently provides an experimental extension mechanism, allowing adding support for new types on-the-fly. See Modules/posixmodule.c in the prototype for an example of its use.

Notes / TBD

• Guido proposed having the "function docstring" be hand-written inline, in the middle of the output, something like this:

/*[clinic]
... prototype and parameters (including parameter docstrings) go here
[clinic]*/
... some output ...
/*[clinic docstring start]*/
... hand-edited function docstring goes here   <-- you edit this by hand!
/*[clinic docstring end]*/
... more output
/*[clinic output end]*/


I tried it this way and don't like it--I think it's clumsy. I prefer that everything you write goes in one place, rather than having an island of hand-edited stuff in the middle of the DSL output.

• Do we need to support tuple unpacking? (The "(OOO)" style format string.) Boy I sure hope not.

• What about Python functions that take no arguments? This syntax doesn't provide for that. Perhaps a lone indented "None" should mean "no arguments"?

• This approach removes some dynamism / flexibility. With the existing syntax one could theoretically pass in different encodings at runtime for the "es"/"et" format units. AFAICT CPython doesn't do this itself, however it's possible external users might do this. (Trivia: there are no uses of "es" exercised by regrtest, and all the uses of "et" exercised are in socketmodule.c, except for one in _ssl.c. They're all static, specifying the encoding "idna".)

• Right now the "basename" flag on a function changes the #define methoddef name too. Should it, or should the #define'd methoddef name always be {module_name}_{function_name} ?

References

 [1] PyArg_ParseTuple() (http://docs.python.org/3/c-api/arg.html#PyArg_ParseTuple)
 [2] PyArg_ParseTupleAndKeywords() (http://docs.python.org/3/c-api/arg.html#PyArg_ParseTupleAndKeywords)
 [3] PyArg format units (http://docs.python.org/3/c-api/arg.html#strings-and-buffers)
 [4] Keyword parameters for extension functions (http://docs.python.org/3/extending/extending.html#keyword-parameters-for-extension-functions)
 [5] Argument Clinic prototype (https://bitbucket.org/larry/python-clinic/)