[issue29299] Argument Clinic: Fix signature of optional positional-only arguments

[Rémi Lapeyre]

Rémi Lapeyre  added the comment:

Thanks for the explanation.

> And *that* would be a lot of code.

Would an effort to make inspect support this be appreciated or do you think 
this is not important?

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nosy: +remi.lapeyre
versions: +Python 3.8

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[issue29299] Argument Clinic: Fix signature of optional positional-only arguments

[Larry Hastings]

Larry Hastings  added the comment:

Argument Clinic can currently only generate signatures for functions whose 
signatures can be represented in Python.  This is because the signatures are 
parsed by the inspect module, which in turn uses the ast module to generate a 
parse tree; it then examines the parse tree and uses that to generate the 
Signature object.

(By the time you see a signature of a function using inspect, the signature has 
actually made a round-trip through the ast module, been turned into a Signature 
object, then str() has been run on it to re-generate the text representation 
from scratch.  The fact that it's usually identical to the original text 
signature buried in the function's docstring is a happy accident.)

Changing Argument Clinic to generate signatures with square brackets in them to 
signify optional parameters is insufficient.  You'd also have to upgrade 
inspect to support this new syntax--otherwise there'd be no point.  And *that* 
would be a lot of code.

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[issue29299] Argument Clinic: Fix signature of optional positional-only arguments

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Please don't change this part of Argument Clinic without Larry. There were 
several attempts to solve this problem, I don't like current status, but this 
is Larry's decision.

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[issue29299] Argument Clinic: Fix signature of optional positional-only arguments

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

The problem is that

  func(mandatory_arg1, mandatory_arg2[, optional_arg3[, optinal_arg4]])

is not compatible with the inspect module.

In many case a meaningful default value was added if this is possible. For 
example the Python default shown in the signature can be set to '', 'utf-8' or 
'strict' while the C default value is NULL for performance. If the parameter is 
upper index in the sequence it can be set to sys.maxsize (Py_SSIZE_T_MAX in C).

This is not always possible. For example there is not default value for 
dict.pop().

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nosy: +yselivanov

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[issue29299] Argument Clinic: Fix signature of optional positional-only arguments

[STINNER Victor]

STINNER Victor added the comment:

This issue is blocking me to convert more functions to Argument Clinic. See for 
example attached getattr_ac.patch which converts getattr() to AC. Without 
ac_optional_positional.patch, AC generates the signature:

   "getattr($module, object, name, default=None, /)\n"

whereas the following signature is expected:

   "getattr($module, object, name[, default])\n"

--
Added file: http://bugs.python.org/file46317/getattr_ac.patch

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[issue29299] Argument Clinic: Fix signature of optional positional-only arguments

[STINNER Victor]

STINNER Victor added the comment:

See also issue #20291: "Argument Clinic should understand *args and **kwargs 
parameters".

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[issue29299] Argument Clinic: Fix signature of optional positional-only arguments

[STINNER Victor]

STINNER Victor added the comment:

List of functions modified when Argument Clinic is run to regenerate .c.h files:

builtin_format
builtin_getattr
builtin_input
builtin_sum
cmath_log
_codecs_ascii_decode
_codecs_ascii_encode
_codecs_charmap_decode
_codecs_charmap_encode
_codecs_code_page_encode
_codecs_escape_decode
_codecs_escape_encode
_codecs_latin_1_decode
_codecs_latin_1_encode
_codecs_mbcs_encode
_codecs_oem_encode
_codecs_raw_unicode_escape_decode
_codecs_raw_unicode_escape_encode
_codecs_readbuffer_encode
_codecs_unicode_escape_decode
_codecs_unicode_escape_encode
_codecs_unicode_internal_decode
_codecs_unicode_internal_encode
_codecs_utf_16_be_encode
_codecs_utf_16_le_encode
_codecs_utf_32_be_encode
_codecs_utf_32_le_encode
_codecs_utf_7_encode
_codecs_utf_8_encode
_dbm_dbm_get
_dbm_dbm_setdefault
fcntl_fcntl
_imp_create_dynamic
_io_FileIO_truncate
os_putenv
os_unsetenv
pyexpat_xmlparser_ExternalEntityParserCreate
_sre_SRE_Match_end
_sre_SRE_Match_span
_sre_SRE_Match_start
_tkinter_create
unicodedata_UCD_decimal
unicodedata_UCD_digit
unicodedata_UCD_name
unicodedata_UCD_numeric
unicode_lstrip
unicode_maketrans
unicode_rstrip

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[issue29299] Argument Clinic: Fix signature of optional positional-only arguments

[STINNER Victor]

New submission from STINNER Victor:

When a function has only positional arguments and at least one argument is 
optional, the expected signature is:

  func(mandatory_arg1, mandatory_arg2[, optional_arg3[, optinal_arg4]])

For example, the signature of format() is inconsistent with its documentation.

Signature:
---
$ python3 -c 'help(format)'|cat
Help on built-in function format in module builtins:

format(value, format_spec='', /)
Return value.__format__(format_spec)

format_spec defaults to the empty string
---

Documentation:
---
.. function:: format(value[, format_spec])
---

Attached patch is a first attempt to fix the issue. The problem is that my 
heuristic to check if an argument is "optional" doesn't seem to work as 
expected in all cases. I chose to check if the C default is NULL.

The problem is that some functions defines a C default to NULL whereas the 
Python default is set to a different value and is correct.

Example with _io.FileIO.truncate:

/*[clinic input]
_io.FileIO.truncate
size as posobj: object = NULL
/

whereas the documentation says that the default is None:

   .. method:: truncate(size=None)

It's easy to fix the default, but in this case my change doesn't fix the 
signature anymore since the C default is still NULL:

/*[clinic input]
_io.FileIO.truncate
size as posobj: object(c_default="NULL") = None
/

We need a different heuristic than C default is NULL, or we should fix 
functions where the heuristic fails.

--
components: Argument Clinic
files: ac_optional_positional.patch
keywords: patch
messages: 285650
nosy: haypo, larry, rhettinger, serhiy.storchaka
priority: normal
severity: normal
status: open
title: Argument Clinic: Fix signature of optional positional-only arguments
versions: Python 3.7
Added file: http://bugs.python.org/file46316/ac_optional_positional.patch

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