[Python-Dev] The curious case of 255 function arguments

[Andrea Griffini]

typo... meant of course foo(*([0]*300))

Andrea

On Mon, Aug 6, 2018 at 9:57 AM Andrea Griffini <agriff at tin.it> wrote:

> With Python 2.7.15 what fails is a call with explicit arguments (e.g.
> `foo(0,0,0 ... 0,0)`), not the function definition.
> Calling with `foo([0]*300)` instead works.
>
>
> On Mon, Aug 6, 2018 at 7:18 AM Stephen McDowell <sjm324 at cornell.edu>
> wrote:
>
>> Hello Python Gurus,
>>
>> TL;DR: 3.7 released functions having greater than 255 arguments.  Despite
>> explicit checks for this in 2.x, no such limit is actually imposed -- why?
>>
>> In the 3.7 release notes "Other Language Changes" section (
>> https://docs.python.org/3.7/whatsnew/3.7.html#other-language-changes),
>> the first bullet point denotes
>>
>> > More than 255 arguments can now be passed to a function, and a function
>> can now have more than 255 parameters. (Contributed by Serhiy Storchaka in
>> bpo-12844 <https://bugs.python.org/issue12844> and bpo-18896
>> <https://bugs.python.org/issue18896>.)
>>
>> Now lets get something straight: unless I want to exclusively support
>> Python 3.7 or higher, I must make sure I obey the <255 rule.  Use *args //
>> **kwargs, etc.  I'm totally ok with that, 2020 is already here in my mind ;)
>>
>> Curiosity is the reason I'm reaching out.  Upon further investigation and
>> some discussion with like-minded Python enthusiasts, the code being patched
>> by Serhiy Storchaka is present in e.g., Python 2.7 (
>> https://github.com/python/cpython/blob/2.7/Python/ast.c#L2013-L2016)
>>
>>     if (nargs + nkeywords + ngens > 255) {
>>       ast_error(n, "more than 255 arguments");
>>       return NULL;
>>     }
>>
>> Despite that code, as demonstrated with the supplemental output in the
>> post script, *no 2.x versions fail with >255 arguments*.  In contrast,
>> 3.x where x<7 all do fail (as expected) with a SyntaxError.  To test
>> this, I tried every minor release of python (excluding v1, arbitrarily
>> choosing the latest patch release of a minor version) with the following
>> snippet via the -c flag
>>
>>     /path/to/pythonX.Y -c 'exec("def foo(" + ", ".join(["a" + str(i) for
>> i in range(1, 300)]) + "): pass")'
>>
>> Which tries to construct a function
>>
>>     def foo(a0, a1, ..., a299): pass
>>
>> I've looked at the C code for a while and it is entirely non-obvious what
>> would lead to python *2* *allowing* >255 arguments.  Anybody happen to
>> know how / why the python *2* versions *succeed*?
>>
>> Thank you for reading, this is not a problem, just a burning desire for
>> closure (even if anecdotal) as to how this can be.  I deeply love python,
>> and am not complaining!  I stumbled across this and found it truly
>> confounding, and thought the gurus here may happen to recall what changed
>> in 3.x that lead the the error condition actually being asserted :)
>>
>> Sincerely,
>>
>> Stephen McDowell
>>
>> P.S. On a Fedora 25 box using GCC 6.4.1, I lovingly scripted the
>> installation of all the python versions just to see if it truly was a 2.x /
>> 3.x divide.  The results of running `python -V` followed by the `python
>> -c 'exec("def foo...")'` described above, with some extra prints for
>> clarity are as follows (script hackily thrown together in ~30minutes not
>> included, so as not to make your eyes bleed):
>>
>>
>> ********************************************************************************
>> Python 2.0.1
>> ==> Greater than 255 Arguments supported
>>
>> ********************************************************************************
>> Python 2.1.3
>> ==> Greater than 255 Arguments supported
>>
>> ********************************************************************************
>> Python 2.2.3
>> ==> Greater than 255 Arguments supported
>>
>> ********************************************************************************
>> Python 2.3.7
>> ==> Greater than 255 Arguments supported
>>
>> ********************************************************************************
>> Python 2.4.6
>> ==> Greater than 255 Arguments supported
>>
>> ********************************************************************************
>> Python 2.5.6
>> ==> Greater than 255 Arguments supported
>>
>> ********************************************************************************
>> Python 2.6.9
>> ==> Greater than 255 Arguments supported
>>
>> ********************************************************************************
>> Python 2.7.15
>> ==> Greater than 255 Arguments supported
>>
>> ********************************************************************************
>> Python 3.0.1
>> Traceback (most recent call last):
>>   File "<string>", line 1, in <module>
>>   File "<string>", line 1
>> SyntaxError: more than 255 arguments
>>
>> ********************************************************************************
>> Python 3.1.5
>> Traceback (most recent call last):
>>   File "<string>", line 1, in <module>
>>   File "<string>", line 1
>> SyntaxError: more than 255 arguments
>>
>> ********************************************************************************
>> Python 3.2.6
>> Traceback (most recent call last):
>>   File "<string>", line 1, in <module>
>>   File "<string>", line 1
>> SyntaxError: more than 255 arguments
>>
>> ********************************************************************************
>> Python 3.3.7
>> Traceback (most recent call last):
>>   File "<string>", line 1, in <module>
>>   File "<string>", line 1
>> SyntaxError: more than 255 arguments
>>
>> ********************************************************************************
>> Python 3.4.9
>> Traceback (most recent call last):
>>   File "<string>", line 1, in <module>
>>   File "<string>", line 1
>> SyntaxError: more than 255 arguments
>>
>> ********************************************************************************
>> Python 3.5.6
>> Traceback (most recent call last):
>>   File "<string>", line 1, in <module>
>>   File "<string>", line 1
>> SyntaxError: more than 255 arguments
>>
>> ********************************************************************************
>> Python 3.6.6
>> Traceback (most recent call last):
>>   File "<string>", line 1, in <module>
>>   File "<string>", line 1
>> SyntaxError: more than 255 arguments
>>
>> ********************************************************************************
>> Python 3.7.0
>> ==> Greater than 255 Arguments supported
>>
>> P.P.S. Seriously, I LOVE PYTHON <3  It was so easy to download,
>> configure, build, and install each of these versions, and run the test!
>> Thank you :)
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>
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