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

[Andrea Griffini]

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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