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 and
bpo-18896.)
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 :)