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 <sjm...@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 :) > _______________________________________________ > Python-Dev mailing list > Python-Dev@python.org > https://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: > https://mail.python.org/mailman/options/python-dev/agriff%40tin.it >
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