autowrap and ufuncify "helpers" arguments are inconsistent and poorly documented

[ronanpaixao]

utilities.autowrap.ufuncify() doesn't explain properly how the helpers argument should be used. The best documentation, of course, is an example. Maybe one can include this in the docstring (it can also work as a test if using doctest):

from sympy.utilities import autowrap
from sympy.abc import x, t
expr = 3*x+f(t)
func_ufunc = autowrap.ufuncify([x, t], expr, helpers=[['f', 4*x, [x]]])
func_ufunc(1, 0.125)
#3.5

It took a long time for me to find the proper "format" for the helpers, which really is like iterable_of(3-tuple(str, SymPy expression, list)).

utilities.autowrap.autowrap(), however, expects only a single helper function!

autowrap.autowrap(expr, args=[x, t], helpers=[['f', 4*x, [x]]])

Traceback (most recent call last):

  File "<ipython-input-105-729cbaf02a5a>", line 1, in <module>
    autowrap.autowrap(expr, args=[x, t], helpers=[['f', 4*x, [x]]])

  File "D:\Anaconda\lib\site-packages\sympy\utilities\autowrap.py", line 510, in autowrap
    for name_h, expr_h, args_h in helpers:

ValueError: need more than 1 value to unpack

func_autow = autowrap.autowrap(expr, args=[x, t], helpers=['f', 4*x, [x]])
func_autow(1, 0.1)
#3.4

[ronanpaixao]

An important note: since autowrap doesn't allow for more helpers and the C/Cython backend doesn't support (out-of-the-box) some builtin function like sympy.Max, this pretty much blocks using C/Cython for expressions with more than one function, any function.

[moorepants]

We recently fixed this #8889 and added an extensive addition with code gen examples. I think the helpers is now in the master branch. It will be released with the 1.0 release that will happen soon.

[moorepants]

This was also recently merged: https://github.com/sympy/sympy/pull/10282

[moorepants]

@ronanpaixao Would you mind submitting a pull request with your docstring additions?