On Wed, 24 Aug 2022 at 08:55, Peter Stahlecker
<peter.stahlec...@gmail.com> wrote:
>
> I have upgraded to sympy 1.11
> I wanted to try the cse keyword.
>
> If I set cse = False, all seems to work fine.
> If I set cse = True, lambdify(..) seems to work fine, but solve_ivp(..) gives
> and error.
It looks like cse=True doesn't work properly when lambdifying a Matrix:
In [3]: lambdify((x,), Matrix([[x**2, 0], [0, x**2]]), cse=False)(1)
Out[3]:
array([[1, 0],
[0, 1]])
In [4]: lambdify((x,), Matrix([[x**2, 0], [0, x**2]]), cse=True)(1)
Out[4]:
[array([[1, 0],
[0, 1]])]
Note that with cse=True the Matrix is in a list of length 1. If the
matrix doesn't have any nontrivial subexpressions then it doesn't
happen:
In [6]: lambdify((x,), Matrix([[x, 0], [0, x]]), cse=True)(1)
Out[6]:
array([[1, 0],
[0, 1]])
Looking at the generated code we have:
In [11]: print(inspect.getsource(lambdify((x,), Matrix([[x**2, 0], [0,
x**2]]), cse=True)))
def _lambdifygenerated(x):
x0 = x**2
return [array([[x0, 0], [0, x0]])]
In [12]: print(inspect.getsource(lambdify((x,), Matrix([[x**2, 0], [0,
x**2]]), cse=False)))
def _lambdifygenerated(x):
return array([[x**2, 0], [0, x**2]])
So for some reason the generated code puts the output in a list. Looks
like this code is responsible:
https://github.com/sympy/sympy/blob/5eb59bdad2f4e1a675acb31e7f0c72c8abca708c/sympy/utilities/lambdify.py#L1114-L1126
It seems to presume that expr (which in this case is a Matrix) is
supposed to be a list or tuple. First it tries to add a list to it and
then if that fails it tries to add a tuple and then if that fails it
just puts expr into a list (expr = [expr]). I wish I could just ban
the use of try/except in the sympy codebase because it's almost never
used in a good way.
Basically that code will fail any time expr is not a tuple or a list e.g.:
In [3]: lambdify(x, x**2 + y*x**2, cse=True)(1)
Out[3]: [y + 1]
A workaround is just to extract the element from the list:
In [4]: lambdify(x, x**2 + y*x**2, cse=True)(1)[0]
Out[4]: y + 1
The bug comes from here:
https://github.com/sympy/sympy/pull/23538
--
Oscar
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