Status: Accepted
Owner: smichr
Labels: Type-Defect Priority-Medium
New issue 1922 by smichr: derivative results in an "illegal derivative"
http://code.google.com/p/sympy/issues/detail?id=1922
I'm not sure if this is a bug or a feature. When you differentiate wrt y in
f(x/y) you end up with an illegal Derivative object (i.e. one that couldn't
be built from scratch). This, like issue 1920, results in an object that
can't be rebuilt:
from sympy import *
var('x y z')
(x, y, z)
f=Function('f')
f(x/y).diff(y)
-x*D(f(x/y), x/y)/y**2
^^^^^^^^^^^^^^
|_now try recreate this:
f(x/y).diff(x/y)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\Documents and Settings\chris\sympy\sympy\core\expr.py", line
1095, in
diff
ret = Derivative(self, *new_symbols, **assumptions)
File "C:\Documents and Settings\chris\sympy\sympy\core\function.py", line
469,
in __new__
raise ValueError('Invalid literal: %s is not a valid variable' % s)
ValueError: Invalid literal: x/y is not a valid variable
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