Status: Valid
Owner: [email protected]
Labels: Type-Defect Priority-Medium
New issue 3923 by [email protected]: Calling cancel() on a Piecewise()
instance.
http://code.google.com/p/sympy/issues/detail?id=3923
from sympy import init_session
init_session()
p1 = Piecewise(((x**2 - 1)/(x + 1), x > 1), (0, True))
p1
/ 2
|x - 1
|------ for x > 1
<x + 1
|
| 0 otherwise
\
cancel(p1)
/ 2
| x 1
|----- - ----- for x > 1
<x + 1 x + 1
|
| 0 otherwise
\
Notice that it did not simplify (x**2 - 1)/(x + 1) to (x - 1). But there is
worse:
A = Symbol('A', commutative=False)
p2 = Piecewise((A*(x**2 - 1)/(x + 1), x > 1), (0, True))
p2
// 2 \
|\x - 1/*A
|---------- for x > 1
< x + 1
|
| 0 otherwise
\
cancel(p2)
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "sympy/polys/polytools.py", line 5988, in cancel
return f.xreplace(dict(reps))
File "sympy/core/basic.py", line 1100, in xreplace
return self.func(*args)
File "sympy/functions/elementary/piecewise.py", line 96, in __new__
pair = ExprCondPair(*ec)
TypeError: __new__() takes exactly 3 arguments (4 given)
The problem is that cancel can take either an Expr or a 2-Tuple as input,
and does something completely different according to the input. But when
given the noncommutative Piecewise, it recurses throught the expression
tree and treats the ExprCondPair as a 2-Tuple and does something very
unexpected with it.
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