Issue 1161: Wrong result if calling Basic.subs() with a dictionary
http://code.google.com/p/sympy/issues/detail?id=1161
New issue report by hastebrot:
Under certain circumstances expr.subs(dictionary.items()) and
expr.subs(dictionary) called with SymPy 0.6.2 give different results.
Here is how I discovered this misbehavior.
In [1]: from sympy import symbols, Eq, tsolve
In [2]: K, L = symbols("KL")
Lets transpose an equation. That is what comes out with SymPy 0.6.0:
In [2]: equation = Eq(K**0.4 * L, 3)
In [3]: tsolve(equation, K)
Out[3]: 15.5884572681199*(1/L)**2.5
But with SymPy 0.6.2 we get:
In [2]: equation = Eq(K**0.4 * L, 3)
In [3]: tsolve(equation, K)
Out[3]: -15.5884572681199*(1/L)**2.5
SymPy 0.6.0 gives us the right result since:
K**0.4 * L = 3 [divide through L]
K**0.4 = 3 * (L**-1) [power 2.5]
K = (3**2.5) * (L**-2.5) [evaluate 3**2.5]
K = 15.5884572681199 * (L**-2.5)
Slightly changing the exponent, and both SymPy versions give us the same
result:
In [4]: equation = Eq(K**0.5 * L, 3)
In [5]: tsolve(equation, K)
Out[5]: 9.0*(1/L)**2.0
I was tracking down this bug with pdb. Extracting the state of the variables
from calling tsolve(Eq(K**0.4 * L, 3), K) with SymPy 0.6.2. Here a code
example
to reproduce the bug with a possible workaround.
In [6]: from sympy import Symbol, Wild
In [7]: x = Symbol("x", dummy=True)
In [8]: a, b, c, d, e = [Wild(t, exclude=[x]) for t in "abcde"]
In [9]: L = Symbol("L")
In [10]: m = {a: L, b: 1, c: 0, d: 0.4, e: -3}
In [11]: sol = (-c + (-e/a)**(1/d))/b
In [12]: sol.subs(m)
Out[12]: -15.5884572681199*(1/L)**2.5
In [13]: sol.subs(m.items())
Out[13]: 15.5884572681199*(1/L)**2.5
We get different results here! There is some trouble with the order in
which variables are substituted. Changing Basic._subs_list() so that it
prints out the substitution:
def _subs_list(self, sequence):
if not isinstance(sequence, (list, tuple)):
raise TypeError("Not an iterable container")
result = self
print "result = %s" % result
for old, new in sequence:
result = result.subs(old, new)
print "result.subs(%s, %3s) -> %s" % (old, new, result)
return result
# sol.subs(m)
result = (-c_ + (-e_/a_)**(1/d_))/b_
result.subs(d_, 0.4) -> (-c_ + I*e_**2.5*(1/a_)**2.5)/b_
result.subs(a_, L) -> (-c_ + I*e_**2.5*(1/L)**2.5)/b_
result.subs(c_, 0) -> I*e_**2.5*(1/L)**2.5/b_
result.subs(e_, -3) -> -15.5884572681199*(1/L)**2.5/b_
result.subs(b_, 1) -> -15.5884572681199*(1/L)**2.5
# sol.subs(m.items())
result = (-c_ + (-e_/a_)**(1/d_))/b_
result.subs(b_, 1) -> -c_ + (-e_/a_)**(1/d_)
result.subs(e_, -3) -> -c_ + (3/a_)**(1/d_)
result.subs(c_, 0) -> (3/a_)**(1/d_)
result.subs(a_, L) -> (3/L)**(1/d_)
result.subs(d_, 0.4) -> 15.5884572681199*(1/L)**2.5
Now changing _subs_list(), so that sequence is delivered in reversed order:
def _subs_list(self, sequence):
...
for old, new in reversed(sequence):
...
# sol.subs(m)
result = (-c_ + (-e_/a_)**(1/d_))/b_
result.subs(b_, 1) -> -c_ + (-e_/a_)**(1/d_)
result.subs(e_, -3) -> -c_ + (3/a_)**(1/d_)
result.subs(c_, 0) -> (3/a_)**(1/d_)
result.subs(a_, L) -> (3/L)**(1/d_)
result.subs(d_, 0.4) -> 15.5884572681199*(1/L)**2.5
# sol.subs(m.items())
result = (-c_ + (-e_/a_)**(1/d_))/b_
result.subs(d_, 0.4) -> (-c_ + I*e_**2.5*(1/a_)**2.5)/b_
result.subs(a_, L) -> (-c_ + I*e_**2.5*(1/L)**2.5)/b_
result.subs(c_, 0) -> I*e_**2.5*(1/L)**2.5/b_
result.subs(e_, -3) -> -15.5884572681199*(1/L)**2.5/b_
result.subs(b_, 1) -> -15.5884572681199*(1/L)**2.5
Issue attributes:
Status: New
Owner: ----
Labels: Type-Defect Priority-Medium
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