Comment #8 on issue 1358 by wflynny: (e*f*e*f).subs(e*f,f*e+h) fails if e*f
is not commutative
http://code.google.com/p/sympy/issues/detail?id=1358
Because I don't know how to actually patch the language, I'll just post the
code I
have written. I ended up using a chunk of yaukwankiu's code. Basically,
I've just
rewritten the _eval_subs method in the mul.py file. Here goes:
def _eval_subs(self, old, new):
if self == old:
return new
if isinstance(old, FunctionClass):
return self.__class__(*[s._eval_subs(old, new) for s in
self.args ])
coeff_self,terms_self = self.as_coeff_terms()
coeff_old,terms_old = old.as_coeff_terms()
if terms_self == terms_old: # (2*a).subs(3*a,y) -> 2/3*y
return new * coeff_self/coeff_old
# BEGIN: MY PATCH 6/11/09
l_temp = [] #temporary
for i in terms_old:
if i.is_Pow and type(i.exp) is Integer:
for j in range(i.exp):
l_temp.append(i.base) #convert (...h**2...) to
(...h,h...)
else:
l_temp.append(i)
terms_old=tuple(l_temp)
l_temp = [] #temporary
for i in terms_self:
if i.is_Pow and type(i.exp) is Integer:
for j in range(i.exp):
l_temp.append(i.base) #convert (...h**2...) to
(...h,h...)
else:
l_temp.append(i)
terms_self=tuple(l_temp)
l1, l2 = len(terms_self), len(terms_old)
if l2 == 0:
# if old is just a number, go through the self.args one by one
return Mul(*[x._eval_subs(old, new) for x in self.args])
elif l2 < l1:
self_set = [element for element in terms_self]
old_set = [element for element in terms_old]
len_old = len(terms_old)
len_self = len(terms_self)
temp = []
if len(old_set) < len(self_set):
# Scan through list of self
for i in range(len_self):
# If a member of self matches the first in the set to
be replaced
# Scan through the list of to-be-replaced set and make
sure the
# Next terms all match. If they don't, break.
if self_set[i] == old_set[0]:
for j in range(len_old):
if self_set[i+j] != old_set[j]:
for k in range(i,i+j):
temp.append(self_set[k])
break
elif j == len_old - 1:
temp.append(new)
for k in range(i+j+1,len_self):
temp.append(self_set[k])
return Mul(coeff_self/coeff_old, *[element
for
element in temp])._eval_subs(old,new)
else: temp.append(self_set[i])
return Mul(coeff_self/coeff_old, *[element for element in
temp])
return self.__class__(*[s._eval_subs(old, new) for s in self.args])
# END: MY PATCH ATTEMPT 6/11/09
I also made the attached test file, which it passes. Here is the code for
that so you
don't have to download it:
import sympy as sp
def subs_test():
# Define symbols
t,w,x,y,z = sp.symbols('twxyz',commutative=False)
# Start off easy
a = x*y
b = y*x
# Add some noise
c = w*x*y*z
# Double
d = x*y*x*y
# Hard
e = x*x*y
# Hardest
f = x*x*y*y
assert a.subs(x*y,t) == t; print 'test A passed'
assert b.subs(x*y,t) == y*x; print 'test B passed'
assert c.subs(x*y,t) == w*t*z; print 'test C passed'
assert d.subs(x*y,t) == t**2; print 'test D passed'
assert e.subs(x*y,t) == x*t; print 'test E passed'
assert f.subs(x*y,t) == x*t*y; print 'test F passed'
subs_test()
I hope this helps!
-wflynny
Attachments:
test.py 658 bytes
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