TypeError: unhashable type: 'dict' in python

Question

I'm getting an error message about dictionaries despite having never used a dictionary anywhere in my code

Here is my code:

mm=[]
soln=[]
for i in range(len(momvec)):
    string = str(momvec[i])
    num = string[2:]
    mm.append(Symbol('x'+num))
    print num
    print mm
    soln.append(solve(mom[i]-momvec[i],mm))
print type(MFK[0])
for m in range(0,len(MFK)):
    for i in range(0,len(mm)):
        print MFK[m]
        MFK[m]= Subs(MFK[m],mm[i], soln[i]).doit()

What I am trying to do is replace some items in the list MFK with what is indicated by mm with the contents of soln iteratively.

The traceback:

Traceback (most recent call last):
  File "MEA.py", line 313, in <module>
    MFK_final(numMoments)
  File "MEA.py", line 242, in MFK_final
    MFK[m]= Subs(MFK[m],mm[i], soln[i]).doit()
  File "/cluster/soft/linux64/epd/lib/python2.7/site-packages/sympy/core/function.py", line 1270, in __new__
    pts = sorted(set(point), key=default_sort_key)
TypeError: unhashable type: 'dict'

values for mom, momvec and MFK respectively:

[x2 - y_0**2]

[ym2]

[-2*c_0*y_0*(y_0 - 1) - 2*c_0*(-y_0**2 + yx1*(y_0**2/yx1 + 1)) + 2*c_1*(0.5*c_2 - 0.5*y_0), 4.0*c_0*y_0**2 - 4.0*c_0*y_0 + 2.0*c_1*c_2 - 2.0*c_1*y_0 - (-y_0**2 + yx1*(y_0**2/yx1 + 1))*(8.0*c_0*y_0 - 8.0*c_0 + 2.0*c_1)]

Printing output to screen it appears as though the first loop goes fine, then it throws the error in the title. Does anyone know how I could solve this?

Many thanks

Can you show the full traceback? And where does the function `solve` come from? — David Robinson, May 31 '13 at 14:27
@user1987097: it's usually helpful to post a [self-contained example](http://sscce.org), one that people can simply copy and paste to try out. Even after adding the missing sympy imports, neither `mom`, nor `momvec`, nor `MFK` are defined. — DSM, May 31 '13 at 14:33
@DSM This is a small part of a much larger program but I will post values for them to take. — user124123, May 31 '13 at 14:35
@richie Do you know how I can coerce it to return something different? — user124123, May 31 '13 at 14:39
Before doing `MFK[m]= Subs(MFK[m],mm[i], soln[i]).doit()` try printing `soln[i]` to see what it returns — richie, May 31 '13 at 14:43
printing soln in the loop gives this list: `[[y_0**2 + ym2], [{x2: (x3 + 2*y_0**3 - ym3)/(3*y_0)}]]` I see the problem... — user124123, May 31 '13 at 14:47

score 1 · Answer 1 · edited May 23 '17 at 12:16

From your description, I can't reproduce your error. That is, I made a Python file:

from sympy import *

x2 = Symbol("x2")
y_0 = Symbol("y_0")
ym2 = Symbol("ym2")
c_0 = Symbol("c_0")
c_1 = Symbol("c_1")
c_2 = Symbol("c_2")
yx1 = Symbol("yx1")

mom = [x2 - y_0**2]
momvec = [ym2]
MFK = [-2*c_0*y_0*(y_0 - 1) - 2*c_0*(-y_0**2 + yx1*(y_0**2/yx1 + 1)) + 2*c_1*(0.5*c_2 - 0.5*y_0), 4.0*c_0*y_0**2 - 4.0*c_0*y_0 + 2.0*c_1*c_2 - 2.0*c_1*y_0 - (-y_0**2 + yx1*(y_0**2/yx1 + 1))*(8.0*c_0*y_0 - 8.0*c_0 + 2.0*c_1)]

mm=[]
soln=[]
for i in range(len(momvec)):
    string = str(momvec[i])
    num = string[2:]
    mm.append(Symbol('x'+num))
    print num
    print mm
    soln.append(solve(mom[i]-momvec[i],mm))
print type(MFK[0])
for m in range(0,len(MFK)):
    for i in range(0,len(mm)):
        print MFK[m]
        MFK[m]= Subs(MFK[m],mm[i], soln[i]).doit()

and it simply outputs

2
[x2]
<class 'sympy.core.add.Add'>
-2*c_0*y_0*(y_0 - 1) - 2*c_0*(-y_0**2 + yx1*(y_0**2/yx1 + 1)) + 2*c_1*(0.5*c_2 - 0.5*y_0)
4.0*c_0*y_0**2 - 4.0*c_0*y_0 + 2.0*c_1*c_2 - 2.0*c_1*y_0 - (-y_0**2 + yx1*(y_0**2/yx1 + 1))*(8.0*c_0*y_0 - 8.0*c_0 + 2.0*c_1)

without any errors or tracebacks.

However, if you do find the point in the code where the hidden dictionary appears, the reason dictionaries shouldn't be hashable (and therefore usable as keys in other dictionaries) is because they're mutable. If you use one as a key and then later change a value somewhere deep in its structure, you could get weird x != x style errors. You could fix such an error with a hashable frozen dictionary...

score 1 · Accepted Answer · answered Jun 02 '13 at 19:49

1

Your soln[i] is a dictionary, but it needs to be a SymPy expression (the one you plan to substitute into MFK[m] for mm[i]).

answered Jun 02 '13 at 19:49

asmeurer

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TypeError: unhashable type: 'dict' in python

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