Now that variables are symbols, we can implement assumptions

[schymans]

The standard domain of Variable is "real", but this is not injected as an assumption when defining the symbol. We should add real = True in BaseVariable(Symbol) or whatever the prescribed domain is.

[schymans]

Example using assumptions in sympy:

from sympy import Symbol, solve
x = Symbol("x")
print(solve(x**2 - 1))
pos = Symbol("pos", positive=True)
print(solve(pos**2 - 1))

gives

[-1, 1] [1]

How could we do this with Variable?

[schymans]

Here is a possible behaviour:

from essm.variables import Variable
from sympy import solve
from sympy.physics.units import second
class x(Variable):
     """Positive real variable."""  
     assumptions = {'positive': True, 'real': True}

print(solve(x**2 - 1))

[1] Under the hood, the variable should be defined as: Symbol(x, **assumptions)

[schymans]

PR #62 solved this issue.