CODEWITHSUNDEEP
pythonsympyreducesimplify
Delfad0r
Delfad0r

Reputation: 155

Python sympy - simplify nonzero factors from an equation

I am using the sympy library for python3, and I am handling equations, such as the following one:

a, b = symbols('a b', positive = True)
my_equation = Eq((2 * a + b) * (a - b) / 2, 0)

my_equations gets printed exactly as I have defined it ((2 * a + b) * (a - b) / 2 == 0, that is), and I am unable to reduce it even using simplify or similar functions.

What I am trying to achieve is simplifying the nonzero factors from the equation (2 * a + b and 1 / 2); ideally, I'd want to be able to simplify a - b as well, if I am sure that a != b.

Is there any way I can reach this goal?

Upvotes: 1

Views: 416

Answers (1)

Francesco Bonazzi
Francesco Bonazzi

Reputation: 1957

The point is that simplify() is not capable (yet) of complex reasoning about assumptions. I tested it on Wolfram Mathematica's simplify, and it works. It looks like it's a missing feature in SymPy.

Anyway, I propose a function to do what you're looking for.

Define this function:

def simplify_eq_with_assumptions(eq):
    assert eq.rhs == 0  # assert that right-hand side is zero
    assert type(eq.lhs) == Mul  # assert that left-hand side is a multipl.
    newargs = []  # define a list of new multiplication factors.
    for arg in eq.lhs.args:
        if arg.is_positive:
            continue  # arg is positive, let's skip it.
        newargs.append(arg)
    # rebuild the equality with the new arguments:
    return Eq(eq.lhs.func(*newargs), 0)

Now you can call:

In [5]: simplify_eq_with_assumptions(my_equation)
Out[5]: a - b = 0

You can easily adapt this function to your needs. Hopefully, in some future version of SymPy it will be sufficient to call simplify.

Upvotes: 1

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