CODEWITHSUNDEEP
pythonsympy
Shashank Agarwal
Shashank Agarwal

Reputation: 2804

How to check if sympy relational holds for certain values

I have defined some constraints using relational GreaterThan -

x = sympy.Symbol('x')
constraint1 = (x >= 0)

I want to now check if the constraint holds for an arbitrary value of 'x'. I tried using sympy.checksol, but get an attribute error, so I'm guessing that's not the way to go - 

In [7]: sympy.checksol(constraint1, {x: 3})
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-7-cc41bd5986e3> in <module>()
----> 1 sympy.checksol(constraint1, {x: 3})

/Library/Python/2.7/site-packages/sympy/solvers/solvers.pyc in checksol(f, symbol, sol, **flags)
    200         if attempt == 0:
    201             val = f.subs(sol)
--> 202             if val.atoms() & illegal:
    203                 return False
    204         elif attempt == 1:

AttributeError: 'bool' object has no attribute 'atoms'

I even tried constraint1.evalf but it always returns itself -

In [10]: constraint1.evalf(subs={x: 3})
Out[10]: x >= 0

In [11]: constraint1.evalf(subs={x: -3})
Out[11]: x >= 0

So how do I evaluate the relational constraint for given values of its symbols?

Upvotes: 1

Views: 437

Answers (1)

asmeurer
asmeurer

Reputation: 91470

evalf is for numerical evaluation of expressions, which is not what you want here. Just use subs, as you discovered

>>> constraint1.subs(x, -3)
False

And be aware that if you plug something in that it can't figure out, it will remain unevaluated:

>>> contraint1.subs(x, y)
y >= 0

Also, subs only does basic checking. If you want more advanced checking, you should use ask (ask currently doesn't translate inequalities, so you'll need to convert them to Q.positive or Q.nonnegative manually).

Upvotes: 5

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