See this issue <https://github.com/sympy/sympy/issues/5031> for previous
discussion.
On Saturday, October 27, 2018 at 12:44:09 PM UTC-5, Oscar wrote:
>
> Hi all,
>
> I find the behaviour of operations involving Eq strange. I would
> really like to be able to use Eqs for algebra but they don't seem to
> do anything useful. Is this behaviour intentional or is it something
> that could be improved?
>
> Setup:
> >>> from sympy import *
> >>> x = Symbol('x')
> >>> y = Symbol('y')
> >>> eq = Eq(x, y)
> >>> eq
> Eq(x, y)
> >>> pprint(eq)
> x = y
>
> I don't understand what any of these mean:
> >>> pprint(2*eq)
> 2⋅(x = y)
> >>> pprint((2*eq).expand())
> 2⋅(x = y)
> >>> exp(eq)
> exp(Eq(x, y))
> >>> pprint(abs(eq))
> │x = y│
> >>> eq - 1
> -1 + Eq(x, y)
> >>> pprint(eq - 1)
> -1 + (x = y)
>
> Integration works but differentiation doesn't:
> >>> pprint(integrate(eq, x))
> ⌠ ⌠
> ⎮ x dx = ⎮ y dx
> ⌡ ⌡
> >>> pprint(integrate(eq, x).doit())
> 2
> x
> ── = x⋅y
> 2
> >>> diff(eq, x)
> Derivative(Eq(x, y), x)
> >>> pprint(diff(eq, x))
> ∂
> ──(x = y)
> ∂x
> >>> pprint(diff(eq, x).doit())
> ∂
> ──(x = y)
> ∂x
>
> Functions of Eq raise errors:
> >>> sin(eq)
> ...
> TypeError: cannot determine truth value of Relational
>
> It looks as if I can chain equations and inequalities but does it
> actually mean what it looks like mathematically?
> >>> eq < 3
> Eq(x, y) < 3
> >>> pprint(eq < 3)
> x = y < 3
>
> Apart from the inequality example at the end I would like it if all of
> the above operations acted on both lhs and rhs separately as in the
> case of integration e.g.:
>
> >>> eq
> x = y
> >>> 2*eq
> 2*x = 2*y
> >>> sin(eq)
> sin(x) = sin(y)
>
> The other thing that I don't understand although it is clearly
> documented is this:
> >>> Eq(1, 1)
> True
> >>> Eq(1, 0)
> False
>
> These True/False values are annoying if you are building up Eqs
> programatically e.g. to pass to solve:
> >>> solve([Eq(1, 1), Eq(x, y), Eq(x, 1)], [x, y])
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> File "/Users/enojb/current/sympy/sympy/sympy/solvers/solvers.py",
> line 980, in solve
> return reduce_inequalities(f, symbols=symbols)
> File "/Users/enojb/current/sympy/sympy/sympy/solvers/inequalities.py",
> line 987, in reduce_inequalities
> rv = _reduce_inequalities(inequalities, symbols)
> File "/Users/enojb/current/sympy/sympy/sympy/solvers/inequalities.py",
> line 907, in _reduce_inequalities
> '''))
> NotImplementedError:
> inequality has more than one symbol of interest.
>
> You can solve this last problem with evaluate=False but I really don't
> understand why any evaluation is desirable here. I think that solve
> has probably gotten confused here for the same reason that any other
> code would: the True/False objects don't have any of the same
> attributes that an Eq would have:
>
> >>> Eq(0, 1).lhs
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> AttributeError: 'BooleanFalse' object has no attribute 'lhs'
>
>
> --
> Oscar
>
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