sympy.Eq, equality versus is

[Paul Royik]

Maybe this is property of Python, but I'm not aware about it.

expr = 1/(2*sin(x/2 + pi/4)*cos(x/2 + pi/4))

equation = Eq(expr,expr)

equation == True # outputs True

equation is True # outputs False

Why True equation is not True?

Moreover, in IDE in which I'm working, when I write equation != True, it tells me "this inspection detects equality comparison with a boolean literal".

Thnak you.

[Aaditya Nair]

equation == True # outputs True

equation is True # outputs False

Why True equation is not True?

The `Eq` function doesn't actually return `True` but returns `S.true` whose value is equal to True.

Hence `is`, which does an object-comparison which results `False` as `True` and `S.true` are separate objects.

But since values of `True` and `S.true` are same, use of `==` operator results `True`.

It is recommended that you use `==` for testing Boolean Equality.

Aaditya M Nair

[Joachim Durchholz]

Am 24.03.2015 um 12:28 schrieb Paul Royik:
> Maybe this is property of Python, but I'm not aware about it.
>
> expr = 1/(2*sin(x/2 + pi/4)*cos(x/2 + pi/4))
> equation = Eq(expr,expr)
> equation == True # outputs True
> equation is True # outputs False
>
> Why True equation is not True?

In Python, "==" is value equality while "is" is object identity.

The differences are relevant in two ways:
1) "==" may change if you modify an object, "is" never does. This is
less relevant for SymPy because SymPy objects usually are not modified.
2) "is" cannot be redefined, "==" can. SymPy's S.true has its "=="
redefined so that it compares equal with "==".

> Moreover, in IDE in which I'm working, when I write equation != True, it
> tells me "this inspection detects equality comparison with a boolean
> literal".

It's telling you that writing
xxx == True
is silly because you can simply write
xxx

[Paul Royik]

Thank you.

I've got it now.