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True is True / False is False?


On Wed, 22 Jul 2020 at 02:12, Chris Angelico <rosuav at gmail.com> wrote:
>
> On Wed, Jul 22, 2020 at 11:04 AM Tim Chase
> <python.list at tim.thechases.com> wrote:
> >
> > I know for ints, cpython caches something like -127 to 255 where `is`
> > works by happenstance based on the implementation but not the spec
> > (so I don't use `is` for comparison there because it's not
> > guaranteed by the language spec). On the other hand, I know that None
> > is a single object that can (and often *should*) be compared using
> > `is`. However I spent some time reading through the language specs and
> > didn't encounter anything about booleans returned from
> > comparisons-operators, guaranteeing that they always return The One
> > True and The One False.
...
>
> That said, though, a comparison isn't required to return a bool. If it
> *does* return a bool, it has to be one of those exact two, but it
> could return anything it chooses. But for built-in types and most
> user-defined types, you will indeed get a bool.

I'm not sure if this is relevant to the question but thought I'd
mention concrete examples. A numpy array will return non-bool for both
of the mentioned operators:


In [2]: import numpy as np

In [3]: a = np.array([2, 2, 2])

In [4]: a == a
Out[4]: array([ True,  True,  True])

In [5]: a > 4
Out[5]: array([False, False, False])


With sympy expressions == will strictly always return a bool but
inequality operators can return instances of Relational. When they can
be evaluated those will give sympy's Booleans rather than bool.


In [6]: import sympy as sym

In [7]: x = sym.Symbol('x')

In [8]: x > 0
Out[8]: x > 0

In [9]: type(_)
Out[9]: sympy.core.relational.StrictGreaterThan

In [10]: (x > 0).subs(x, 2)
Out[10]: True

In [11]: type(_)
Out[11]: sympy.logic.boolalg.BooleanTrue


--
Oscar