[Python-ideas] Programming recommendations (PEP 8) and boolean values
steve at pearwood.info
Thu Aug 9 03:20:24 CEST 2012
On 09/08/12 08:37, Alexander Belopolsky wrote:
> With respect to bool, an argument can be made that it having only two
> preallocated instances is a language feature rather than an
> implementation detail and user can rely on the fact that True, bool(1)
> and say not  all return the same instance. If this is so, I think
> this fact should be mentioned somewhere, but probably not in PEP 8.
Yes, this is documented:
However, the question is not whether
if x is True:
if isinstance(x, bool) and x:
are equivalent, or whether there are obscure edge-cases where one genuinely
needs to test whether or not an object is identical to True. The question
is what is the idiomatic Python code for testing a flag. That is simply
which duck-types truthiness and accepts any true value, not just True.
Like almost all identity tests, testing "x is True" puts the emphasis on
the wrong thing: object identity instead of object value. It restricts
the value's type instead of duck-typing, and that's usually unpythonic.
If you do have a special need to do so, okay, that's fine, but there's
no need for PEP 8 to cover your special use-case. PEP 8 is for
conventions for idiomatic code, not every exception. PEP 8 already
gives its blessing to break the rules when you need to.
Besides, even when it is not, "x is True" looks like a rookie mistake.
Beginners often write "if flag == True" even in statically-typed
languages where flags cannot be other than a bool.
More information about the Python-ideas