On Mon, 09 Jun 2014 08:34:42 -0700, Roy Smith wrote:
> We noticed recently that:
>>>> None in 'foo'
> raises (at least in Python 2.7)
That goes back to at least Python 1.5, when member tests only accepted a
single character, not a substring:
>>> None in "abc"
Traceback (innermost last):
File "<stdin>", line 1, in ?
TypeError: string member test needs char left operand
It's a matter of taste whether predicate functions should always return a
bool, or sometimes raise an exception. Would you be surprised that this
A predicate function could swallow any exception, e.g. be the logical
return True if the condition holds, else return False
return False # or True as needed
but that is, I think, an anti-pattern, as it tends to hide errors rather
than be useful. Most of the time, doing ` in "xyz"` is an error, so
returning False is not a useful thing to do.
I think that Python has been moving away from the "swallow exceptions"
model in favour of letting errors propagate. E.g. hasattr used to swallow
a lot more exceptions than it does now, and order comparisons (less than,
greater than etc.) of dissimilar types used to return a version-dependent
arbitrary but consistent result (e.g. all ints compared less than all
strings), but in Python 3 that is now an error.