Vedran Čačić <ved...@gmail.com> added the comment:
Matthew: can you then answer the same question I asked Serhiy?
The example usually given when advocating strong typing is whether 2 + '3'
should be '23' or 5. Our uneasiness with it doesn't stem from coercions between
int and str, but from the fact that + has two distinct meanings.
Of course, binary operators are always like that, even if it's not obvious,
since there's always a tension created by difference of types of the left and
right operand. Even if it's obvious that 2 - '3' should coerce the second
argument to int since str doesn't define -, this can't be a general rule
because e.g. set does (what about 2 - {3}?).
But method calls (and many protocols) are _not_ of that kind. As I said above,
my_set ^ some_list makes us uneasy (even though list doesn't implement ^), but
my_set.symmetric_difference(some_list) doesn't, simply because there is no
ambiguity: there is only one thing we could have meant.
The same can be said about "for x in not_an_iterator", or "if not_a_bool".
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<https://bugs.python.org/issue43535>
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