A little stricter type checking

[Peter Otten]

Andrew Dalke wrote:

[examples of isinstance() usage in the standard library]

> The only examples I found along the lines you wanted were
> in pickletools.py, copy.py, sets.py, and warnings.py.
> It just isn't needed that often in Python programming.

There are at least three more idioms that could (sometimes) be replaced
by type-checking:

# "casting"
s = str(s)
it = iter(seq)

# check for an "interface"
try:
    lower = s.lower
except AttributeError:
    raise TypeError
else:
    lower()

# check for an "interface", LBYL-style
if not hasattr(foo, "bar"):
    raise TypeError

These are more difficult to hunt and I'm lazy. 

However, I don't think you can tell from the structure alone whether an
explicit type declaration would be a superior solution in these cases.

Should Python be extended to allow type declarations, I expect them to
appear in places where they reduce the usefulness of the code while
claiming to make it safe...

Peter