Matching Python 2 int behavior on Python 2

[henryiii]

This is from a Gitter discussion. Pybind11's default conversion to int always produces a long on Python 2 (ints and longs were unified in Python 3). This patch fixes int handling to match Python 2 on Python 2; for short types (size_t or smaller), the number will be returned as an int if possible, otherwise long. Requires Python 2.5+.

An example of where this matters: The sys.exit function must have an int. So if you have this:

.def("__int__", &Thing::operator int)

You currently need this to work;

sys.exit(int(int(thing)))

My Python 2 numpy is having issues, so can't test locally yet.

[henryiii]

I believe this works now, but Clang 5.0 is broken for everyone right now. Someone will need to restart it tomorrow.

[jagerman]

Perhaps elaborate a bit on why:

In Python 2, a C++ int should return a Python int rather than long if possible: int's can always be used where a long is required, but longs are not always accepted where ints are used (such as the argument to sys.exit()).

[henryiii]

It is possible (though extremely rare) that a long could be required; you could just check the type, for example. But now you can force a long in C++ by casting to long long.

[henryiii]

Updated.

[jagerman]

Instead of the #ifdef here, let's add these to detail/common.h like we have for various other python API calls that differ across python versions:

#define PYBIND11_LONG_FROM_SIGNED(o) PyLong_FromSize_t((size_t) o)
#define PYBIND11_LONG_FROM_UNSIGNED(o) PyLong_FromSsize_t((ssize_t) o)

for Python 3 and

#define PYBIND11_LONG_FROM_SIGNED(o) PyInt_FromSize_t((size_t) o)     // Returns long if needed.
#define PYBIND11_LONG_FROM_UNSIGNED(o) PyInt_FromSsize_t((ssize_t) o) // Likewise.

for Python 2; then this part of the cast.h code becomes much cleaner:

            if (std::is_signed<T>::value)
                return PYBIND11_LONG_FROM_SIGNED(o);
            else
                PYBIND11_LONG_FROM_UNSIGNED(o);

[henryiii]

Okay, I was considering something like that. I'll add when the CI build finishes.

[henryiii]

Done.

[henryiii]

Repushed, now that Clang 5 is working again.

[wjakob]

This looks great.

[jagerman]

Shouldn't these be assert isinstance(...)?

[henryiii]

Oops!

[jagerman]

Aside from the missing asserts in the test, this looks good.

[jagerman]

Looks like apt.llvm.org is back to being broken, but more importantly, the added tests don't appear to be passing under Python 2.

[henryiii]

The problem is long long == ssize_t. Which is nice, that means that everything is covered on most systems though just that one Python API call. The current method of a backup call is still best just in case size_t is 32 bits, such as on some 32 bit systems.

Python ints are signed, so the max unsigned long long should trigger a python long return.

[jagerman]

The i386 build should hit it; I think there long long is 64 bits but size_t is 32.

[henryiii]

Can this be merged? I think it is finished from my end.

[jagerman]

Yeah, thanks for the reminder (I'm pretty swamped with other things at the moment).