Add Python 3 support

[HamedSabri-adsk]

PR Description

This PR fixes various issues when building MayaUsd with Python 3. All changes are Python 2 and 3 compatible.

[mattyjams]

A few tiny gripes, but otherwise looks good to me!

[mattyjams]

I noticed that we've got find_package() calls both here and in cmake/python.cmake. Do we need all of this here? Could everything from lines 71-86 be replaced with simply include(cmake/python.cmake)? Or could it at least all be moved into cmake/python.cmake to avoid the redundancy?

I'd be ok with it going in as is for now, but it'd be nice to see that in a follow-up PR.

[seando-adsk]

This block here from lines 71-76 is for our internal Maya build where we build maya-usd inside. We are passing in the 3 python vars (line 71) and then calling find_package() which doesn't really do anything since all the vars were set. Unless something bad happens in which case we reset (line 75). We cannot remove this, but it could probably be moved inside of cmake/python.cmake.

[HamedSabri-adsk]

Good point @mattyjams , I am planning to revisit python.cmake and some of the logic here. There is definitely some redundancy here but let's leave it for another PR.

[mattyjams]

It's not obvious to me how these Qt changes are related to Python 3 support.

[HamedSabri-adsk]

In Python 3 headers, there are certain identifiers such as 'signals' and/or 'slots' that conflict with Qt's identifiers. To get around this one needs to replace all special Qt keywords ( signals, slots, emit ) with their Q_ macro counterparts.

This is a known issue. Please see Qt's help ( Using Qt with 3rd Party Signals and Slots )

https://doc.qt.io/qt-5/signalsandslots.html#using-qt-with-3rd-party-signals-and-slots

Here is the error that one gets before the fix:

C:\Python37\include\object.h(448): error C2059: syntax error: ';'
error C2238: unexpected token(s) preceding ';'

and here is the line in question is PyType_Slot slots; / terminated by slot==0. */

typedef struct{
    const char* name;
    int basicsize;
    int itemsize;
    unsigned int flags;
    PyType_Slot *slots; /* terminated by slot==0. */
} PyType_Spec;

[mattyjams]

I don't think you should need this. range() exists in Python 2, it's just not a generator like it is in Python 3, and for the purposes of this test, the difference shouldn't matter.

[HamedSabri-adsk]

Right, I don't think it is a biggie but will get rid of line 20 in another PR.

[HamedSabri-adsk]

Please see d10a527

[mattyjams]

For the way print() is used below, I don't think you need this. It was only needed in build.py because of the parameter-style usage: print("Message:", msg).

[HamedSabri-adsk]

Hey Matt, I am not sure if I see the connection with build.py.

This is a separate python file and without this change almost all ufe tests fail.

Perhaps there is something I don't see?

[mattyjams]

I meant that I don't think you should need this import statement. You do need the changes below to use print() as a function.

I didn't mean to imply that there was a connection to build.py. I just meant that because of the way build.py uses print(), you do need the from __future__ import print_function statement there.

[HamedSabri-adsk]

Ahhh I see now. My apologies for the confusion.

For the way print() is used below, I don't think you need this

which print() are you referring to? There are two in here:

print(sys.exc_info()[1])
print("Unable to load %s" % pluginName)

[mattyjams]

I was trying to highlight the difference in the single parameter version of print() vs. the multiple-parameter version, and how the behavior there is different between Python 2 and Python 3.

Single vs. Multi-parameter In Python 2:

> python --version
Python 2.7.10
> python -c "msg = 'This is a string'; print('Message: %s' % msg)"
Message: This is a string
> python -c "msg = 'This is a string'; print('Message:', msg)"
('Message:', 'This is a string')

Single vs. Multi-parameter In Python 3:

> python3 --version
Python 3.7.7
> python3 -c "msg = 'This is a string'; print('Message: %s' % msg)"
Message: This is a string
> python3 -c "msg = 'This is a string'; print('Message:', msg)"
Message: This is a string

Notice that the multi-parameter version in Python 2 prints tuple-like output, which we don't want. That's what requires importing from the future:

In Python 2:

> python -c "from __future__ import print_function; msg = 'This is a string'; print('Message:', msg)"
Message: This is a string

So since build.py is using the multi-parameter version, we need to import from the future to get the right output. Since mayaUtils.py is not, we should not need to import from the future.

[HamedSabri-adsk]

@mattyjams Ughh, I didn't know this. Thanks, you convinced me.

Done. See d10a527