Changes under src/ptf for python3 compatible changes

[saynb]

Major changes include

1. Running lib2to3 on .py files under src/ptf
2. Changing manually str() to bytes() internally on pkt objects. In python2, bytes() and str() is the same thing but in python3 str() will convert it into a unicode object.

[cc10512]

I'm afraid this is not fully tested, as there are still typos. How are you validating that your changes?
You should at least add unit tests that exercise simple_ipv4ip_packet and simple_ipv6ip_packet. The more unit tests that cover your changes, the better.

[cc10512]

import .ptfutils works as well and it is more readable in my view.

[cc10512]

If you want to keep compatibility with Python2 (as below), you need the following:

if sys.version_info.major == 2:
    import ptfutils
else:
    import .ptfutils```

[saynb]

Implicit relative import is not supported in python 3. However, explicit relative import is supported in both python 2 and 3 so the import style from . import ptfutils still works. The import style import .ptfutils is not supported in python2. So rather than littering the entire codebase with p2 vs p3 imports we can just choose the common supported one.
One thing that I missed earlier is that in python 3, imports inside function level has been discouraged now with deprecation warnings. In order to get use of lazy importing, we need to use importlib which is unnecessary for the usecases here

[cc10512]

typo: inner_frame.
Also, AFAICT, you can directly construct a bytearray, without first converting to bytes: https://docs.python.org/3.7/library/stdtypes.html?highlight=bytearray#bytearray

[saynb]

I tried it with just bytearray() directly but if inner_frame is a scapy pkt object, then it won't work. So converting it to bytes first is a safe way of converting to a common representation.

[saynb]

Still testing out these changes on both python2 and python3 while adding more to unit tests. I raised the PR early to get some greatly appreciated feedback. Thanks!

[saynb]

Implicit relative import is not supported in python 3. However, explicit relative import is supported in both python 2 and 3 so the import style from . import ptfutils still works. The import style import .ptfutils is not supported in python2. So rather than littering the entire codebase with p2 vs p3 imports we can just choose the common supported one.
One thing that I missed earlier is that in python 3, imports inside function level has been discouraged now with deprecation warnings. In order to get use of lazy importing, we need to use importlib which is unnecessary for the usecases here

[saynb]

Change this to check for long when python 2

[cc10512]

Thanks for adding the tests.

[saynb]

I have added a few tests but the ones with SimpleIpv4 are failing on python2 with a pktlength of >74. For now, I have made the pktlen to be 70 for now so that the test passes. I have checked in another branch https://github.com/p4lang/ptf/tree/ipv
which only adds a simple IPv4 test with a pktlen of 75 on top of master and the travis job fails. It mostly looks like some kind of nanomsg/nnpy issue where some extra bytes are being added. @antoninbas Do you know of anything about this issue?