Qiskit-terra: Pulse Schedule should have a __len__ attribute which returns its number of instructions

[taalexander]

What is the expected enhancement?

The pulse schedule should have a length attribute and return the number of instructions contained in the schedule. This will enable checking if a Schedule is empty and statements such as.

if not Schedule(): print('Empty')

A test should be included for this feature.

[neonipun]

Hello, this looks very doable to me as my first issue for Qiskit! Can I be assigned to this?!
If so let me know what kind of additional help and prerequisites are needed from me.
Looking forward to it :).

[1ucian0]

Hi Nipun! Great to see around here! Assigned to you. Let me know if I can help.

[neonipun]

Hey @taalexander and @1ucian0!
I think I am done with the code but need help to decide on a few things, which are:

• I found two ways to make __len__ work, which I think are right but correct me if I am wrong. Also, one of those two is a bit efficient since self.instructions gives back a sorted list while self._children is a property that just gives back the tuple that self.instructions uses. Code and some %timeit examples are below, and here is the example code I tried it on. I will finalize on either one of them after I hear from you.
• Do you think adding an @abstractmethod __len__ to ScheduleComponent interface is a good idea? Then changes to the Instruction class would also be required. Let me know about that or maybe this can be another issue?
• I need help on where and how a test should be added to the codebase. I saw a couple """Test __len__ method.""" inside 'test', but any more information on that would help me.

Here are the images/code for reference:

[neonipun]

Hey @1ucian0 @taalexander! Any comments/Updates? Should I move forward with a PR of just len(self.instructions) in the Schedule class?

[taalexander]

Hi @neonipun, my apologies for the delay,

1. For now __len__ should be based on instructions. I agree it is inefficient, however, there is an upcoming refactor that will mitigate this problem.
2. Testing with a method test__len__ in test/python/pulse/test_schedule.py seems fine.
3. I do not think we need to add this to ScheduleComponent as this will be deprecated shortly.

[1ucian0]

Fixed via #4394