[TYPE_CHECKING] Use cirq.* instead of cirq submodules for imports that are only for type-annotations

[dabacon]

[dabacon]

When a dependency only exists for type annotations, you can use

if TYPE_CHECKING:
    from cirq import ops

and then you can use the type as an annotation as a string

ops.Operation

[mpharrigan]

I thought the proposed convention was

if TYPE_CHECKING:
  import cirq

def my_func(op: 'cirq.Operation'):
  pass

i.e., use top-level imports (stringified)

[dabacon]

Oh that is probably better yes.

[andbe91]

What about imports from outside of Cirq? I can imagine that there are some numpy.ndarray used for type checking only, for example. Should we do the same to those?

Also, is there a good way to automatically find all instances of this?

[dabacon]

The main goal would be to reduce our internal module dependencies. The drawback of the string is likely that it makes using and IDE to navigate a bit harder. For external dependencies I wonder how many people use their IDE to navigate to these methods. Otherwise they don't really add much to our dependency graph, so I'd leave them.

[balopat]

Not sure when it started to do it but pylint reports cases when an import is only used for type annotations:

e.g. if I move the import cirq statement out of TYPE_CHECKING in cirq/devices/device.py, pylint will complain:

cirq/devices/device.py:18:0: W0611: Unused import cirq (unused-import)

I think we can close this but I'll wait for your input @dabacon

[mpharrigan]

There are still (many?) locations in the code where a type annotation uses a cirq submodule that is only ever used in the type annotation and should be turned into the "cirq.XXX" string idiom. As an example, I opened a random file (simulator.py) and saw immediately that we

from cirq import circuits

but only ever use circuits.Circuit in type annotations where "cirq.Circuit" would be better

@dabacon -- at least pycharm handles the string form just as well as if it were a real object.

[balopat]

Oh, okay, that's fair - I'll rename the title then to reflect that standardization effort.