[SDS-335] Resolve import cycles using postponed annotations (#137)

* [SDS-335] Resolve import cycles using postponed annotations

Postponed annotations allow us to use forward code references without having access to them statically, but we can have access at runtime. Using the TYPE_CHECKING constant from typing makes sure that we don't have circular dependencies in the actual code.

The "correct" approach would be to extract interfaces. For example, we could have a generic API interface or a generic Job interface that exposes the methods and properties that we need access to. However, this is not particularly "Pythonese", so this approach seemed more lightweight.

* Address review comments

* Add a .coveragerc so we can ignore TYPE_CHECKING warning lines for coverage

.coveragerc

@@ -0,0 +1,3 @@
+[report]
+exclude_lines =
+    if TYPE_CHECKING:

src/quantuminspire/job.py

@@ -22,13 +22,18 @@
    :members:
 """
 
-from typing import Dict, Any
+from __future__ import annotations
+
+from typing import Dict, Any, TYPE_CHECKING
 from coreapi.exceptions import ErrorMessage
 
+if TYPE_CHECKING:
+    from api import QuantumInspireAPI
+
 
 class QuantumInspireJob:
 
-    def __init__(self, api: Any, job_identifier: int) -> None:
+    def __init__(self, api: QuantumInspireAPI, job_identifier: int) -> None:
         """ Encapsulation of a job.
 
         The :py:class:`QuantumInspireJob` class encapsulates the base job of the API and has
@@ -39,10 +44,10 @@ def __init__(self, api: Any, job_identifier: int) -> None:
         """
         QuantumInspireJob.__check_arguments(api, job_identifier)
         self.__job_identifier: int = job_identifier
-        self.__api: Any = api
+        self.__api: QuantumInspireAPI = api
 
     @staticmethod
-    def __check_arguments(api: Any, job_identifier: int) -> None:
+    def __check_arguments(api: QuantumInspireAPI, job_identifier: int) -> None:
         """ Checks whether the supplied arguments are of correct type.
 
         :param api: An instance to the API.

src/quantuminspire/qiskit/backend_qx.py

@@ -14,13 +14,15 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import annotations
+
 import copy
 import io
 import json
 import uuid
 import warnings
-from collections import defaultdict, OrderedDict, Counter
-from typing import Any, Dict, List, Tuple, Optional, Union
+from collections import defaultdict, Counter
+from typing import Any, Dict, List, Tuple, Optional, Union, TYPE_CHECKING
 
 import numpy as np
 from qiskit.circuit import QuantumCircuit
@@ -39,6 +41,9 @@
 from quantuminspire.qiskit.qi_job import QIJob
 from quantuminspire.version import __version__ as quantum_inspire_version
 
+if TYPE_CHECKING:
+    from quantum_inspire_provider import QuantumInspireProvider
+
 
 class QuantumInspireBackend(Backend):  # type: ignore
     DEFAULT_CONFIGURATION = QasmBackendConfiguration(
@@ -59,7 +64,7 @@ class QuantumInspireBackend(Backend):  # type: ignore
     )
     qobj_warning_issued = False
 
-    def __init__(self, api: QuantumInspireAPI, provider: Any,
+    def __init__(self, api: QuantumInspireAPI, provider: QuantumInspireProvider,
                  configuration: Optional[QasmBackendConfiguration] = None) -> None:
         """ Python implementation of a quantum simulator using Quantum Inspire API.
 

src/quantuminspire/qiskit/qi_job.py

@@ -14,11 +14,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import annotations
+
 import time
-from typing import List, Optional, Any, Dict, Callable
+from typing import List, Optional, Any, Dict, Callable, TYPE_CHECKING
 
 from qiskit.providers import JobError, JobTimeoutError, JobV1 as Job
-from qiskit.providers.backend import Backend
 from qiskit.providers.jobstatus import JobStatus, JOB_FINAL_STATES
 from qiskit.result.models import ExperimentResult
 from qiskit.qobj import QasmQobj, QasmQobjExperiment
@@ -27,6 +28,9 @@
 from quantuminspire.qiskit.qi_result import QIResult
 from quantuminspire.version import __version__ as quantum_inspire_version
 
+if TYPE_CHECKING:
+    from backend_qx import QuantumInspireBackend
+
 
 class QIJob(Job):  # type: ignore
     """
@@ -68,7 +72,8 @@ class QIJob(Job):  # type: ignore
 
     """
 
-    def __init__(self, backend: Backend, job_id: str, api: QuantumInspireAPI, qobj: Optional[QasmQobj] = None) -> None:
+    def __init__(self, backend: QuantumInspireBackend, job_id: str, api: QuantumInspireAPI,
+                 qobj: Optional[QasmQobj] = None) -> None:
         """
         A QIJob object is normally not constructed directly.