don't try to use token if it is falsy (#234)

* don't try to use `token` if it is falsy

Today it checks specifically if `token` is `None`. It might as well check that it's not `""` either.

* add test for env var with empty token

* changelog entry

CHANGELOG.md

@@ -10,10 +10,15 @@
 
 ### Bug fixes
 
+* Do not try to connect with an IBMQX token if it is falsy.
+  [(#234)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/234)
+
 ### Contributors
 
 This release contains contributions from (in alphabetical order):
 
+Matthew Silverman
+
 ---
 # Release 0.27.0
 

pennylane_qiskit/ibmq.py

@@ -106,7 +106,7 @@ def connect(kwargs):
     url = kwargs.get("ibmqx_url", None) or os.getenv("IBMQX_URL")
 
     # TODO: remove "no cover" when #173 is resolved
-    if token is not None:  # pragma: no cover
+    if token:  # pragma: no cover
         # token was provided by the user, so attempt to enable an
         # IBM Q account manually
         def login():

tests/test_ibmq.py

@@ -88,6 +88,13 @@ def test_load_kwargs_takes_precedence(token, monkeypatch):
     assert dev.provider.credentials.is_ibmq()
 
 
+def test_load_env_empty_string_has_short_error(monkeypatch):
+    """Test that the empty string is treated as a missing token."""
+    monkeypatch.setenv("IBMQX_TOKEN", "")
+    with pytest.raises(IBMQAccountError, match="No active IBM Q account"):
+        IBMQDevice(wires=1)
+
+
 def test_account_already_loaded(token):
     """Test loading an IBMQ device using
     an already loaded account"""