diff --git a/swh/lister/opam/lister.py b/swh/lister/opam/lister.py
--- a/swh/lister/opam/lister.py
+++ b/swh/lister/opam/lister.py
@@ -51,7 +51,12 @@
             scheduler=scheduler, credentials=credentials, url=url, instance=instance,
         )
         self.env = os.environ.copy()
+        # Opam root folder is initialized in the :meth:`get_pages` method as no
+        # side-effect should happen in the constructor to ease instantiation
         self.opamroot = tempfile.mkdtemp(prefix="swh_opam_lister")
+
+    def get_pages(self) -> Iterator[PageType]:
+        # Initialize the opam root directory with the opam instance data to list.
         call(
             [
                 "opam",
@@ -61,13 +66,12 @@
                 "--no-setup",
                 "--root",
                 self.opamroot,
-                instance,
-                url,
+                self.instance,
+                self.url,
             ],
             env=self.env,
         )
-
-    def get_pages(self) -> Iterator[PageType]:
+        # Actually list opam instance data
         proc = Popen(
             [
                 "opam",
diff --git a/swh/lister/opam/tests/test_lister.py b/swh/lister/opam/tests/test_lister.py
--- a/swh/lister/opam/tests/test_lister.py
+++ b/swh/lister/opam/tests/test_lister.py
@@ -6,24 +6,41 @@
 import io
 from unittest.mock import MagicMock
 
-from swh.lister.opam.lister import OpamLister
+import pytest
 
+from swh.lister.opam.lister import OpamLister
 
-def test_urls(swh_scheduler, mocker):
+module_name = "swh.lister.opam.lister"
 
-    instance_url = "https://opam.ocaml.org"
 
-    lister = OpamLister(swh_scheduler, url=instance_url, instance="opam")
+@pytest.fixture
+def mock_opam(mocker):
+    """Fixture to bypass the actual opam calls within the test context.
 
+    """
+    # inhibits the real `subprocess.call` which prepares the required internal opam
+    # state
+    mock_init = mocker.patch(f"{module_name}.call", return_value=None)
+    # replaces the real Popen with a fake one (list origins command)
     mocked_popen = MagicMock()
     mocked_popen.stdout = io.BytesIO(b"bar\nbaz\nfoo\n")
+    mock_open = mocker.patch(f"{module_name}.Popen", return_value=mocked_popen)
+    return mock_init, mock_open
+
 
-    # replaces the real Popen with a fake one
-    mocker.patch("swh.lister.opam.lister.Popen", return_value=mocked_popen)
+def test_urls(swh_scheduler, mock_opam):
+    mock_init, mock_popen = mock_opam
+
+    instance_url = "https://opam.ocaml.org"
+
+    lister = OpamLister(swh_scheduler, url=instance_url, instance="opam")
 
     # call the lister and get all listed origins urls
     stats = lister.run()
 
+    assert mock_init.called
+    assert mock_popen.called
+
     assert stats.pages == 3
     assert stats.origins == 3
 
@@ -41,7 +58,6 @@
 
 
 def test_opam_binary(datadir, swh_scheduler):
-
     instance_url = f"file://{datadir}/fake_opam_repo"
 
     lister = OpamLister(swh_scheduler, url=instance_url, instance="fake")