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
@@ -52,6 +52,12 @@
         )
         self.env = os.environ.copy()
         self.opamroot = tempfile.mkdtemp(prefix="swh_opam_lister")
+
+    def prepare(self):
+        """Prepare internal state that lister opam requires. This actually does network access
+           to bootstrap the self.opamroot folder.
+
+        """
         call(
             [
                 "opam",
@@ -61,8 +67,8 @@
                 "--no-setup",
                 "--root",
                 self.opamroot,
-                instance,
-                url,
+                self.instance,
+                self.url,
             ],
             env=self.env,
         )
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 +6,40 @@
 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.
 
+    """
+    # replaces the real prepare method (this prepares the internal opam state)
+    mock_init = mocker.patch(f"{module_name}.OpamLister.prepare", 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 +57,6 @@
 
 
 def test_opam_binary(datadir, swh_scheduler):
-
     instance_url = f"file://{datadir}/fake_opam_repo"
 
     lister = OpamLister(swh_scheduler, url=instance_url, instance="fake")
diff --git a/swh/lister/pattern.py b/swh/lister/pattern.py
--- a/swh/lister/pattern.py
+++ b/swh/lister/pattern.py
@@ -113,6 +113,18 @@
         self.state = self.get_state_from_scheduler()
         self.updated = False
 
+    def prepare(self) -> None:
+        """Prepare some state that the lister requires.
+
+        This should centralize all external state configurations that do side effects;
+        reading from network or disk... Those should not run within the class
+        constructor and be moved within their dedicated prepare call.
+
+        See opam lister prepare call for an example
+
+        """
+        pass
+
     def run(self) -> ListerStats:
         """Run the lister.
 
@@ -121,6 +133,7 @@
           of the lister.
 
         """
+        self.prepare()
         full_stats = ListerStats()
 
         try: