Only run conda_tests session by default if user has conda installed (#…

…521)

noxfile.py

@@ -23,6 +23,11 @@
 
 ON_WINDOWS_CI = "CI" in os.environ and platform.system() == "Windows"
 
+# Skip 'conda_tests' if user doesn't have conda installed
+nox.options.sessions = ["tests", "cover", "blacken", "lint", "docs"]
+if shutil.which("conda"):
+    nox.options.sessions.append("conda_tests")
+
 
 def is_python_version(session, version):
     if not version.startswith(session.python):

tests/test_virtualenv.py

@@ -233,6 +233,7 @@ def test_condaenv_bin_windows(make_conda):
     assert [dir_.strpath, dir_.join("Scripts").strpath] == venv.bin_paths
 
 
+@pytest.mark.skipif(not HAS_CONDA, reason="Missing conda command.")
 def test_condaenv_(make_conda):
     venv, dir_ = make_conda()
     assert not venv.is_offline()