fix (and cover with test) the case

Author: oesteban

nipype/utils/config.py

@@ -202,14 +202,16 @@ def _mock():
             Xvfb = namedtuple('Xvfb', ['vdisplay_num', 'stop'])
             self._display = Xvfb(ndisp, _mock)
             return sysdisplay
-
         else:
+            # If $DISPLAY is empty, it confuses Xvfb so unset
+            if sysdisplay == '':
+                del os.environ['DISPLAY']
             try:
                 from xvfbwrapper import Xvfb
             except ImportError:
                 raise RuntimeError(
                     'A display server was required, but $DISPLAY is not defined '
-                    ' and Xvfb could not be imported.')
+                    'and Xvfb could not be imported.')
 
             self._display = Xvfb(nolisten='tcp')
             self._display.start()

nipype/utils/tests/test_config.py

@@ -38,4 +38,12 @@ def test_display_noconfig_nosystem(monkeypatch):
     if config.has_option('execution', 'display_variable'):
         config._config.remove_option('execution', 'display_variable')
     monkeypatch.delitem(os.environ, 'DISPLAY', raising=False)
-    assert int(config.get_display().split(':')[-1]) > 80
+    assert int(config.get_display().split(':')[-1]) > 80
+
+def test_display_empty(monkeypatch):
+    """Check that when no display is specified, a virtual Xvfb is used"""
+    config._display = None
+    if config.has_option('execution', 'display_variable'):
+        config._config.remove_option('execution', 'display_variable')
+    monkeypatch.setitem(os.environ, 'DISPLAY', '')
+    assert int(config.get_display().split(':')[-1]) > 80