Fix adjusting log level called with uninitialized logger (#263)

* Fix adjusting log level called with uninitialized logger
* Add test

pyam/logger.py

@@ -1,5 +1,5 @@
-from contextlib import contextmanager
 import logging
+from contextlib import contextmanager
 
 # globally accessible logger
 _LOGGER = None
@@ -18,7 +18,8 @@ def logger():
 @contextmanager
 def adjust_log_level(level='ERROR'):
     """Context manager to change log level"""
-    old_level = _LOGGER.getEffectiveLevel()
-    _LOGGER.setLevel(level)
+    _logger = logger()
+    old_level = _logger.getEffectiveLevel()
+    _logger.setLevel(level)
     yield
-    _LOGGER.setLevel(old_level)
+    _logger.setLevel(old_level)

tests/test_logger.py

@@ -1,9 +1,18 @@
-from pyam import logger
-from pyam.logger import adjust_log_level
+import sys
+
+from pyam.logger import logger, adjust_log_level
 
 
 def test_context_adjust_log_level():
     assert logger().getEffectiveLevel() == 20
     with adjust_log_level():
         assert logger().getEffectiveLevel() == 40
     assert logger().getEffectiveLevel() == 20
+
+
+def test_adjusting_level_for_not_initialized_logger():
+    # de-initialize logger to simulate adjust_log_level called before logger
+    pyam_logger = sys.modules['pyam.logger']
+    pyam_logger._LOGGER = None
+    with adjust_log_level():
+        pass