add example for logging

DEBUG.md

@@ -62,11 +62,58 @@ Lessions:
 
 More on floating point problems [here](https://floating-point-gui.de/) of as [pdf](https://www.phys.uconn.edu/~rozman/Courses/P2200_15F/downloads/floating-point-guide-2015-10-15.pdf)
 
+# How to improve programs step-ny-step: debuggability 
+If `print()` is so useful, why not integrating it into the program? 
+
+## Logging
+
+```python
+import sys
+import logging
+logging.basicConfig(level=logging.INFO)
+# or 
+# logging.basicConfig(filename='example.log',
+#   encoding='utf-8',
+#   format='%(asctime)s :: %(levelname)s :: %(message)s',
+#   level=logging.DEBUG)
+
+
+def sum(a,b):
+  logging.debug("started method sum()")
+  logging.info("Got a = {0} of type {1}".format(a,type(a)))
+  logging.info("Got b = {0} of type {1}".format(b,type(b)))
+  return a+b
+
+if __name__ == "__main__":
+  a,b = sys.argv[1], sys.argv[2]
+  print("{1}+{2}= {0}".format(sum(a,b)),a,b)
+```
+
+### basic log levels
+
+- DEBUG: Detailed information, for diagnosing problems
+- INFO: Confirm things are working as expected
+- WARNING: Something unexpected happened, or indicative of some problem. But the software is still working as expected
+- ERROR: More serious problem, the software is not able to perform some function
+- CRITICAL: A serious error, the program itself may be unable to continue running
+
+
+## Exceptions: try/except
+
+Python has means to deal with non-intended behaviour called Exceptions. Using them will make your scripts easier to understand by others (and by yourself next week).
+
+```python
+try:
+  filehandle = open(filename)
+except IOError:
+  print('File cannot be opened:', filename)
+  exit()
+```
+
+- Think about what could possibly go wrong
+- Think about which requirements must be met for your code work
+
 
 # python's own debugger: pdb/pudb
 
 [list of python debugging tools](https://wiki.python.org/moin/PythonDebuggingTools)
-# How to improve programs step-ny-step: debuggability 
-
-## logging
-## try/except