Question

def getvalue(dictionary, name):
    try:
        return dictionary[name]
    except Exception as e:
        log = open('logfile.txt', 'w')
        log.write('%s\n' % e)
        log.close()

What does this code do? (I understand all of it except for the log.write part, no need to explain the rest, I just added the rest for context)

Answer 1

This code is roughly equivalent to dictionary.get(name), except that in the case when a non-existent key is looked up, the name of the non-existent key is also written to file logfile.txt. Similarly to dict.get, the object None will be returned by the function in this case, and the exception will not be re-raised.

You can clear things up for yourself by experimenting in the interpreter:

>>> d = {'some_key_which_exists': 'some_value'}
>>> d['some_key_which_exists']
'some_value'
>>> d['some_key_which_does_not_exist']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'some_key_which_does_not_exist'
>>> try:
...   d['some_key_which_does_not_exist']
... except Exception as e:
...   print '%s\n' % e
... 
'some_key_which_does_not_exist'

>>> e
KeyError('some_key_which_does_not_exist',)

• Note it over-writes the file, it doesn’t append the file.