added item 20

Author: craigem

item_20.py

@@ -0,0 +1,110 @@
+#!/usr/bin/env python3
+
+'''Item 20 from Effective Python'''
+
+
+# Example 1
+''' print logging messages that are marked with the time of the logged event
+'''
+print('Example 1:\n==========')
+
+from datetime import datetime
+from time import sleep
+
+
+def log(message, when=datetime.now()):
+    print('%s: %s' % (when, message))
+
+log('Hi there!')
+sleep(0.1)
+log('Hi again!')
+
+
+# Example 2
+''' The convention for achieving the desired result in Python is to provide a
+default value of None and to document the actual behavior in the docstring.
+When your code sees an argument value of None , you allocate the default value
+accordingly '''
+print('\nExample 2:\n==========')
+
+
+def log(message, when=None):
+    """Log a message with a timestamp.
+
+    Args:
+        message: Message to print.
+        when: datetime of when the message occurred.
+            Defaults to the present time.
+    """
+    when = datetime.now() if when  is None else when
+    print('%s: %s' % (when, message))
+
+log('Hi there!')
+sleep(0.1)
+log('Hi again!')
+
+
+# Example 3
+''' load a value encoded as JSON data. If decoding the data fails, you want an
+empty dictionary to be returned by default '''
+print('\nExample 3:\n==========')
+
+import json
+
+
+def decode(data, default={}):
+    try:
+        return json.loads(data)
+    except ValueError:
+        return default
+
+
+# Example 4
+''' The dictionary specified for default will be shared by all calls to decode
+because default argument values are only evaluated once (at module load time).
+This can cause extremely surprising behavior '''
+print('\nExample 4:\n==========')
+
+foo = decode('bad data')
+foo['stuff'] = 5
+bar = decode('also bad')
+bar['meep'] = 1
+print('Foo:', foo)
+print('Bar:', bar)
+
+
+# Example 5
+''' The culprit is that foo and bar are both equal to the default parameter.
+They are the same dictionary object '''
+print('\nExample 5:\n==========')
+
+assert foo is bar
+
+
+# Example 6
+''' The fix is to set the keyword argument default value to None and then
+document the behavior in the function's docstring '''
+print('\nExample 6:\n==========')
+
+
+def decode(data, default=None):
+    """ Load JSON data from a string.
+
+    Args:
+        data: JSON data to decode.
+        default: Value to return if decoding fails.
+            DEfaults to an empty dictionary,
+    """
+    if default is None:
+        default = {}
+    try:
+        return json.loads(data)
+    except ValueError:
+        return default
+
+foo = decode('bad data')
+foo['stuff'] = 5
+bar = decode('also bad')
+bar['meep'] = 1
+print('Foo:', foo)
+print('Bar:', bar)