Added item_18

Author: craigem

item_18.py

@@ -0,0 +1,83 @@
+#!/usr/bin/env python3
+
+'''Item 18 from Effective Python'''
+
+
+# Example 1
+''' say you want to log some debug information. With a fixed number of
+arguments, you would need a function that takes a message and a list of values
+'''
+print('Example 1:\n==========')
+
+def log(message, values):
+    if not values:
+        print(message)
+    else:
+        values_str =  ', '.join(str(x) for x in values)
+        print('%s: %s' % (message, values_str))
+
+log('My numbers are', [1, 2])
+log('Hi there', [])
+
+
+# Example 2
+''' The first parameter for the log message is required, whereas any number of
+subsequent positional arguments are optional. The function body doesn't need to
+change, only the callers do '''
+print('\nExample 2:\n==========')
+
+def log(message, *values):  # The only difference
+    if not values:
+        print(message)
+    else:
+        values_str = ', '.join(str(x) for x in values)
+        print('%s: %s' % (message, values_str))
+
+log('My numbers are', 1 ,2)
+log('Hi there')  # Much better
+
+
+# Example 3
+''' If you already have a list and want to call a variable argument function
+like log , you can do this by using the * operator. This instructs Python to
+pass items from the sequence as positional arguments '''
+print('\nExample 3:\n==========')
+
+favourites = [7, 33, 99]
+log('Favourite colours', *favourites)
+
+
+# Example 4
+''' the variable arguments are always turned into a tuple before they are
+passed to your function. This means that if the caller of your function uses
+the * operator on a generator, it will be iterated until it's exhausted '''
+print('\nExample 4:\n==========')
+
+def my_generator():
+    for i in range(10):
+        yield i
+
+
+def my_func(*args):
+    print(args)
+
+
+it = my_generator()
+my_func(*it)
+
+
+# Example 5
+''' If you try to add a positional argument in the front of the argument list,
+existing callers will subtly break if they aren't updated '''
+print('\nExample 5:\n==========')
+
+def log(sequence, message, *values):
+    if not values:
+        print('%s: %s' % (sequence, message))
+    else:
+        values_str = ', '.join(str(x) for x in values)
+        print('%s: %s: %s' % (sequence, message, values_str))
+
+
+log(1, 'Favourites', 7, 33)         # New usage is OK
+log('Favourite numbers', 7, 33)     # Old usage breaks