Powerful Python, 2nd Edition

1. Multiple Sources and Filters
   
2. You can have several for VAR in SEQUENCE clauses. This lets you
   
3. construct lists based on pairs, triplets, etc., from two or more source
   
4. sequences:
   
5. >>> colors = ["orange", "purple", "pink"]
   
6. >>> toys = ["bike", "basketball", "skateboard", "doll"]
   
7. >>>
   
8. >>> [ color + " " + toy
   
9. ... for color in colors
   
10. ... for toy in toys ]
    
11. ['orange bike', 'orange basketball', 'orange skateboard',
    
12. 'orange doll', 'purple bike', 'purple basketball',
    
13. 'purple skateboard', 'purple doll', 'pink bike',
    
14. 'pink basketball', 'pink skateboard', 'pink doll']

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1. >>> # User is a class with "email" and "is_active" fields.
   
2. ... # all_users is a list of User objects.
   
3. >>> # Sorted list of active user's email addresses.
   
4. ... # Passing in a generator expression.
   
5. >>> sorted((user.email for user in all_users
   
6. ... if user.is_active))
   
7. ['fred@a.com', 'sandy@f.net', 'tim@d.com']
   
8. >>>
   
9. >>> # Omitting the inner parentheses.
   
10. ... # Still passing in a generator expression!
    
11. >>> sorted(user.email for user in all_users
    
12. ... if user.is_active)
    
13. ['fred@a.com', 'sandy@f.net', 'tim@d.com']

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