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 #!/usr/bin/env python3
 #
 # So, I've got some free python training for you! This code works both
 # with python2 and python3. I love python3's print function, so let's add
 # it...
 
 from __future__ import print_function
 
 # Ok. First thing, did you know about namedtuple? It works as a regular
 # tuple, but you can access the fields by name (hence "named" tuple).
 # Besides, it is optimized for producing a large amount of instances.
 
 from collections import namedtuple
 
 # It works as a meta-type: you define types by calling a function.
 Row = namedtuple("Row", "name age city")
 
 # ...but refer to the documentation (pydoc collections.namedtuple) for
 # that. Here a proof of how it works:
 
 r = Row("Luke", 40, "Amsterdam")
 print("Showing Row:", r)
 print("Showing fields of the row:", r.name, r.age, r.city)
 
 # Let's take an hypotetical list of results. This Could be the output of
 # some stats file, or maybe from a database (even if the database will
 # probably have some inner type!).
 
 records = [
     Row("Jane", 15, "London"),
     Row("Georg", 15, "Berlin"),
     Row("Urlika", 20, "Stockholm"),
     Row("Johan", 17, "Stockholm"),
     Row("Aldo", 9, "Rome"),
-    Row("Hans", 17, "Berlin")
+    Row("Hans", 17, "Berlin"),
     Row("Petra", 21, "Berlin")
 ]
 
 # About itertools? It's an extremely nice library of python, with useful
 # lazy-evaluated functions [ https://en.wikipedia.org/wiki/Lazy_evaluation ].
 # Iterators in general allow to iterate (hence the word) on objects like
 # tuples or lists ("iterables").
 
 import itertools as it
 
 # Iterators are very elegant, and if wisely used they can reduce the
 # memory footprint of a program, but on the minus side they get consumed:
 # you cannot use the same iterator twice. Example:
 
 # I can print lists twice:
 
 print('-' * 80)
 print("Records")
 for r in records:
     print("\t", r)
 print("...second shot:")
 for r in records:
     print("\t", r)
 print("end.")
 
 print('-' * 80)
 
 # But iterators get consumed
 records_iterator = iter(records)
 print("Iterated records")
 for r in records_iterator:
     print("\t", r)
 print("...second shot (will be empty):")
 for r in records_iterator:
     print("\t", r)
 print("end.")
 print('-' * 80)
 
 # Also I'll include the "attrgetter" operator. Quoting the documentation:
 #
 #   After, f=attrgetter('name'), the call f(r) returns r.name.
 
 from operator import attrgetter as aget
 
 # For example, this is get_city:
 get_city = aget('city')
 
 # Since the records are namedtuple with a field called `city`,
 # `aget('city')` will produce a function returning the `city` field:
 r = Row("Luke", 40, "Amsterdam")
 print("The city of Luke is:", get_city(r))
 
 # Now some interesting use of the groupby function.
 #
 # It basically works as the "Group By" operator on SQL: group together
 # items of a table (in this case an iterator), and allow iteration over
 # single groups. The only caveat: items of the same group must be
 # contiguous, as for the "uniq" command of unix, which is often preceded
 # by the "sort" command, in pipe.
 #
 # For instance, let's take the `records` list, we defined previously, and
 # group the rows by the `city` attribute.  The `get_city` we defined
 # before turns out to be a good grouping function So, those items have to
 # be sorted by the relevant field...
 records_sorted = sorted(records, key=get_city)
 
 # ...Then we can use the groupby:
 for city, rows in it.groupby(records_sorted, get_city):
     print("City:", city, end="\n\t")
     print(*rows, sep="\n\t", end="\n\n")
 
 # Nice thing: this allows us to do some aggregation (e.g. sum, or average)
 # quite easily. Want to know the average age by city?
 for city, rows in it.groupby(records_sorted, get_city):
     print(
         "City:", city,
         "Avg.age:", sum(map(aget('age'), rows)),
         end="\n\n"
     )
 
 # Note: sorted() will return *a list*, not an iterator. That's why we can
 # use the it.groupby() function twice on it. If line 99 were
 #
 #   records_sorted = iter(sorted(records, key=get_city))
 #
 # Then the second cycle at line 108 would have not worked. Also you may
 # try to cycle with it.groupby on records instead of records_sorted: you
 # will see the results! :)
 
 # Happy hacking.
 
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