Python - list comprehension and string formatting procedure to take a list and add specific string to all the list.

list_comprehension from mattmakesmap

"""
Question One:
Why does the join method not add ".cso" to "king"
"""
 
a=('seattle','chelan','king')
print ".cso ".join(a) # returns "seattle.cso chelan.cso king"
 
"""
Answer:
The .join() method is used here to combine multiple elements, e.g.
the three in your tuple, into a single string element, joined
together by whatever you specify (in your case, '.cso'). Since you
have three elements in your tuple, the join method only needs to
insert two '.cso' entries.
 
.join() is more commonly used to create comma (or any other type of
delimited) strings from a list or tuple.
"""
 
a=('seattle','chelan','king')
a_joined_string = ','.join(a)
print a_joined_string # returns 'seattle,chelan,king'
len(a_joined_string) # returns 19, the number of characters in the string
 
"""
Question Two: Why does wrapping a join() call within a list constructor
create a list of length 1?
"""
 
a=['seattle','chelan','harbor']
b=[".cso ".join(a)] # returns ['seattlecso. chelancso. harbor']
 
"""
Answer: Looking at lines 21-24, you can see that when you call the join
method on a list, you're saying something like. "Create a single string
that contains every element in this list, but seperate(join) each element
with a specific delimiter."
 
So when you wrap that call to join() within a list constructor (the brackets),
you're saying something like, "Create a single string using join(), but wrap that
single string in a list".
 
What you know you really want however, is to ask
something like, "Create a new list containing individual strings from each element in
list a, while adding '.cso' to the end of each element from a".
 
List comprehension is a great way to do that.
"""
 
a=['seattle','chelan','harbor']
b = ["%s.cso" % city for city in a] # list comprehension
print b # returns ['seattle.cso', 'chelan.cso', 'harbor.cso'] This is what you want?
 
"""
Line 52 introduces two concepts here, list comprehension and string formatting.
Let's talk about the string formatting first.
 
As opposed to using the join() method which creates a single string, combining
all elements in a list, we use string formatting to modify an individual string, 
adding '.cso' to the end. Here are the docs on that syntax:
http://docs.python.org/2/library/stdtypes.html#string-formatting-operations
 
Secondly is the list comprehension syntax. You're basically creatting a small
for-loop inside a list construction. notice in line 52 the part 'city for city in a'?
You're saying, "for each city, in list a, add a copy to me (list b).
 
So wrapping those two things together you get this phrase that is like 'add the suffix
.cso to a copy of each city from list a, then add it to me (list b)
"""