I will read this when I get a minute, but I must say thanks for the explanation. tutor@python really is the most helpful mailing list EVER! > Date: Tue, 24 Jun 2014 00:54:30 +1000 > From: st...@pearwood.info > To: tutor@python.org > Subject: Re: [Tutor] write dictionary to file > > On Mon, Jun 23, 2014 at 09:17:44AM +0000, Ian D wrote: > > > > for row in spamreader: > > > if row['year'] == '40': > > > email = row['user'] + '@email.com' > > > output = [ row[fieldname] for fieldname in fields ] > > > > I am unsure about this syntax [ row[fieldname] for fieldname in fields ] > > > That's called a "list comprehension". It is a way of building up a list > as a single expression. Let's start with the old-fashioned way to build > a list: > > output = [] > for fieldname in fields: > output.append( row[fieldname] ) > > > Or, we can do exactly the same thing, in a single line, with a list > comprehension, and get rid of the temporary variables: > > output = [row[fieldname] for fieldname in fields] > > If you remember your high school maths classes, the form of the list > comp is rather close to that of mathematical set builder notation: > > http://www.mathsisfun.com/sets/set-builder-notation.html > > Here is a set builder notation, and its English translation: > > {2n+1 : n ∈ {1,2,3,4}} > > This reads as "the set of 2n plus 1, such that n is an element of > {1,2,3,4}". Don't be confused by the fact that there is a set inside a > set -- this just tells us how to build a new set from an old set. > > So we start with a set {1,2,3,4}, and let n equal each of those numbers > in turn. Then we calculate 2n+1, and use that inside the new set we're > creating: > > n = 1, so 2n+1 => 3 > n = 2, so 2n+1 => 5 > n = 3, so 2n+1 => 7 > n = 4, so 2n+1 => 9 > > and so the final result is the set {3, 5, 7, 9}. > > Enough about sets and mathematics! Now we're going to talk about Python! > Python uses the same sort of expression, except it builds a list, not a > set, and calls it a "list comprehension" instead of a "list builder". > (Don't blame me for that, blame the Haskell programming language which > invented this.) > > So, we start with the mathematical set builder: > > {2n+1 : n ∈ {1,2,3,4}} > > > Turn the sets into lists: > > [2n+1 : n ∈ [1,2,3,4]] > > > Use Python syntax for the formula: > > [2*n+1 : n ∈ [1,2,3,4]] > > > Replace the "such that" and "element of" with "for" "in": > > [2*n+1 for n in [1,2,3,4]] > > > and now you have a Python list comprehension. If you copy and paste that > into a Python interpreter, you'll see this: > > py> [2*n+1 for n in [1,2,3,4]] > [3, 5, 7, 9] > > > > > > # DictWriter needs a dict, not a list. > > > spamwriter.writerow({name: row[name] for name in fields}) > > > > And this writerow syntax is something new for me, as are dictionaries > > (which I have tried to read up and understand.) > > List comprehensions were added to Python in, I think, version 2.2, which > was about ten years ago. They turned out to be so useful and popular > that now, in Python 3, we have FOUR different kinds of "comprehensions" > or builder syntax: > > List comprehensions: > [2*n+1 for n in [1,2,3,4]] > > Generator expressions (like a list comp, except values are created > lazily on demand, rather than all at once): > (x+1 for x in [2, 4, 6, 8]) > > Set comprehensions (like a list comp, except it builds a set rather than > a list): > {char.lower() for char in "Hello World!"} > > Dict comprehensions (like a list comp, except it builds a dictionary of > key:value pairs): > {char: char.lower() for char in "Hello World!"} > > > So what is a dict? A dict, short for dictionary, is a table of keys with > associated values. Think of them as being like words and definitions: > > words = { > "cat": "a small mammal that purrs", > "dog": "man's best friend", > "snake": "a lizard with no legs", > "ostrich": "biggest bird in the world" > } > > > The first part, before the colon, is the key. The part after the colon > is the value associated with that key. If you want to know the meaning > of a word, you look it up in the dictionary: > > py> print(words['dog']) > man's best friend > > > To add a new word: > > py> words['kangeroo'] = 'jumping marsupial from Australia' > py> print(words['kangeroo']) > jumping marsupial from Australia > > > So I've already shown you the dict comprehension form: > > spamwriter.writerow({name: row[name] for name in fields}) > > > Here is how we can do it the old-fashioned way without a dict comp: > > > table = {} > for name in fields: > value = row[name] > table[name] = value > > spamwriter.writerow(table) > > > > > Does this help? Ask any further questions you may have. > > > > > > -- > Steven > _______________________________________________ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > https://mail.python.org/mailman/listinfo/tutor
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