[Tutor] help with exercise 15 of zed shaw's LPTHW

[lohecn at tuta.io]

first, sorry everyone for having attached the file instead of just typing it 
here.
second, thanks a lot for the replies; even though I gave you no code it was 
quite helpful!
the code was this:

from sys import argv

script, filename = argv
txt = open (filename)

print "Here's your file %r: " % filename
print txt.read()

print "Type the filename again: "
file_again = raw_input("> ")

txt_again = open(file_again)

print txt_again.read()

Peter Otten explained it to me line by line [thanks so much :)] however, I do 
have one more question:
why do I have to create a variable txt_again to assign it to the open 
function and them print the file?
why is it that I can't only write something like open(file_again).read()?


6. Jul 2016 05:22 by __peter__ at web.de:


> lohecn at tuta.io>  wrote:
>
>> hey everyone. this is my first time trying this -- actually, I've been
>> studying python only for some days now, and I'm afraid my questions are
>> going to be reeeeally simple, but I can't seem to understand this piece of
>> code and thus can't move on.
>
> You seem to be talking about
>
> http://learnpythonthehardway.org/book/ex15.html
>
> """
> from sys import argv
>
> script, filename = argv
>
> txt = open(filename)
>
> print "Here's your file %r:" % filename
> print txt.read()
>
> print "Type the filename again:"
> file_again = raw_input("> ")
>
> txt_again = open(file_again)
>
> print txt_again.read()
> """
>
> As others said, always provide the code you are asking about, or if that is
> not possible at least provide a link.
>
>> you probably know the book, so you know that zed always makes us write
>> code so that then we can understand how it works, and it's great, but in
>> this exercise there are just too many new functions and without
>> explanation they are a bit hard to understand... so I'm having trouble
>> with most of the lines here.
>>
>> it's not that I want the full explanation to that code, but since I'm
>> unfamiliar with some of its concepts, I'm just going to tell you all the
>> things that I don't understand (sorry for it being a lot):
>> 1. the need to put script into an estipulation for argv (line 3)
>
> Write a script tmp.py containing
>
> from sys import argv
> print argv
>
> then call it with with one parameter, e. g.
>
> $ python tmp.py somefile.txt
> ['tmp.py', 'somefile.txt']
>
> As you can see argv is a list with two items, the first being "tmp.py", the
> name of the script you are invoking. You are only interested in the second
> one, the filename. The easy way to get that is
>
> filename = argv[1]
>
> the hard way is to use "unpacking"
>
> script, filename = argv
>
> where python will assign one item from the list on the right to every name
> on the left:
>
>>>> items = ["foo", "bar"]
>>>> one, two = items
>>>> one
> 'foo'
>>>> two
> 'bar'
>
> What happens if the number of names on the left doesn't match the number of
> items in the list?
>
>>>> one, two, three = items
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> ValueError: need more than 2 values to unpack
>
> You get an exception. That is why you have to provide the name "script" in
> Zed's example even though you are not actually interested in the script
> name.
>
>> 2. the what is txt and why it has to be used there (line 4)
>
> txt is a file object and
>
>> 3. txt.read() -- which are all new functions(? I dont even know what they
>> are)  (line 7)
>
> read() is a method that here reads the whole file into a string. You use 
> the
> open() function to open a file and usually assign the file object that is
> returned by open to a name. You can freely choose that name. The structure
> is the same for every object, be it a number:
>
> x = 42  # assign a number to x
> y = x + x  # do some arithmetic with x and assign the result to y
> print y  # print the result
>
> a list:
>
> mynumbers = list()  # create a list
> mynumbers.append(42)  # append a number to the list
> print mynumbers  # print the list
>
> or a file:
>
> myfile = open("example.txt")  # open the file example.txt in the current
>                               # working directory. If the file doesn't 
> exist
>                               # you get an error
>
> print "first line:", myfile.readline()  # read the first line and print it
> print "rest of the file:"
> print myfile.read()  # read the rest of the file and print it
>
> myfile.close()  # close the file
>
>> 4. file_again (line 10)
>> 5. txt_again (line 12)
>> and line 14.
>
> 4. and 5. are just a repetition of the first part, with the variation that
> the filename, assigned to file_again is read interactively with raw_input()
> instead of passing it as a commandline argument to the script.
>
> The names used can be freely chosen by the programmer, a script
>
> from sys import argv
>
> red_apple, my_hat = argv
>
> blue_suede_shoes = open(my_hat)
> print blue_suede_shoes.read()
> blue_suede_shoes.close()
>
> would work exactly like the first part of the hard-way example. However,
> picking descriptive names and using them consistently makes it much easier
> for a human reader to understand what's going on.
>
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