.. Objectives
.. ----------

.. By the end of this tutorial, you will be able to 
.. 1. Open and read the contents of a file. 
.. #. Read files line by line. 
.. #. Read all the contents of the file at once. 
.. #. Close open files. 

.. Prerequisites
.. -------------

.. 1. getting started with ipython
.. #. getting started with lists
.. #. getting started with for
     
.. Author              : Puneeth
   Internal Reviewer   : Anoop Jacob Thomas<anoop@fossee.in>
   External Reviewer   :
   Language Reviewer    : Bhanukiran
   Checklist OK?       : <06-11-2010, Anand, OK> [2010-10-05]

Script
------

{{{ Show the slide containing title }}}

Hello Friends. Welcome to the tutorial on getting started with files. 

{{{ Show the outline for this tutorial }}} 

In this tutorial we shall learn to read files, and do some basic
actions on the file, like opening and reading a file, closing a
file, iterating through the file line-by-line, and appending the
lines of a file to a list. 

{{{ switch back to the terminal }}}

As usual, we start IPython, using 
::

  ipython -pylab 

Let us first open the file, ``pendulum.txt`` present in
``/home/fossee/``. 
::

  f = open('/home/fossee/pendulum.txt')

``f`` is called a file object. Let us type ``f`` on the terminal to
see what it is. 
::

  f

The file object shows, the file which is open and the mode (read
or write) in which it is open. Notice that it is open in read only
mode, here. 

We shall first learn to read the whole file into a single
variable. Later, we shall look at reading it line-by-line. We use
the ``read`` method of ``f`` to read, all the contents of the file
into the variable ``pend``. 
::

  pend = f.read()

Now, let us see what is in ``pend``, by typing 
::

  print pend

We can see that ``pend`` has all the data of the file. Type just ``pend``
to see more explicitly, what it contains. 
::

  pend

Following is an exercise that you must do. 

{{ show slide with Question 1 }}

%%1%% Split the variable into a list, ``pend_list``, of the lines in
the file. Hint, use the tab command to see what methods the string
variable has.

Please, pause the video here. Do the exercise and then continue. 

{{ show slide with Solution 1 }}

::

  pend_list = pend.splitlines()

  pend_list

Now, let us learn to read the file line-by-line. But, before that we
will have to close the file, since the file has already been read till
the end.

Let us close the file opened into f.
::

  f.close()

Let us again type ``f`` on the prompt to see what it shows. 
::

  f

Notice, that it now says the file has been closed. It is a good
programming practice to close any file objects that we have
opened, after their job is done.

Let us, now move on to reading files line-by-line. 

Following is an exercise that you must do. 

%%2%% Re-open the file ``pendulum.txt`` with ``f`` as the file object.

Please, pause the video here. Do the exercise and then continue. 

We just use the up arrow until we reach the open command and issue
it again. 
::

  f = open('/home/fossee/pendulum.txt')

Now, to read the file line-by-line, we iterate over the file
object line-by-line, using the ``for`` command. Let us iterate over
the file line-wise and print each of the lines. 
::

  for line in f:
      print line

As we already know, ``line`` is variable, sometimes called the loop
variable, and it is not a keyword. We could have used any other
variable name, but ``line`` seems meaningful enough.

Instead of just printing the lines, let us append them to a list,
``line_list``. We first initialize an empty list, ``line_list``. 
::

  line_list = [ ]

Let us then read the file line-by-line and then append each of the
lines, to the list. We could, as usual close the file using
``f.close`` and re-open it. But, this time, let's leave alone the
file object ``f`` and directly open the file within the for
statement. This will save us the trouble of closing the file, each
time we open it. 

::

  for line in open('/home/fossee/pendulum.txt'):
      line_list.append(line)

Let us see what ``line_list`` contains. 
::

  line_list

Notice that ``line_list`` is a list of the lines in the file, along
with the newline characters. If you noticed, ``pend_list`` did not
contain the newline characters, because the string ``pend`` was
split on the newline characters. 

Using some string methods, that we shall look at in the tutorial on
strings, we can strip out the newline characters from the lines. 

.. #[[Anoop: I think the code that are required to be typed can be
   added to the slide.]]

{{{ show the summary slide }}}

That brings us to the end of this tutorial. In this tutorial we
have learnt to open and close files, read the data in the files as
a whole, using the read command or reading it line by line by
iterating over the file object. 

{{{ Show the "sponsored by FOSSEE" slide }}}

This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India

Hope you have enjoyed and found it useful.
Thank you!