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Welcome to the tutorial on getting started with files. 

{{{ Screen shows welcome slide }}}

{{{ 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. 

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 file. Type just ``pend``

to see more explicitly, what it contains. 

::

  pend

%%1%% Pause the video here and split the variable into a list,

``pend_list``, of the lines in the file and then resume the

video. Hint, use the tab command to see what methods the string

variable has. 

#[punch: should this even be put? add dependency to strings LO,

where we mention that strings have methods for manipulation. hint:

use splitlines()]

::

  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. 

#[punch: should we mention file-pointer?]

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. 

%%1%% Pause the video here and re-open the file ``pendulum.txt``

with ``f`` as the file object, and then resume the video.

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 just a dummy variable, and 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. 

{{{ 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. 

Thank you!