1 ========

     2  Script

     3 ========

     4 

     5 Welcome to the tutorial on getting started with files. 

     6 

     7 {{{ Screen shows welcome slide }}}

     8 

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

    10 

    11 In this tutorial we shall learn to read files, and do some basic

    12 actions on the file, like opening and reading a file, closing a

    13 file, iterating through the file line-by-line, and appending the

    14 lines of a file to a list. 

    15 

    16 {{{ switch back to the terminal }}}

    17 

    18 As usual, we start IPython, using 

    19 ::

    20 

    21   ipython -pylab 

    22 

    23 Let us first open the file, ``pendulum.txt`` present in

    24 ``/home/fossee/``. 

    25 ::

    26 

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

    28 

    29 ``f`` is called a file object. Let us type ``f`` on the terminal to

    30 see what it is. 

    31 ::

    32 

    33   f

    34 

    35 The file object shows, the file which is open and the mode (read

    36 or write) in which it is open. 

    37 

    38 We shall first learn to read the whole file into a single

    39 variable. Later, we shall look at reading it line-by-line. We use

    40 the ``read`` method of ``f`` to read, all the contents of the file

    41 into the variable ``pend``. 

    42 ::

    43 

    44   pend = f.read()

    45 

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

    47 ::

    48 

    49   print pend

    50 

    51 We can see that ``pend`` has all the data of file. Type just ``pend``

    52 to see more explicitly, what it contains. 

    53 ::

    54 

    55   pend

    56 

    57 %%1%% Pause the video here and split the variable into a list,

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

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

    60 variable has. 

    61 

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

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

    64 use splitlines()]

    65 ::

    66 

    67   pend_list = pend.splitlines()

    68 

    69   pend_list

    70 

    71 Now, let us learn to read the file line-by-line. But, before that

    72 we will have to close the file, since the file has already been

    73 read till the end. 

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

    75 

    76 Let us close the file opened into f.

    77 ::

    78 

    79   f.close()

    80 

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

    82 ::

    83 

    84   f

    85 

    86 Notice, that it now says the file has been closed. It is a good

    87 programming practice to close any file objects that we have

    88 opened, after their job is done.

    89 

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

    91 

    92 %%1%% Pause the video here and re-open the file ``pendulum.txt``

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

    94 

    95 We just use the up arrow until we reach the open command and issue

    96 it again. 

    97 ::

    98 

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

   100 

   101 Now, to read the file line-by-line, we iterate over the file

   102 object line-by-line, using the ``for`` command. Let us iterate over

   103 the file line-wise and print each of the lines. 

   104 ::

   105 

   106   for line in f:

   107       print line

   108 

   109 As we already know, ``line`` is just a dummy variable, and not a

   110 keyword. We could have used any other variable name, but ``line``

   111 seems meaningful enough.

   112 

   113 Instead of just printing the lines, let us append them to a list,

   114 ``line_list``. We first initialize an empty list, ``line_list``. 

   115 ::

   116 

   117   line_list = [ ]

   118 

   119 Let us then read the file line-by-line and then append each of the

   120 lines, to the list. We could, as usual close the file using

   121 ``f.close`` and re-open it. But, this time, let's leave alone the

   122 file object ``f`` and directly open the file within the for

   123 statement. This will save us the trouble of closing the file, each

   124 time we open it. 

   125 

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

   127 line_list.append(line)

   128 

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

   130 ::

   131 

   132   line_list

   133 

   134 Notice that ``line_list`` is a list of the lines in the file, along

   135 with the newline characters. If you noticed, ``pend_list`` did not

   136 contain the newline characters, because the string ``pend`` was

   137 split on the newline characters. 

   138 

   139 {{{ show the summary slide }}}

   140 

   141 That brings us to the end of this tutorial. In this tutorial we

   142 have learnt to open and close files, read the data in the files as

   143 a whole, using the read command or reading it line by line by

   144 iterating over the file object. 

   145 

   146 Thank you!   

   147