1 .. Objectives

     2 .. ----------

     3 

     4 .. By the end of this tutorial, you will be able to 

     5 .. 1. Open and read the contents of a file. 

     6 .. #. Read files line by line. 

     7 .. #. Read all the contents of the file at once. 

     8 .. #. Close open files. 

     9 

    10 .. Prerequisites

    11 .. -------------

    12 

    13 .. 1. getting started with ipython

    14 .. #. getting started with lists

    15 .. #. getting started with for

    16      

    17 .. Author              : Puneeth

    18    Internal Reviewer   : Anoop Jacob Thomas<anoop@fossee.in>

    19    External Reviewer   :

    20    Language Reviewer    : Bhanukiran

    21    Checklist OK?       : <06-11-2010, Anand, OK> [2010-10-05]

    22 

    23 Script

    24 ------

    25 

    26 {{{ Show the slide containing title }}}

    27 

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

    29 

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

    31 

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

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

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

    35 lines of a file to a list. 

    36 

    37 {{{ switch back to the terminal }}}

    38 

    39 As usual, we start IPython, using 

    40 ::

    41 

    42   ipython -pylab 

    43 

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

    45 ``/home/fossee/``. 

    46 ::

    47 

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

    49 

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

    51 see what it is. 

    52 ::

    53 

    54   f

    55 

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

    57 or write) in which it is open. Notice that it is open in read only

    58 mode, here. 

    59 

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

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

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

    63 into the variable ``pend``. 

    64 ::

    65 

    66   pend = f.read()

    67 

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

    69 ::

    70 

    71   print pend

    72 

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

    74 to see more explicitly, what it contains. 

    75 ::

    76 

    77   pend

    78 

    79 Following is an exercise that you must do. 

    80 

    81 {{ show slide with Question 1 }}

    82 

    83 %%1%% Split the variable into a list, ``pend_list``, of the lines in

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

    85 variable has.

    86 

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

    88 

    89 {{ show slide with Solution 1 }}

    90 

    91 ::

    92 

    93   pend_list = pend.splitlines()

    94 

    95   pend_list

    96 

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

    98 will have to close the file, since the file has already been read till

    99 the end.

   100 

   101 Let us close the file opened into f.

   102 ::

   103 

   104   f.close()

   105 

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

   107 ::

   108 

   109   f

   110 

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

   112 programming practice to close any file objects that we have

   113 opened, after their job is done.

   114 

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

   116 

   117 Following is an exercise that you must do. 

   118 

   119 %%2%% Re-open the file ``pendulum.txt`` with ``f`` as the file object.

   120 

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

   122 

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

   124 it again. 

   125 ::

   126 

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

   128 

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

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

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

   132 ::

   133 

   134   for line in f:

   135       print line

   136 

   137 As we already know, ``line`` is variable, sometimes called the loop

   138 variable, and it is not a keyword. We could have used any other

   139 variable name, but ``line`` seems meaningful enough.

   140 

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

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

   143 ::

   144 

   145   line_list = [ ]

   146 

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

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

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

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

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

   152 time we open it. 

   153 

   154 ::

   155 

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

   157       line_list.append(line)

   158 

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

   160 ::

   161 

   162   line_list

   163 

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

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

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

   167 split on the newline characters. 

   168 

   169 Using some string methods, that we shall look at in the tutorial on

   170 strings, we can strip out the newline characters from the lines. 

   171 

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

   173    added to the slide.]]

   174 

   175 {{{ show the summary slide }}}

   176 

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

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

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

   180 iterating over the file object. 

   181 

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

   183 

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

   185 

   186 Hope you have enjoyed and found it useful.

   187 Thank you!

   188 

   189