getting-started-files/script.rst
author anand
Thu, 11 Nov 2010 00:03:57 +0530
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Checklist OK for `The other kinds of plots`
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.. Objectives
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.. ----------
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.. By the end of this tutorial, you will be able to 
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.. 1. Open and read the contents of a file. 
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.. #. Read files line by line. 
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.. #. Read all the contents of the file at once. 
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.. #. Close open files. 
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.. Prerequisites
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.. -------------
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.. 1. getting started with ipython
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.. #. getting started with lists
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.. #. getting started with for
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.. Author              : Puneeth
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   Internal Reviewer   : Anoop Jacob Thomas<anoop@fossee.in>
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   External Reviewer   :
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   Language Reviewer    : Bhanukiran
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   Checklist OK?       : <06-11-2010, Anand, OK> [2010-10-05]
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Script
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------
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{{{ Show the slide containing title }}}
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Hello Friends. Welcome to the tutorial on getting started with files. 
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{{{ Show the outline for this tutorial }}} 
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In this tutorial we shall learn to read files, and do some basic
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actions on the file, like opening and reading a file, closing a
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file, iterating through the file line-by-line, and appending the
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lines of a file to a list. 
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{{{ switch back to the terminal }}}
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As usual, we start IPython, using 
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::
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  ipython -pylab 
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Let us first open the file, ``pendulum.txt`` present in
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``/home/fossee/``. 
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  f = open('/home/fossee/pendulum.txt')
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``f`` is called a file object. Let us type ``f`` on the terminal to
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see what it is. 
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::
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  f
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The file object shows, the file which is open and the mode (read
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or write) in which it is open. Notice that it is open in read only
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mode, here. 
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We shall first learn to read the whole file into a single
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variable. Later, we shall look at reading it line-by-line. We use
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the ``read`` method of ``f`` to read, all the contents of the file
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into the variable ``pend``. 
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::
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  pend = f.read()
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Now, let us see what is in ``pend``, by typing 
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::
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  print pend
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We can see that ``pend`` has all the data of the file. Type just ``pend``
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to see more explicitly, what it contains. 
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::
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  pend
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Following is an exercise that you must do. 
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{{ show slide with Question 1 }}
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%%1%% Split the variable into a list, ``pend_list``, of the lines in
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the file. Hint, use the tab command to see what methods the string
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variable has.
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Please, pause the video here. Do the exercise and then continue. 
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{{ show slide with Solution 1 }}
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::
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  pend_list = pend.splitlines()
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  pend_list
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Now, let us learn to read the file line-by-line. But, before that we
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will have to close the file, since the file has already been read till
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the end.
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Let us close the file opened into f.
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::
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  f.close()
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Let us again type ``f`` on the prompt to see what it shows. 
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::
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  f
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Notice, that it now says the file has been closed. It is a good
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programming practice to close any file objects that we have
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opened, after their job is done.
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Let us, now move on to reading files line-by-line. 
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Following is an exercise that you must do. 
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%%2%% Re-open the file ``pendulum.txt`` with ``f`` as the file object.
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Please, pause the video here. Do the exercise and then continue. 
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We just use the up arrow until we reach the open command and issue
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it again. 
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::
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  f = open('/home/fossee/pendulum.txt')
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Now, to read the file line-by-line, we iterate over the file
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object line-by-line, using the ``for`` command. Let us iterate over
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the file line-wise and print each of the lines. 
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::
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  for line in f:
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      print line
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As we already know, ``line`` is variable, sometimes called the loop
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variable, and it is not a keyword. We could have used any other
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variable name, but ``line`` seems meaningful enough.
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Instead of just printing the lines, let us append them to a list,
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``line_list``. We first initialize an empty list, ``line_list``. 
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::
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  line_list = [ ]
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Let us then read the file line-by-line and then append each of the
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lines, to the list. We could, as usual close the file using
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``f.close`` and re-open it. But, this time, let's leave alone the
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file object ``f`` and directly open the file within the for
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statement. This will save us the trouble of closing the file, each
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time we open it. 
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::
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  for line in open('/home/fossee/pendulum.txt'):
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      line_list.append(line)
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Let us see what ``line_list`` contains. 
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::
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  line_list
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Notice that ``line_list`` is a list of the lines in the file, along
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with the newline characters. If you noticed, ``pend_list`` did not
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contain the newline characters, because the string ``pend`` was
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split on the newline characters. 
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Using some string methods, that we shall look at in the tutorial on
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strings, we can strip out the newline characters from the lines. 
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.. #[[Anoop: I think the code that are required to be typed can be
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   added to the slide.]]
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{{{ show the summary slide }}}
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That brings us to the end of this tutorial. In this tutorial we
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have learnt to open and close files, read the data in the files as
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a whole, using the read command or reading it line by line by
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iterating over the file object. 
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{{{ Show the "sponsored by FOSSEE" slide }}}
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This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India
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Hope you have enjoyed and found it useful.
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Thank you!
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