Python-3.4.3/C2/Getting-started-with-files/English

Title of script: Getting started with files

Author: Trupti Kini

Keywords: IPython, files, read, write, append, mode, splitlines()

Visual Cue	Narration
Show Slide	Hello Friends. Welcome to the tutorial on "Getting started with files".
Show Slide Objectives	At the end of this tutorial, you will learn to - Open a file. Read the contents of the file line by line. Read the entire content of the file at once. Append the lines of a file to a list and Close the file.
Show slide System Specifications	To record this tutorial, I am using Ubuntu Linux 16.04 operating system Python 3.4.3 and IPython 5.1.0
Show Slide: Pre-requisite	To practice this tutorial, you should know about Lists and for statement If not, see the pre-requisite Python tutorials on this website.
Show slide Open a file Highlight according to narration: (open and file object) syntax: file_object = open(filename, mode) Filename - name of the file to be opened. Mode- indicates how the file is going to be opened.	To open a file for reading or writing, we can use a built in function called open(). Open() function returns a file object. The syntax is shown here: Filename - is the name of the file to be opened. Mode- This indicates how the file is going to be opened.
Show slide Open a file syntax: file_object = open(filename, mode) r- Read mode w- Write mode a -Appending mode r+ - Read and Write	r- represent the Read mode w- is for Write mode a - represents Appending mode and r+ - for both Read and Write mode Specifying mode is optional.
Open pendulum.txt in text editor	Let us open a file, pendulum.txt in a text editor.
Point to the columns in the file	This file contains 2 data columns, length and time of pendulum. We will be using this text file for our demonstration.
	The file pendulum.txt is available in the code file link of this tutorial. Please download it in Home directory and use it.
[Terminal] Press Ctrl+Alt+T keys Type ipython3	Let us first open the Terminal by pressing Ctrl+Alt+T keys simultaneously. Type ipython3 and press Enter.
[IPython console] Type %pylab and press Enter.	Let us initialise the pylab package. Type %pylab and press Enter. Let us clear the terminal.
	Let us open the file, pendulum.txt.
[IPython Terminal] Type f = open('pendulum.txt')	Type f is equal to open inside parentheses inside quotes pendulum dot txt Here the mode is not specified. By default, it is ‘r’.
[IPython Terminal] Type f	Let us type f on the terminal to see what it is. The file object f, shows the filename and mode of the file, which is open.
[IPython Terminal] Highlight filename and mode 'r'	'r' stands for read only mode. As you can see, this file is open in read only mode.
[IPython Terminal]	Now let us learn to read the whole file into a single variable.
Type pend = f.read() Highlight pend= f.read()	Type pend equal to f dot read open and close parentheses. We use the read method to read all the contents of the file into the variable, pend Press Enter To use read method we use the file object dot read method.
[IPython Terminal] Type print(pend)	Now, let us see what pend contains, by typing print inside parentheses pend. Press Enter
Point to the data	We can see that pend has all the data of the file pendulum.txt.
[IPython Terminal] Type pendHighlight data with newline characters	Type just pend to see more explicitly, what it contains. We can see the newline characters as well in the output.
[IPython Terminal]	Let us learn to split the variable pend into a list of lines in the file. We use the method splitlines to split a file of data into list of lines.
[IPython Terminal] Type pend_list = pend.splitlines() pend_list Highlight the data into list of lines	For this we need to store this list in a variable, say pend_list Type pend_list equal to pend dot splitlines open and close parentheses Press Enter Type pend underscore list Press Enter We got the data into list of lines.
Highlight the data into list of lines	pend_list does not contain newline characters like \n. This is because the string pend is split on the newline characters.
	Now, let us learn to read the file line-by-line.
[IPython Terminal] f.close()	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. Type f dot close open and close parentheses Press Enter
[IPython Terminal] Highlight closed file	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.
Show Slide Exercise 1	Pause the video here, try out the following exercise and then resume the video. Re-open the file pendulum.txt with f as the file object. Recall, that we have closed the file earlier.
[IPython Terminal] Typef = open('pendulum.txt')	Let us switch back to the terminal. To re-open the file again, type f is equal to open inside parentheses inside quotes pendulum.txt
[IPython Terminal] Type for line in f: print(line)	Now, to read the file line-by-line, we iterate over the file object line-by-line, using the for loop. Let us iterate over the file line-wise and print each of the lines. Let us clear the terminal. Type for line in f colon press Enter four spaces print inside parentheses line
[IPython Terminal] Highlight line	Here, line is a loop variable, and it is not a keyword. We could have used any other variable name, but line seems meaningful enough. Press Enter twice.
[IPython Terminal] Highlight line	Instead of just printing the lines, let us append them to a list, say line_list.
[IPython Terminal] Type line_list = []	We first initialize a line_list as an empty-list Type, line underscore list is equal to open and close square brackets.
[IPython Terminal] Highlight according to narration. Type for line in open('pendulum.txt'): line_list.append(line)	Type the code as for line in open inside parentheses inside quotes pendulum dot txt colon press enter four space line underscore list dot append inside parentheses line Press Enter twice. Here, the for loop reads the file pendulum.txt line-by-line . The append method will add each of the line to the list, line_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.
[IPython Terminal] Type line_list and press Enter Highlight data	Let us see what line_list contains. Type line underscore list and press Enter.
[IPython Terminal] Highlight newline characters	If you noticed, line underscore list is a list of the lines in the file, along with the newline characters.
	We can strip out the newline characters from the lines by using some string methods. This will be covered in the further tutorial on strings.
Show Slide Summary	This brings us to the end of this tutorial. In this tutorial, we learnt to - Open and close files using the open and close methods respectively. Read the data in the files as a whole, by using the read method.
Show Slide Summary	Read the data in the files line by line by iterating over the file object using the for loop. Append the lines of a file to a list using the append method within the for loop.
Show Slide Assignment	Here are some self assessment questions for you to solve The open function returns a string list file object function 2. What does the function splitlines() do. Displays the data as strings, all in a line Displays the data line by line as strings Displays the data line by line but not as strings
Show Slide Solutions	And the answers are, open function returns a file object splitlines() displays the data line by line as strings
Show Slide Forum	Please post your timed queries in this forum.
Show Slide Fossee Forum	Please post your general queries on Python in this forum.
Show Slide Textbook Companion	FOSSEE team coordinates the TBC project.
Show Slide Acknowledgment http://spoken-tutorial.org	Spoken Tutorial Project is funded by NMEICT, MHRD, Govt. of India. For more details, visit this website.
Previous slide	This is Trupti Kini from IIT Bombay (or FOSSEE, if you wish) signing off. Thank you.

Contributors and Content Editors

Nancyvarkey, Trupti