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

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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 -
  1. Open a file.
  2. Read the contents of the file line by line.
  3. Read the entire content of the file at once.
  4. Append the lines of a file to a list and
  5. 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- is for 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.

Text box on the screen as per the narration 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.

Open pendulum.txt 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()

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_list

Type pend underscore list

Press Enter

Highlight the data into list of lines 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.

But, before that we will have to close the file, since the file has already been read till the end.

[IPython Terminal]


f.close()

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.
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.

Let us switch back to the terminal.

Let us now move on to reading files line- by - line.
[IPython Terminal]

Typef = open('pendulum.txt')

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 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 into 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. Press Enter

[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 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

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

1. 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,
  1. open function returns a file object
  2. splitlines() displays the data line by line as strings
Show Slide

Forum

Please post your timed queries in this forum.
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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