Difference between revisions of "Python-3.4.3/C2/Getting-started-with-files/English"

Objectives

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.

System Specifications

• Ubuntu Linux 16.04 operating system
• Python 3.4.3 and
• IPython 5.1.0
  

Pre-requisite

• Lists and
• for statement

If not, see the pre-requisite Python tutorials on this website.

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.

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.

Open a file

syntax:

file_object = open(filename, mode)

r- Read mode

w- Write mode

a -Appending mode

r+ - Read and Write

w- is for Write mode

a - represents Appending mode and

r+ - for both Read and Write mode

Specifying mode is optional.

We will be using this text file for our demonstration.

Please download it in Home directory and use it.

Press Ctrl+Alt+T keys

Type ipython3

Type ipython3 and press Enter.

Type

%pylab and press Enter.

Type %pylab and press Enter.

Let us clear the terminal.

Type f = open('pendulum.txt')

f is equal to open inside parentheses inside quotes pendulum dot txt

Here the mode is not specified.

By default, it is ‘r’.

Type f

The file object f shows the filename and mode of the file which is open.

Highlight filename and mode 'r'

As you can see, this file is open in read only mode.

pend = f.read()

Highlight pend= f.read()

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.

Type

print(pend)

print inside parentheses pend.

Press Enter

Type

pendHighlight data with newline characters

We can see the newline characters as well, in the output.

We use the method splitlines to split a file of data into list of lines.

Type

pend_list = pend.splitlines()

Type

pend_list equal to pend dot splitlines open and close parentheses

Press Enter

Type pend underscore list

Press Enter

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.

f.close()

Type f dot close open and close parentheses

Press Enter

Highlight closed file

• to close any file objects that we have opened
• after their job is done.

Exercise 1

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.

Typef = open('pendulum.txt')

To re-open the file again, type

f is equal to open inside parentheses inside quotes pendulum.txt

Type

for line in f:

print(line)

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

Highlight line

We could have used any other variable name, but line seems meaningful enough.

Press Enter twice.

Highlight line

Type

line_list = []

Type,

line underscore list is equal to open and close square brackets. Press Enter

Highlight according to narration.

Type

for line in open('pendulum.txt'):

line_list.append(line)

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.

This will save us the trouble of closing the file, each time we open it.

Type line_list and press Enter

Highlight data

Type line underscore list and press Enter.

Highlight newline characters

This will be covered in the further tutorial on strings.

Summary

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.

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.

Assignment

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

Solutions

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

Forum

Fossee Forum

Textbook Companion

Acknowledgment

http://spoken-tutorial.org

For more details, visit this website.

Thank you.