File handling

A file is some information or data which stays in the computer storage devices. You already know about different kinds of file , like your music files, video files, text files. Python gives you easy ways to manipulate these files. Generally we divide files in two categories, text file and binary file. Text files are simple text where as the binary files contain binary data which is only readable by computer.

File opening

To open a file we use open() function. It requires two arguments, first the file path or file name, second which mode it should open. Modes are like

  • “r” -> open read only, you can read the file but can not edit / delete anything inside
  • “w” -> open with write power, means if the file exists then delete all content and open it to write
  • “a” -> open in append mode

The default mode is read only, ie if you do not provide any mode it will open the file as read only. Let us open a file

>>> fobj = open("love.txt")
>>> fobj
<_io.TextIOWrapper name='love.txt' mode='r' encoding='UTF-8'>

Closing a file

After opening a file one should always close the opened file. We use method close() for this.

>>> fobj = open("love.txt")
>>> fobj
<_io.TextIOWrapper name='love.txt' mode='r' encoding='UTF-8'>
>>> fobj.close()

Important

Important

Always make sure you explicitly close each open file, once its job is done and you have no reason to keep it open. Because - There is an upper limit to the number of files a program can open. If you exceed that limit, there is no reliable way of recovery, so the program could crash. - Each open file consumes some main-memory for the data-structures associated with it, like file descriptor/handle or file locks etc. So you could essentially end-up wasting lots of memory if you have more files open that are not useful or usable. - Open files always stand a chance of corruption and data loss.

Reading a file

To read the whole file at once use the read() method.

>>> fobj = open("sample.txt")
>>> fobj.read()
'I love Python\nPradeepto loves KDE\nSankarshan loves Openoffice\n'

If you call read() again it will return empty string as it already read the whole file. readline() can help you to read one line each time from the file.

>>> fobj = open("sample.txt")
>>> fobj.readline()
'I love Python\n'
>>> fobj.readline()
'Pradeepto loves KDE\n'

To read all the lines in a list we use readlines() method.

>>> fobj = open("sample.txt")
>>> fobj.readlines()
['I love Python\n', 'Pradeepto loves KDE\n', 'Sankarshan loves Openoffice\n']

You can even loop through the lines in a file object.

>>> fobj = open("sample.txt")
>>> for x in fobj:
...     print(x, end=' ')
...
I love Python
Pradeepto loves KDE
Sankarshan loves Openoffice

Let us write a program which will take the file name as the input from the user and show the content of the file in the console.

#!/usr/bin/env python3
name = input("Enter the file name: ")
fobj = open(name)
print(fobj.read())
fobj.close()

In the last line you can see that we closed the file object with the help of close() method.

The output

$ ./showfile.py
Enter the filename: sample.txt
I love Python
Pradeepto loves KDE
Sankarshan loves Openoffice

Using the with statement

In real life scenarios we should try to use with statement. It will take care of closing the file for you.

>>> with open('setup.py') as fobj:
...     for line in fobj:
...         print line,
...
#!/usr/bin/env python3
"""Factorial project"""
from setuptools import find_packages, setup

setup(name = 'factorial',
    version = '0.1',
    description = "Factorial module.",
    long_description = "A test module for our book.",
    platforms = ["Linux"],
    author="Kushal Das",
    author_email="kushaldas@gmail.com",
    url="https://pymbook.readthedocs.io/en/latest/",
    license = "http://www.gnu.org/copyleft/gpl.html",
    packages=find_packages()
    )

Writing in a file

Let us open a file then we will write some random text into it by using the write() method.

>>> fobj = open("ircnicks.txt", 'w')
>>> fobj.write('powerpork\n')
>>> fobj.write('indrag\n')
>>> fobj.write('mishti\n')
>>> fobj.write('sankarshan')
>>> fobj.close()

Now read the file we just created

>>> fobj = open('ircnicks.txt')
>>> s = fobj.read()
>>> print(s)
powerpork
indrag
mishti
sankarshan

copyfile.py

In this example we will copy a given text file to another file.

#!/usr/bin/env python3
import sys
if len(sys.argv) < 3:
    print("Wrong parameter")
    print("./copyfile.py file1 file2")
    sys.exit(1)
f1 = open(sys.argv[1])
s = f1.read()
f1.close()
f2 = open(sys.argv[2], 'w')
f2.write(s)
f2.close()

Note

This way of reading file is not always a good idea, a file can be very large to read and fit in the memory. It is always better to read a known size of the file and write that to the new file.

You can see we used a new module here sys. sys.argv contains all command line parameters. Remember cp command in shell, after cp we type first the file to be copied and then the new file name.

The first value in sys.argv is the name of the command itself.

#!/usr/bin/env python3
import sys
print("First value", sys.argv[0])
print("All values")
for i, x  in enumerate(sys.argv):
    print(i, x)

The output

$ ./argvtest.py Hi there
First value ./argvtest.py
All values
0 ./argvtest.py
1 Hi
2 there

Here we used a new function enumerate(iterableobject), which returns the index number and the value from the iterable object.

Count spaces, tabs and new lines in a file

Let us try to write an application which will count the spaces, tabs, and lines in any given file.

#!/usr/bin/env python3

import os
import sys


def parse_file(path):
    """
    Parses the text file in the given path and returns space, tab & new line
    details.

    :arg path: Path of the text file to parse

    :return: A tuple with count of spacaes, tabs and lines.
    """
    fd = open(path)
    i = 0
    spaces = 0
    tabs = 0
    for i,line in enumerate(fd):
        spaces += line.count(' ')
        tabs += line.count('\t')
    #Now close the open file
    fd.close()

    #Return the result as a tuple
    return spaces, tabs, i + 1

def main(path):
    """
    Function which prints counts of spaces, tabs and lines in a file.

    :arg path: Path of the text file to parse
    :return: True if the file exits or False.
    """
    if os.path.exists(path):
        spaces, tabs, lines = parse_file(path)
        print("Spaces %d. tabs %d. lines %d" % (spaces, tabs, lines))
        return True
    else:
        return False


if __name__ == '__main__':
    if len(sys.argv) > 1:
        main(sys.argv[1])
    else:
        sys.exit(-1)
    sys.exit(0)

You can see that we have two functions in the program , main and parse_file where the second one actually parses the file and returns the result and we print the result in main function. By splitting up the code in smaller units (functions) helps us to organize the codebase and also it will be easier to write test cases for the functions.

Let us write some real code

Do you know how many CPU(s) are there in your processor? or what is the model name? Let us write some code which can help us to know these things.

If you are in Linux, then you can actually view the output of the lscpu command first. You can actually find the information in a file located at /proc/cpuinfo.

Now try to write code which will open the file in read only mode and then read the file line by line and find out the number of CPU(s).

Tip

Always remember to read files line by line than reading them as a whole. Sometimes you may have to read files which are way bigger than your available RAM.

After you do this, try to write your own lscpu command in Python :)