Python Tutorial 17 --- "Open(), Read() & Readline() For Reading File"

 Python Tutorial 17 --- 

"Open(), Read() & Readline() For Reading File"

This PROGRAM or TUTORIAL is about "Open(), Read() & Readline() For Reading File"

DOC:

As we now have an idea of what files(text or binary) are and their access modes, we are now ready to dive into the discussion of file handling methods. When we want to read or write a file (say on our hard drive), we must first open the file. When we open a file, we are asking the operating system to find the file by name, making sure the file exists.

How to open a file?

Python has a built-in open() function to open a file.

The syntax of the function is:

open("filename" ,"mode")

To open a file, we must specify two things,

  • Name of the file and its extension
  • Access mode where we can specify in which mode file has to be opened, it could either be read (r), write (w) or append(a), etc. For more information regarding access modes, refer to the previous tutorial.

For Example, 

open("myfile.txt")

The file “myfile.txt” will open in "rt" mode as it is the default mode. But the best practice is to follow the syntax to avoid errors.

The open function returns a file object. We store this file object into a variable which is generally called as a file pointer/file handler. Here is a code snippet to open the file using file handing in Python,

     

  f=open("myfile.txt," "w")

You can use this file pointer to further add modifications in the file. An error could also be raised if the operation fails while opening the file. It could be due to various reasons like trying to access a file that is already closed or trying to read a file open in write mode.

How to read a file?

To read a file in Python, there are various methods available,

  • We can read a whole file line by line using a for loop in combination with an iterator. This will be a fast and efficient way of reading data.
  • When opening a file for reading, Python needs to know exactly how the file should be opened. Two access modes are available reading (r), and reading in binary mode (rb). They have to be specified when opening a file with the built-in open() method.

                                        

f = open("myfile.txt", "r")

 

  • The read() method reads the whole file by default. We can also use the read(size) method where you can specify how many characters you want to return i.e.
  • f.read(2); #Here, you will get the first two characters of the file.

     

    • You can use the readline() method to read individual lines of a file. By calling readline() a second time, you will get the next line.
    • readlines() method reads until the end of the file and returns a list of lines of the entire file. It does not read more than one line.

                                            

    f=open("myfile.txt","r");
f.readlines() #Returns a list object

    Note: The default mode to read data is text mode. If you want to read data in binary format, use ''rb".

    Is it necessary to close a file?

    The answer is yes, it is always the best practice to close a file after you are done performing operations on it. However, Python runs a garbage collector to clean up the unused objects, but as good programmers, we must not rely on it to close the file. Python has a build-in close() function to close a file i.e;

    f.close()

    I hope you like this tutorial. Here, we have discussed different file handling in Python with examples that will help you while working on real-world projects.

CODE:

NOTE - I have comment out many things in this code, you have remove the comments(#) to see output of all the codes. Any problem, ask me in the comment below.
#Open(), Read() & Readline() For Reading File

#first we are going to make .txt file and write 2-3 lines in it;
open("Saswata.txt") #To open file in python
# it will not print anything unless you  make an file pointer,
f = open("Saswata.txt", "rt") #rb read the file in binary;
# but the default mode is txt mode 'rt'.
#content = f.read() #it is a function, it helps us to read a document

#content = f.read(3) #Only read 3 characters,

# if we put huge number like 100000 then it will just print all chars by loop
#print(content)

#To itreate
#for line in content:
    #print(line) #this will print chars line by line,

# to line; line by line we have to us f
#for line in f:
#   print(line, end="")

#Another line by line read method is read_by_line
# print(f.readline()) #one read_by_line use one line
# print(f.readline())
# print(f.readline())

#Another function is read_lines
# print(f.readlines())


f.close() #you also have to close a file

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