Question

( Python Program) explain how Input/Output data files work in a Python program, and how you would use a loop to process input and/or output. Define the differences in a field, record and file, and define what an exception is.

Answer 1

Reading from a file

There are three ways to read from a file.

• read([n])
• readline([n])
• readlines()

Note: that n is the number of bytes to be read.

Create a file as below:

1st line
2nd line
3rd line
4th line
5th line

Let's see what each read method does:

my_file=open("D:\\new_dir\\multiplelines.txt","r")
my_file.read()

The read() method just outputs the entire file if number of bytes are not given in the argument. If you execute my_file.read(3), you will get back the first three characters of the file

my_file=open("D:\\new_dir\\multiplelines.txt","r")
my_file.read(3)

readline(n) outputs at most n bytes of a single line of a file. It does not read more than one line.

my_file.close()
my_file=open("D:\\new_dir\\multiplelines.txt","r")
#Use print to print the line else will remain in buffer and replaced by next statement
print(my_file.readline())
# outputs first two characters of next line
print(my_file.readline(2))

Use the close() method with file handle to close the file. When you use this method, you clear all buffer and close the file.

my_file.close()

You can use a for loop to read the file line by line:

my_file=open("D:\\new_dir\\multiplelines.txt","r")
#Use print to print the line else will remain in buffer and replaced by next statement
for line in my_file:
    print(line)
my_file.close()

The readlines() method maintains a list of each line in the file:

my_file=open("D:\\new_dir\\multiplelines.txt","r")
my_file.readlines()

Writing to a file

You can use three methods to write to a file in Python:

• write(string) (for text) or write(byte_string) (for binary)
• writelines(list)

Let's create a new file within a folder in the "D" drive. Following will create a new file in the specified folder because it does not exist. Remember to give correct path with correct filename otherwise you will get error:

Create a notepad file and write some text in it. Make sure to save file as .txt and save it to the working directory of Python.

my_file_handle=open("mynewtextfile.txt")
my_file_handle.read()

new_file=open("D:\\new_dir\\newfile.txt",mode="w",encoding="utf-8")

new_file.write("Writing to a new file\n")
new_file.write("Writing to a new file\n")
new_file.write("Writing to a new file\n")
new_file.close()

Now let's write a list to this file with a+ mode:

fruits=["Orange\n","Banana\n","Apple\n"]
new_file=open("D:\\new_dir\\newfile.txt",mode="a+",encoding="utf-8")
new_file.writelines(fruits)
for line in new_file:
    print(line)
new_file.close()

Note that reading from a file does not print anything because the file cursor is at the end of the file. To set the cursor at the beginning, you can use the seek() method of file object:

cars=["Audi\n","Bentely\n","Toyota\n"]
new_file=open("D:\\new_dir\\newfile.txt",mode="a+",encoding="utf-8")
for car in cars:
    new_file.write(car)
print("Tell the byte at which the file cursor is:",new_file.tell())
new_file.seek(0)
for line in new_file:
    print(line)

The tell() method of file object tells at which byte the file cursor is located. In seek(offset,reference_point) the reference points are 0 (the beginning of the file and is default), 1 (the current position of file) and 2 (the end of the file).

Let's try out passing another reference point and offset and see the output:

new_file.seek(4,0)
print(new_file.readline())
new_file.close()

Note the use of .seek() and .truncate(): the argument in .truncate() is 5 that says that truncate the file till 5 bytes of text are left. And output shows exactly 5 bytes of text left including space. You are only left with next() method so let's complete this section of the tutorial! Here you are using same file created above with name multiplelines.txt.

End-relative seeks such as seek(-2,2) are not allowed if file mode does not include 'b', which indicates binary format. Only forward operations such seek(0,2) are allowed when file object is dealt as text file.

file=open("D:\\new_dir\\multiplelines.txt","r")
for index in range(5):
    line=next(file)
    print(line)
file.close()

You can think of a traditional database as an electronic filing system, organized by fields, records, and files. A field is a single piece of information; a record is one complete set of fields; and a file is a collection of records. For example, a telephone book is analogous to a file. It contains a list of records, each of which consists of three fields: name, address, and telephone number.

Python has many built-in exceptions which forces your program to output an error when something in it goes wrong.

When these exceptions occur, it causes the current process to stop and passes it to the calling process until it is handled. If not handled, our program will crash.

For example, if function A calls function B which in turn calls function C and an exception occurs in function C. If it is not handled in C, the exception passes to B and then to A.

If never handled, an error message is spit out and our program come to a sudden, unexpected halt.