Python Programming

1. UNIT 5
Files and Exception
Handling

2. Introduction – Text Input and Output – File
Dialogs – Retrieving Data from the Web –
Exception Handling - Raising Exceptions –
Processing Exception using Exception Objects
– Defining Custom Exception Classes.

3. Files – Definition
• A collection of data or information write into
notepad and save those information called file.
• All the information stored in a computer must be
in a file.
• File is a named location on disk to store related
information.
• It is used to permanently store data in non-
volatile memory (e.g. hard disk), Since, random
access memory (RAM) is volatile which loses its
data when computer is turned off,
• we use files for future use of the data.
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4. There are different types of files, they
are listed below:
I. Data files
II. Text files
III. Program files
IV. Directory files, and so on.
Different types of files store different types
of information.
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5. 1. Sequential access
This type of file is to be accessed sequentially ,
Here we want to access the last record in a file you
must read all the records sequentially . It takes more
time for accessing the record.
2. Random access
This type of file allows access to the specific
data directly with out accessing its preceding data
items.
Here the data can be accessed and modified
randomly.
Types of file accessing

6. File Operations /Text input and
output
Whenever the user wants to read or write
into a file we need to open it first, after read
write process, then it needs to be closed,
then only you avoid data losses.
1. Open the file
2. Read or write
3. Close the file
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7. File open function
Python has a built-in function open() to open a file.
This function returns a file object, so it is most commonly
used with two arguments.
Syntax:
file_object = open(“filename”, [“access
mode”],[“buffering”])
Access mode – it determine the mode in which the file has to
be opened
Buffering – 0 , 1
if 0 no buffering
if 1 line buffering is performed while accessing a file
file_object - Once a file is opened and you have one file object
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8. Example
file object name = open(“file name.txt”,
“accessing Mode”)
Example:
f= open(“student.txt”, “w”)

9. With /as Statement
The user can also work with file objects using the with
statement. It is designed to provide much cleaner syntax
and exceptions handling when you are working with code.
Advantages :
• If any files opened will be closed automatically after you
are done.
To use the with statement to open a file:
Syntax:
with open(“filename” , “access mode”) as file:
Ex :
with open(“sam.txt” , “w”) as file:
file.write(“hai”)
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10. File Opening Methods
MODES DESCRIPTION
r Open the file for reading . The file pointer is
placed at beginning of the file .
w Open a file for writing only , overwrites the file
if the file already exist, If the file does not exist ,
create a new file for writing
rb Open the file for reading only in binary format .
The file pointer is placed at the beginning of the
file .
wb Open a file for writing only binary files ,
overwrites the file if the file already exist, If the
file does not exist , create a new file for writing
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11. r+ Open the file for reading and writing . The file
pointer is placed at beginning of the file .
w+ Open a file for writing and reading ,
overwrites the file if the file already exist, If
the file does not exist , create a new file for
writing and reading
a Opens the file for appending, The file pointer is
placed at the end of the line .
ab Opens the file for appending in binary format ,
The file pointer is placed at the end of the line .
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12. Reading and Writing Files
The file object is used to access a file for
reading and writing. There are two functions
to do this operation
read()
write()
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13. Write a text file
Write() method
• This method writes any string to an opened
text file .
• The write() method does not add a newline to
the character(‘n’) to the end of the string
Syntax:
File _object . write(“string”)
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14. Example
>>>f = open(“sample.txt”, “w+”) #open a file
>>>f . write(“Problem solving python
programming”) #write a file
>>>print(f.read()) # read a file
>>>f.close() #close a file
OUTPUT:
Problem solving python programming
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15. Read the text file
Read() method
• This method reads a string from the opened
text file.
• The read method starts reading from the
beginning of the file .
• The parameter is the number of bytes to be
read from the opened file.
• Syntax:
• fileobject.read(count)
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16. readline() function
whenever the user run the method, it will
return a string of characters that contains a
single line of information from the file
file = open(“kk.txt”, “r”)
print file.readline():
Output
Hello World
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17. Example
>>> f = open(“sample.txt”,”r+”)
>>> Str=f.read(10)
>>> Print(“the value is “, str)
>>> f.close()
OUTPUT:
Problem py
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18. CLOSE THE TEXT FILE
The close() method
The close method closes the file object ,
after the completion of read write process.
Python automatically closes a file when
the reference object of a file reassigned to
another file.
It is good practice to use the close()
method to close a file.
Syntax:
file _ object . close()
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19. Random access file /File
Manipulation
There are two major methods,
• Tell()
• Seek()
Tell():
Tells you the current position within the file.
file_object.tell()
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20. Seek() -this function read the string based on the offset and
from where into a text file
file_object.seek(offset , from where)
offset – how many bytes to move (eliminate)
from where – which position to move( 0 represent
beginning of the file)
Ex:
file.seek(10,0)
10- move first 10 character
0 – from beginning of file
file.seek(0,0)
0 – move 0 charater
0- read data from the beginning of file
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21. Example:
>>> f1=open("w.txt", "w")
>>> f1.write(“ragulraj")
>>> f1.write("ramkumar")
>>> f1.close()
>>> f1=open("w.txt", "r")
>>> f1.read()
‘ragulrajramkumar'
>>> f1.seek(7,0)
>>> f1.read()
'ramkumar‘
>>> f1.seek(0,0)
>>> f1.read()
‘ragulrajramkumar' ANGURAJU AP/CSE

22. File Dialog
Python Tkinter offer a set of dialogs that
you can use when working with files.
By using these you don’t have to design
standard dialogs your self.
Example dialogs include an open file
dialog, a save file dialog and many others.
Besides file dialogs there are other
standard dialogs,

23. File dialogs help you open, save files or
directories.
This is the type of dialog you get when you
click file,open.
This dialog comes out of the module,
there’s no need to write all the code manually.

24. Example:
import tkinter.filedialog
tkinter.filedialog.asksaveasfilename()
tkinter.filedialog.asksaveasfile()
tkinter.filedialog.askopenfilename()
tkinter.filedialog.askopenfile()
tkinter.filedialog.askdirectory()
tkinter.filedialog.askopenfilenames()
tkinter.filedialog.askopenfiles()

25. Retrieving Data from the web:
You can use the urlopen function to open a
Uniform Resource Locator (URL) and read data
from the Web.
Using Python, you can write simple code to
read data from a Web site.
All you need to do is to open a URL by using
the urlopen function, as follows:
infile =
urllib.request.urlopen("http://www.yahoo.com")
The urlopen function (defined in the
urllib.request module) opens a URL resource like
a file.

26. Example:
import urllib.request
infile =
urllib.request.urlopen("http://www.yahoo.com
/index.html")
print(infile.read().decode())
The data read from the URL using
infile.read() is raw data in bytes. Invoking the
decode() method converts the raw data to a
string

27. Errors and Exceptions
Errors:
Errors or mistakes in a program are often
referred to as bugs.
This is made by the programmer.
The process of finding and eliminating
errors is called debugging.
Errors can be classified into three major
groups:
a) Syntax errors
b) Runtime errors
c) Logical errors
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28. Syntax error:
Python will find these kinds of errors when
it tries to parse your program,
Syntax errors are mistakes in the use of
the Python language, and spelling or
grammar mistakes in a keyword .
Example:
• leaving out a symbol, such as a colon, comma
or brackets
• misspelling a keyword
• incorrect indentation ….. etc
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29. Runtime errors
If a program is syntactically correct that is,
free of syntax errors it will be run by the Python
interpreter.
However, the program may exit unexpectedly
during execution if it encounters a runtime error
which was not detected when the program
was parsed, but is only stop working when a
particular line is executed.
When a program comes to a halt because of a
runtime error, problem
Example:
• division by zero
• performing an operation on incompatible types
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30. Logical errors
Logical errors are the most difficult to fix.
They occur when the program runs without
crashing, but produces an incorrect result.
The error is caused by a mistake in the
program’s logic.
Example:
 using the wrong variable name
 indenting a block to the wrong level
 getting operator precedence wrong
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31. Handling Exception
An exception is an event, which occurs
during the execution of a program , that
disrupts the normal flow of the program’s
instructions
An exception is a Python object that
represents an error.
When a Python script raises an exception,
it must be handle immediately otherwise it
terminates and quits.
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32. Python provides two important features to
handle any unexpected error in the Python
programs.
There are two kinds of exception :
Predefined exception
predefined exception are build in
exception ,it is also called as python object
that handle the errors in the program.
User defined exception :
Assertions
Try .. . Except
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33. ANGURAJU AP/CSE

34. ANGURAJU AP/CSE

35. Example:
>>> 10 * (1/0)
Traceback (most recent call last):
File "<pyshell#0>", line 1, in <module>
10 * (1/0)
ZeroDivisionError: division by zero
>>> 4 * a+3
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
4 * a+3
NameError: name 'a' is not defined
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36. User defined exception
Assertions
Assertions are carried out by the assert
statement ,
An assertion is a sanity-check that you can
turn on or turn off when you are done with
your testing of the program.
Syntax
assert Expression , [ Arguments]
Example:
assert (no>=0) , [ “ negative number”]
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37. Example:
def celtofar(temp):
assert(temp>=0), "Celsius is negative"
return(((temp*1.8)+32))
print(celtofar(36))
print(int(celtofar(37.6)))
print(celtofar(-5))
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38. Handling Exception( By using try
except block)
Here using Try Except block to write programs
that handle selected exception.
The try except statement:
First the try execute the statement between
the try
and except keywords is executed.
If no exception accurse, the except portion is
skipped and exeution of the try statement is
finished.
If an exception accurse during execution of
the try portion, the entire statement in try block
are skipped.

39. EXAMPLE:
try:
x=int(input(“Enter the number”))
except ValueError:
print("That is not a valid number try
again")
Output:
Enter the number “ram”
That is not a valid number try again

40. Raising an Exception
An exception is thrown by executing the
raise statement.
This will designates the problem.
It can raise a exception using raise
command:
Sysntax:
raise ValueError(“x cannot be negative”)
This syntax raises a newly created instance
of the ValueError class
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41. >>> raise KeyboardInterrupt
Traceback (most recent call last):
KeyboardInterrupt
>>> raise MemoryError("This is an argument")
Traceback (most recent call last):
MemoryError: This is an argument

42. try:
a = int(input("Enter a positive integer: "))
if a <= 0:
raise ValueError("That is not a positive
number!")
except ValueError as ex:
print(ex)
Output:
Enter a positive integer: -2
That is not a positive number!

43. Processing Exceptions Using Exception
Object
In Python possible to access the exception
object in the except clause.
Here we assign the exception object
assigned to variable then it will be handled.

44. we can use the following syntax to assign the
exception object to a variable.
When the except clause catches the exception,
the exception object is assigned to a new variable
named as ex
Now you can use the object in the exception
handler.
Syntax:
try:
<body of try block>
except Exception Type as ex:
< Handling exception statement>

45. Example:
try:
n=eval(input("Enter the number"))
print("Entered number is",n)
except NameError as ex:
print("Exception:", ex)
Output:
Enter the number one
Exception: name 'one' is not defined

46. Defining Custom Exception
Creating Custom Exceptions
In Python, users can define custom
exceptions by creating a new class.
This exception class has to be derived,
either directly or indirectly, from the built-in
Exception class.
Most of the built-in exceptions are also
derived from this class.

48. Example:
>>> class CustomError(Exception):
pass
>>> raise CustomError
Traceback (most recent call last):
...
__main__.CustomError
>>> raise CustomError("An error occurred")
Traceback (most recent call last):
...
__main__.CustomError: An error occurred

49. In this to create a class named as
CustomError is derived from the base class
Exception
By using the new class, now raise the
exception using raise keyword.

50. Example:
class LogError(Exception):
pass
c=int(input("Enter the number"))
if(c>=0):
print("the number is", c)
else:
raise LogError("Enter only passitive number",)