Exception handling in python

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5. Python Exceptions Handling
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6. What is Exception?
●
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An exception is an event, which occurs during the execution of a program, that
disrupts the normal flow of the program's instructions
When a method encounters an abnormal condition (an exception condition) that it
can't handle itself, it may throw an exception.
Exception classes
process
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In Java, exceptions are objects
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When you throw an exception, you throw an object
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Throwable serves as the base class for an entire family of classes, declared
in java
Throwable has two direct subclasses
-Exception
-Error

7. ●
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Exceptions (members of the Exception family) are thrown to signal
abnormal conditions that can often be handled by some catcher
Though it's possible they may not be caught and therefore could
result in a dead thread.
Errors (members of the Error family) are usually thrown for more
serious problems, such as OutOfMemoryError
In general, code you write should throw only exceptions, not errors.
Errors are usually thrown by the methods of the Java API, or by the
Java virtual machine itself

8. Catching Exceptions:
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A method catches an exception using a combination othe try and catch keywords
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A try/catch block is placed around the code that might generate an exception.
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Code within a try/catch block is referred to as protected code
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Syntax:
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try
{
//Protected code
}
catch(ExceptionName e1)
{
//Catch block
}

9. // File Name : ExcepTest.java
import java.io.*;
public class ExcepTest{
public static void main(String args[]){
try{
int a[] = new int[2];
System.out.println("Access element three :" + a[3]);
}
catch(ArrayIndexOutOfBoundsException e){
System.out.println("Exception thrown :" + e);
}
System.out.println("Out of the block");
}
}
Output:
Exception thrown
Out of the block
:java.lang.ArrayIndexOutOfBoundsException: 3

10. Errors and Exceptions:
Syntax Errors
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It is also known as parsing errors, are perhaps the most common kind of
complaint you get while you are still learning Python
>>> while True print 'Hello world'
File "<stdin>", line 1, in ?
while True print 'Hello world'
^
SyntaxError: invalid syntax
●
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The parser repeats the offending line and displays a little ‘arrow’ pointing
at the earliest point in the line where the error was detected
The error is caused by (or at least detected at) the token preceding the
arrow: in the example, the error is detected at the keyword print, since a
colon (':') is missing before it
File name and line number are printed so you know where to look in
case the input came from a script.

11. Exceptions:
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Exceptions should be class objects
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The exceptions are defined in the module exceptions
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For class exceptions, in a try statement with an except clause that
mentions a particular class
Even if a statement or expression is syntactically correct, it may cause
an error when an attempt is made to execute it
Errors detected during execution are called exceptions and are not
unconditionally fatal

12. ●
>>> 10 * (1/0)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ZeroDivisionError: integer division or modulo by zero
>>> 4 + spam*3
Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: cannot concatenate 'str' and 'int' objects

13. Handling Exceptions:
.
>>> while True:
...
try:
...
x = int(raw_input("Please enter a number: "))
...
break
...
...
except ValueError:
print "Oops! That was no valid number. Try again..."
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The try statement works as follows:
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First, the try clause (the statement(s) between the try and except keywords) is executed.
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If no exception occurs, the except clause is skipped and execution of the try statement is
finished.
If an exception occurs during execution of the try clause, the rest of the clause is skipped.
Then if its type matches the exception named after the except keyword, the except clause
is executed, and then execution continues after the try statement.
If an exception occurs which does not match the exception named in the except clause, it
is passed on to outer try statements; if no handler is found, it is an unhandled exception
and execution stops with a message as shown above.

14. Raising Exceptions:
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The raise statement allows the programmer to force a specified exception
to occur
>>> raise NameError('HiThere')
Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: HiThere
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If you need to determine whether an exception was raised but don’t
intend to handle it, a simpler form of the raise statement allows you to reraise the exception:
>>> try:
...
raise NameError('HiThere')
... except NameError:
...
print 'An exception flew by!'
...
raise
...
An exception flew by!
Traceback (most recent call last):
File "<stdin>", line 2, in ?
NameError: HiThere

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