Exception Handling in Python — NCERT Solutions

A syntax error is detected when the rules of the programming language are not followed while writing the program. Such errors are also called parsing errors. The text says that syntax errors are also handled as exceptions, so every syntax error is an exception.

But many exceptions occur after the program has started executing, even when the program is syntactically correct. Examples are ZeroDivisionError when dividing by zero, NameError when a variable is not defined, and IOError when a file cannot be opened. These are exceptions, but not syntax errors. So, every exception cannot be a syntax error.

• ImportError is raised when the requested module definition is not found. Example: trying to import a module that does not exist.
• IOError is raised when the file specified in a program statement cannot be opened. Example: trying to open a file with a wrong name or from a wrong location.
• NameError is raised when a local or global variable name is not defined. Example: using a variable before assigning any value to it.
• ZeroDivisionError is raised when the denominator in a division operation is zero. Example: 10/0.

The raise statement is used to forcefully throw an exception in a program. Once an exception is raised, the normal flow of execution stops and control moves to the exception handler.

Example code to accept two numbers and display the quotient, raising an exception if the denominator is zero:

num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))

if num2 == 0:
    raise ZeroDivisionError("Denominator cannot be zero")
else:
    quotient = num1 / num2
    print("Quotient =", quotient)

If the second number is 0, the program raises ZeroDivisionError and does not continue with the division.

Using assert statement, we can test whether the denominator is valid before division. If the condition is false, an AssertionError is raised.

num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))

assert(num2 != 0), "Denominator should not be zero"
quotient = num1 / num2
print("Quotient =", quotient)

Here, assert(num2 != 0) checks the division expression indirectly by ensuring the denominator is not zero. If num2 is 0, the message "Denominator should not be zero" is displayed and the program stops.

• Exception Handling: the process of writing additional code to give proper messages or instructions to the user when an exception occurs, so that the program does not crash abruptly.
• Throwing an exception: creating an exception object and handing it over to the runtime system when an error occurs; this interrupts the normal flow of execution.
• Catching an exception: executing a suitable handler code for the raised exception, usually in the except block.