Exception Handling in Python

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: Raised when the requested module definition is not found. For example, importing a module that does not exist.
- IOError: Raised when the file specified in a program statement cannot be opened. For example, trying to open a file that is not present.
- NameError: Raised when a local or global variable name is not defined. For example, using a variable before assigning a value to it.
- ZeroDivisionError: Raised when the denominator in a division operation is zero. For 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:

```python
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.

```python
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.

a) 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.

b) Throwing an exception: Creating an exception object and handing it over to the runtime system when an error occurs; this causes control to jump to an exception handler.

c) Catching an exception: Executing a code block designed to handle a particular exception when that exception is raised. In Python, exceptions are caught in the try block and handled in the except block.