5.1 Types of Exceptions/Errors

In Python there are three main types of exceptions/errors that a programmer has to deal with. There are different ways to handle each one according to their types, but first let's go through them.

1. Compile Time Errors

• >These exceptions are raised due to the syntactical mistakes in code and are usually easier to spot and fix. They are raised when the Python Interpreter is compiling a program. A user at this point has to fix the error to be able to execute the program.
• >The interpreter raises a SyntaxError/IndentationError and also indicates the line causing the error when found. SyntaxError is raised due to a syntactical error in code such as missing colon in compound statements, invalid condition checking in if...else statements, missing string quote or bracket operator's termination, empty import statement, missing/misspelling keywords, empty function/class definition etc. IndentationError is raised when using a invalid indentation in compound statements.

## SyntaxError
## Example 1: Missing closing brackets in list and
# missing inverted comma in a string
data = [232,54,65  # SyntaxError
data = "this string is not complete  # SyntaxError


## Example 2: Missing colon in function
def myfun()  # SyntaxError
  print("my function")


## IndentationError
## Example 1: Wrong indentation in condition
a = 30
if a == 30:
  print("Execute this")  # Execute this
   print("This will raise a Indentation error")  # IndentationError

2. Runtime Error

• >Are raised at runtime due to some illegal invocation/operation on objects. If a program is syntactically correct, the interpreter starts to execute the program and if an exception is raised while executing it is a runtime error.
• >The program executes till the part of the runtime error line and the rest of the execution is stopped. Along with the Exception type, the interpreter also prints appropriate messages on the screen. A user at this point can fix the error or can bypass and continue the rest of the execution by using Exception Handling.

Some common runtime errors in Python.

• >For more exceptions check the exception hierarchy on python doc.
• >Python also has a warnings module which is a subclass of Exception class and unlike all other exceptions they don't terminate the program. They are only meant to warn the user by showing some message.

## Example 1: IndexError: Indexing error
a = [34, 56, 32, 87]
print(a[6])  # IndexError


## Example 2: ZeroDivisionError: Dividing by zero
print(34 / 0)  # ZeroDivisionError


## Example 3: AttributeError: Accessing unknown attribute
import math
math.square  # AttributeError


## Example 4: Raise a warning
import warnings
warnings.warn("Something is not right.")  # UserWarning: Something \
                                        # is not right.
print("This can execute")  # This can execute

3. Logical Error

• >Are not raised, but the program output is not an expected behaviour. They usually get difficult to fix as the program grows. They occur when the program's logic is incorrect. Common examples such as using the wrong variable/operator, calling the wrong function/method instead, subclassing a wrong class etc.
• >To avoid these errors it is recommended to debug a program normally or use unit testing framework unittest to test the program before integrating it into an application.

## Example 1: Accessing wrong index
my_list = [82, 92, 38, 42, 54, 23, 64, 87]
# printing last 3 values
print(my_list[-2:])  # [64, 87]
# here our program has no error, but instead of printing 3 numbers
# it's only printing 2 numbers, because we provided wrong index
# this example is not hard to fix, but in larger programs it might consume some
# time to find out which object/variable/function has caused the wrong output