Python Errors and Exceptions

There are three main types of errors in Python programming:

• Syntax errors: errors where the code is not valid Python

• Runtime errors: errors where syntactically valid code fails to execute

• Semantic errors: errors in logic – the code executes but the result is not expected.

Python has an exception handling framework to deal with runtime errors.

Runtime errors typically cause an exception to occur

Examples of exceptions:

• NameError – results from referencing an undefined variable

• TypeError – results from undefined operations

• IndexError – results from accessing an element that does not exist.

The try ... except clause is used to handle runtime exceptions:

try:
    print("this gets executed first")
except:
    print("this gets executed on runtime error")

The except clause can specify which exception it handles

def safe_divide(a, b):
    try:
        return a / b
    except ZeroDivisionError:
        return 1E100

This will not handle other types of exceptions (which is typically what you want)

>>> safe_divide(1, '2')
TypeError

The raise statement is used to make an exception occur

def fibonacci(N):
    if N < 0:
        raise ValueError("N must be non-negative")
    L = []
    a, b, = 0, 1
    while len(L) < N:
        a, b = b, a + b
        L.append(a)
    return L

The error message that an exception contains can be referred to explicitly:

try:
    x = 1 / 0
except ZeroDivisionError as err:
    print("Error class is:  ", type(err)
    print("Error message is:", err)

The else and finally keywords can be used for more exception handling control

try:
    print("try something")
except:
    print("this happens only if it fails")
else:
    print("this happens only if it succeeds")
finally:
    print("this happens no matter what")