Errors and Exception Handling#

Python raises many different types of errors to inform you of what went wrong. The following are the most common errors that you should know.

SyntaxError – invalid Python syntax#

def func(()):
   pass

for index in some_list
    print(index)

NameError – using a variable before it has been defined#

def func():
   print(variable)

TypeError – mismatch data type#

'3' + 3

IndexError – accessing an index that does not exist#

my_list = [1, 2, 3]
my_list[4]

ValueError – calling a built-in function with invalid value type#

int('3')
int('3a')

KeyError – like by index error for lists#

student = {'username': 'john', 'grade': 50}
student['class']

AttributeError – missing attribute – variables or methods#

class TestClass:
    class_variable1 = 2

print(TestClass.class_variable1)
print(TestClass.class_variable2)

string = 'test1234'
string.upper()

x = [1,2,3]
x.upper()

More errors#

For more errors see https://docs.python.org/3/library/exceptions.html

Raising an error#

You can raise an error to indicate to the user that something is wrong.

Raising error – with a message or without message#

raise ValueError('a message')
raise ValueError

def calculate_bmi(height, weight):

    if type(height) not in [float, int]:
        raise TypeError('Height has to be float or an int')

    if type(weight) not in [float, int]:
        raise TypeError('Weight has to be float or an int')


    if height <= 0:
        raise ValueError('Height cannot be less than or equal to 0')

    if weight <= 0:
        raise ValueError('Weight cannot be less than or equal to 0')

    return 703 * weight / height**2

Exception Handling#

You can gracefully catch errors that are raised and handle them using the try/except block. Try and Except blocks – a way to handle errors When you raise an error, your program exits. What you rather want to do is to catch that error and give user feedback.

try:
    statements              # Run this main action first
except name1:
    statements              # Run if exception name1 is raised during try block
except (name2, name3):
    statements              # Run if any of these exceptions occur
except name4 as var:
    statements              # Run if exception name4 is raised, assign instance raised to var
except:
    statements              # Run for all other exceptions raised
else:
    statements              # Run if no exception was raised during try block
finally:
    statements              # Always run

Example1#

while True:
    try:
        height = float(input('Please enter height: '))
        weight = float(input('Please enter weight: '))
    except Exception as error:
        print(error)
        print('Please enter a number')
    else:
        print('I run when there is no error')
        break
    finally:
        print('I always run!')

print(calculate_bmi(height, weight))

Example2#

student = {'username': 'john', 'grade': 50}

def get_key(some_dict, key, default_value=None):
    try:
        return some_dict[key]
    except KeyError:
        return default_value


print(get_key(student, 'username1', default_value='Error'))
print(get_key(student, 'username1'))

Example3 – combining exceptions#

filename = 'abcd.txt'
try: 
    file = open(filename) 
except OSError: 
    print('OS error') 
except FileNotFoundError: 
    print('File not found') 

EAS503

Errors and Exception Handling

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