Python Errors

Python Errors#

There are (at least) two distinguishable kinds of errors:

syntax errors
exceptions

Syntax errors#

Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while you are still learning Python. It is an error in the syntax or the ‘grammar’ of a programming language.

Exceptions#

Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions and are not unconditionally fatal Most exceptions are not handled by programs, however, and result in error messages.

- Python documentation

Exception example#

def divtwoby(a):
    return 2/a

Now try divtwoby(10) and divtwoby(0). There is no syntax error in this function, so the first try should work and return the result 0.2, but the second should return an error message like ZeroDivisionError: division by zero.

Identifying errors#

The code bellow has an intentional error. Try to see whats is wrong and then, compile to see the python message.

# This code has an intentional error.
def favourite_ice_cream():
    ice_creams = [
        'chocolate',
        'vanilla',
        'strawberry'
    ]
    print(ice_creams[3])

favourite_ice_cream()

Output

    ---------------------------------------------------------------------------

    IndexError                                Traceback (most recent call last)

    <ipython-input-1-777c68a47889> in <module>
          8     print(ice_creams[3])
          9 
    ---> 10 favourite_ice_cream()
    

    <ipython-input-1-777c68a47889> in favourite_ice_cream()
          6         'strawberry'
          7     ]
    ----> 8     print(ice_creams[3])
          9 
         10 favourite_ice_cream()


    IndexError: list index out of range

Errors in Python have a very specific form, called a traceback.

Example: Syntax errors#

def some_function()
    msg = 'hello, world!'
    print(msg)
     return msg

Example: Tabs and spaces#

def some_function():
 msg = 'hello, world!'
 print(msg)
        return msg

Examples: Variable name errors#

Define variable#

print(a)

Initialise a counter#

for number in range(10):
    count = count + number
print('The count is:', count)

Misspelling#

Count = 0
for number in range(10):
    count = count + number
print('The count is:', count)

Example: Index errors#

letters = ['a', 'b', 'c']
print('Letter #1 is', letters[0])
print('Letter #2 is', letters[1])
print('Letter #3 is', letters[2])
print('Letter #4 is', letters[3])

Example: File errors#

Open missing files#

file_handle = open('myfile.txt', 'r')

Wrong file operations#

file_handle = open('myfile.txt', 'w') # Opened a file for writing (which will create an empty file)
file_handle.read() # Try to read from it

Activity: `print_message`#

Read the Python code and the resulting traceback below, and answer the following questions:

How many levels does the traceback have?
What is the function name where the error occurred?
On which line number in this function did the error occur?
What is the type of error?
What is the error message?

# This code has an intentional error. Do not type it directly;
# use it for reference to understand the error message below.
def print_message(day):
    messages = {
        'monday': 'Hello, world!',
        'tuesday': 'Today is Tuesday!',
        'wednesday': 'It is the middle of the week.',
        'thursday': 'Today is Donnerstag in German!',
        'friday': 'Last day of the week!',
        'saturday': 'Hooray for the weekend!',
        'sunday': 'Aw, the weekend is almost over.'
    }
    print(messages[day])

def print_friday_message():
    print_message('Friday')

print_friday_message()

Activity: Identifying syntax errors#

Read the code below, and (without running it) try to identify what the errors are.
Run the code, and read the error message.
Is it a SyntaxError or an IndentationError?
Fix the error.

Repeat steps 2, 3, and 4 until you have fixed all the errors.

def another_function
  print('Syntax errors are annoying.')
   print('But at least Python tells us about them!')
  print('So they are usually not too hard to fix.')

Activity: Identifying variable name errors#

Read the code below, and (without running it) try to identify what the errors are.
Run the code, and read the error message. What type of NameError do you think this is? In other words, is it a string with no quotes, a misspelled variable, or a variable that should have been defined but was not?
Fix the error.
Repeat steps 2 and 3, until you have fixed all the errors.

for number in range(10):
    # use a if the number is a multiple of 3, otherwise use b
    if (Number % 3) == 0:
        message = message + a
    else:
        message = message + 'b'
print(message)

Activity: Identifying index errors#

Read the code below, and (without running it) try to identify what the errors are.
Run the code, and read the error message. What type of error is it?
Fix the error.

seasons = ['Spring', 'Summer', 'Fall', 'Winter']
print('My favourite season is ', seasons[4])

Tip

You can find a Jupyter Notebook to run all functions presented above in this link.