21.error handling

• try: This is the block of the code to be attempted(may lead to an error).
• except: Block of code will execute in case there is an error in try block.
• finally: A final block of code to be executed, regardless of an error.

Exception Handling

When an error occurs, python will normally stop and generate an error message.
These exceptions can be handled using try statement.

Without error handling

print(x)

Output:
NameError: name 'x' is not defined

With error handling

try:
  print(x)
except:
  print("An exception occurred")

Output:
An exception occured

Without the try block, the program will crash and raise an error.

With try block, since the try block raises an error, the except block will be executed.

Many Exceptions

We can define as many exception blocks as we want e.g. if we want to execute a special block of code for a special kind of error.

Example

try:
  print(x)
except NameError:
  print("Variable x is not defined")
except:
  print("Something else went wrong")

Output:
Variable x is not defined

Else

We can use else keyword to define a block of code to be executed if no errors were raised.

Using else block

try:
  print("Python")
except:
  print("Something went wrong")
else:
  print("Nothing went wrong")

Finally

The finally block, if specified, will be executed regardless if the try block raises an error or not.

Example

try:
  print(x)
except:
  print("Something went wrong")
finally:
  print("The 'try except' is finished")

Continue and Break

We can use continue and break statements, when we are using try..except blocks in a loop.

Example

while True:
  try:
    result = int(input('Please provide number: '))
  except:
    print('Whoops! That is not a number')
    continue
  else:
    print('Yes Thank You')
    break
  finally:
    print('End of try/except/finally')
    print('I will always run at the end!')

Output:
Please provide number: b
Whoops! That is not a number
End of try/except/finally
I will always run at the end!
Please provide number: 1
Yes Thank You
End of try/except/finally
I will always run at the end!

Raise an exception

As a python developer, we can choose to throw an exception if a condition occurs

To throw or raise an exception, we use raise keyword.

Example

x = -1

if x < 0:
  raise Exception("Sorry, no numbers below zero")

Output:
Exception: Sorry, no numbers below zero

The above program raises an error and stop the program if x is lower than 0.

We can define the kind of error to raise, and the text to print to the user.

Example

x = "Bhavya"

if not type(x) is int:
  raise TypeError("Only integers are allowed")

Output:
TypeError: Only integers are allowed