• The try block lets you test a block of code for errors
• The except block lets you handle the error
• The else block lets you execute code when there is no error
• The finally block lets you execute code, regardless of the result of the try block and except block

When an error (exception) occurs, Python will normally stop and generate an error message.

However, these exceptions can be handled using the try statement.

try:
    print(x) #an error will occur because x is not defined
except:
    print("An exception occurred")

Output:

An exception occurred

The following will print one message if the try block raises a NameError, and another message for other errors.

try:
    print(x) #an error will occur because x is not defined
except NameError:
    print("x is not defined")
except:
    print("An exception occured")

Output:

x is not defined

The else keyword can be used to define a block of code to be executed if no errors are raised.

try:
    print("Hello World")
except:
    print("There was an exception")
else:
    print("There were no exceptions")

Output:

Hello World
There were no exceptions

The finally keyword, if specified, will execute a block regardless if the try block raises an error or not.

try:
    print(x)
except:
    print("There was an exception")
finally:
    print("The code has finished running")

Output:

There was an exception
The code has finished running

The finally block can be useful to close objects and clean up resources.

try:
    f = open("file.txt")
    try:
        f.write("Hello World")
    except:
        print("Something went wrong writing to the file")
    finally:
        f.close()
except:
      print("Something went wrong opening the file") 

Output:

Something went wrong writing to the file

As a Python developer, there may be times when you want to intentionally throw an exception. In these cases, the raise keyword is used.

x = -1

if x < 0:
    raise Exception("Sorry, numbers below zero are not allowed")

Output:

Traceback (most recent call last):
  File "./prog.py", line 4, in <module>
Exception: Sorry, numbers below zero are not allowed

The type of error can also be defined and printed.

x = "hello world"

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

Output:

Traceback (most recent call last):
  File "demo_ref_keyword_raise2.py", line 4, in <module>
    raise TypeError("Only integers are allowed")
TypeError: Only integers are allowed

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