Python Try Except

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- and except blocks.

Exception Handling

When an error occurs, or exception as we call it, Python will normally stop and generate an error message.

These exceptions can be handled using the try statement:

The try block will generate an exception, because x is not defined:

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

Since the try block raises an error, the except block will be executed. Without the try block, the program will crash and raise an error.

This statement will raise an error, because x is not defined:

print(x)

Many Exceptions

Print one message if the try block raises a NameError and another for other errors:

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

Else

In this example, the try block does not generate any error:

try:
  print("Hello")
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.

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

Try to open and write to a file that is not writable:

try:
  f = open("demofile.txt")
  try:
    f.write("Lorum Ipsum")
  except:
    print("Something went wrong when writing to the file")
  finally:
    f.close()
except:
  print("Something went wrong when opening the file")

Raise an exception

As a Python developer you can choose to throw an exception if a condition occurs.

To throw (or raise) an exception, use the raise keyword.

Raise an error and stop the program if x is lower than 0:

x = -1

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

Raise a TypeError if x is not an integer:

x = "hello"

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