PYTHON EXCEPTIONS

Python Try Except

The try block lets you test a block of code for errors.

The except block lets you handle the 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:

Example

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

 

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

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:

Example

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

print(x)

Traceback (most recent call last):
  File "demo_try_except_error.py", line 3, in <module>
    print(x)
NameError: name 'x' is not defined

 

Many Exceptions

You can define as many exception blocks as you want, e.g. if you want to execute a special block of code for a special kind of error:

 

Example

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")

 

Variable x is not defined

Else

You can use the else keyword to define a block of code to be executed if no errors were raised:

Example

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

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

Hello
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")

Something went wrong
The 'try except' is finished

This can be useful to close objects and clean up resources:

Example

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

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

Something went wrong when writing to the file

The program can continue, without leaving the file object open.

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.

Example

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")

Traceback (most recent call last):
  File "demo_ref_keyword_raise.py", line 4, in <module>
    raise Exception("Sorry, no numbers below zero")
Exception: Sorry, no numbers below zero

The raise keyword is used to raise an exception.

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

Example

Raise a TypeError if x is not an integer:

 

x = "hello"

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

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