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.

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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")

Show Python »
C:\Users\My Name>python demo_try_except.py
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)

Show Python »
C:\Users\My Name>python demo_try_except_error.py      
Traceback (most recent call last):                    
  File "demo_try_except_error.py", line 3, in <module>
    print(x)                                          
NameError: name 'x' is not defined                    
                                                      
                                                      

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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")

Show Python »
C:\Users\My Name>python demo_try_except2.py
Variable x is not defined                  
                                           
                                           

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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")

Show Python »
C:\Users\My Name>python demo_try_except3.py
Hello                                      
Nothing went wrong                         
                                           
                                           

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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")

Show Python »
C:\Users\My Name>python demo_try_except4.py
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("Lorum Ipsum")
except:
  print("Something went wrong when writing to the file")
finally:
  f.close()

Show Python »
C:\Users\My Name>python demo_try_except5.py  
Something went wrong when writing to the file
                                             
                                             

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