Python - Try Except

When Python interpreter executes a program and encounters an error or exception, it usually stops and throws error message. Handling such situation is termed as error handling or exception handling and Python has three keywords for this purpose:

• try: It is used to test a block of statements for error.
• except: It has a block of statements which executes when error occurs and handles error.
• finally: It has a block of statements which executes regardless of try-except blocks result.

try-except blocks

In the example below, x is not defined anywhere in the program, which raises an error. As the error occurred in try block of statement, it will be handled by its except block of statement. Without try block, the program will stop immediately after an error and throw error message.

Syntax

try:
  statements
except:
  statements

Example:

try:
  x = x + 1
  print(x)
except:
  print("An error occurred.")

The output of the above code will be:

An error occurred.

Many except blocks

A program can have multiple except blocks to handle each type of exception differently.

Syntax

try:
  statements
except errortype_1:
  statements
except errortype_2:
  statements
...
...
except:
  statements

Example:

try:
  x = 1
  y = 0
  x = x / y 
  print(x)
except ZeroDivisionError:
  print("Zero Division Error")
except (ValueError, TypeError):
  print("Value/Type Error")
except:
  print("An error occurred.")

The output of the above code will be:

Zero Division Error

except block with else

else block statement can also be added with try-except blocks which is executed when no error has occurred.

Syntax

try:
  statements
except:
  statements
else:
  statements

Example:

try:
  x = 10
  y = 2
  x = x / y 
  print(x)
except ZeroDivisionError:
  print("Zero Division Error")
except:
  print("An error occurred.")
else:
  print("No error has occurred.")

The output of the above code will be:

5.0
No error has occurred.

finally block

In the example below, finally block of statement is executed regardless of try-except block results, which facilitates the program to close the file before proceeding further.

Syntax

try:
  statements
except:
  statements
else:
  statements
finally:
  statements

Example:

MyFile = None
try:
  MyFile = open("math_example.txt")
  x = MyFile.readline()
  y = MyFile.readline()
  x = x/y
  print(x)
except:
  print("An error occurred.")
finally:
  if MyFile == None:
    print("MyFile does not exist.")
  else:
    print("MyFile has been closed.")
    MyFile.close()

The output of the above code will be:

An error occurred.
MyFile does not exist.

Exception Name & Description