Python Try Except

Exceptions in Python applications can happen for many of reasons; and if they aren’t handled well, these exceptions can cause the program to crash or causing data loss.

• With the try function you can test a block of code for errors.

• The except block allow you handle or manipulate the error.

• The finally block lets you execute code, regardless of the result of the try- and except blocks.

Try

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 e is not defined,

Example:

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

# Output:
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:

print(e)

# Output:
NameError: name 'e' is not defined

Some Exceptions

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

For example you can print one message if the try block raises a NameError and another for other errors:

Example:

try:
  print(e)
except NameError:
  print('You need to define the variable "e" ')
except:
  print("Something else went wrong") 

# Output:
You need to define the variable "e"

Else

The else clause is executed if and only if no exception is raised. This is different from the finally clause that’s always executed.

Example:

try:
    e = "Hello"
except:
    print('"e" is not definite')
else:
    print('No exception occurred')

# Output:
No exception occurred

Else with many exceptions

You can catch many types of exceptions in this way, but remember that the else clause is executed only if no exception happens.

Example:

try:
    e = "Hello"
except NameError:
    print('"e" was not defined')
except ValueError:
    print(' A value error ocurred')
except TypeError:
    print( 'A TypeError ocurred')
else:
    print('No exception occurred')
finally:
    print("It's all")

Output:

No exception occurred
It's all

Finally

A try-except block can have the finally clause (optionally). The finally clause is always executed. So the general idea is this.

Example:

try:
    e = "Hello"
except:
    print('"e" is not definite')
else:
    print('No exception occurred')
finally:
    print("It's all")

Output:

No exception occurred
It's all

Raise Exception

-Exceptions are raised when an error occurs. But in Python you can also force an exception to occur with the keyword raise. -You can define what kind of error to raise, and the text to print to the user.

Any type of exception can be raised.

Example:

number = "1.0"
if type(number) is not int:
  raise TypeError("Only integers are allowed")

# Output:
TypeError: Only integers are allowed

Example - Raise an error and stop the program if e is lower than 50:

e = 26
if e < 50:
  raise Exception("Sorry, you cannot continue!")

# Output:
Exception: Sorry, you cannot continue!