Exception handling in Python

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If we can catch an error then we can write code to handle it. An uncaught error will crash our program.

We can keep our code inside a block placed within try and then keep another code block under except to handle in case of error. If the code within try block generate some error , the code block within except will be executed. If there is no error then except part of the code will not be executed.

Example with try & except:

my_dict={1:'Alex',2:'Ronald'}try:    print(my_dict[3]) # 3rd element is not there except :    print (" some error occured ")

Output is here

some error occured 

All types of error

Capturing any type of error and displaying the system returned message

x=5y=0try:  print(x/y)except Exception as e:  print(e)   except:  print('Hi there is an error')

Output ( this message is because of the line print(e) )

division by zero

We can catch some specific error and display appropriate message.
If you are trying to catch any specific error then that code block should be above of generic exception handling block

my_dict={1:'Alex',2:'Ronald'}try:    print(my_dict[3])except (KeyError):    print (" This is a key error  ")except :    print (" some error occured ")

Output is here

 This is a key error

finally

The code block within finally code block is executed in all conditions ( error or no error )

my_dict={1:'Alex',2:'Ronald'}try:    print(my_dict[3])except (KeyError):    print (" This is a key error  ")except :    print (" some error occured ")finally :    print ( " I came out " )

Output is here

 This is a key error   I came out 

else:

If there is no error then code block within else will be executed.

my_dict={1:'Alex',2:'Ronald'}
try:    print(my_dict[2])
except (KeyError):    print (" This is a key error  ")except :    print (" some error occured ")else :    print ( " within else ")

Output is here

Ronald within else 

else with finally

my_dict={1:'Alex',2:'Ronald'}try:    print(my_dict[2])    except (KeyError):    print (" This is a key error  ")except :    print (" some error occured ")else :    print ( " within else ")finally:    print ( " I am within finally")

Output

Ronald within else  I am within finally

Examples of exception handling

We will separate integer part from a string.

my_str="Welcome to 45 town "my_list=my_str.split() # Create a list using split string methodfor i in my_list:      # iterating through list    try:        r=1/int(i)     # Generate error if i is not integer        print("The interger is :",i)    except:        print ("not integer : ",i) # Print only if integer 

Output is here

not integer :  Welcomenot integer :  toThe interger is : 45not integer :  town

User generated exception by using raise

In the code below there is nothing wrong with syntax but user can create its own exception by using raise

x=13if x >10:    raise Exception ('Value of x is greater than 10 ')else:    print('Value is fine ')

Output

Value of x is greater than 10

Specific error handling

from sqlalchemy.exc import SQLAlchemyErrordef my_update(p_id): # receives the p_id on button click to update    try:        data=(p_name.get(),unit.get(),price.get() )        id=my_conn.execute("UPDATE plus2_products SET p_name=%s			,unit=%s,price=%s WHERE p_id=%s",data)            #print(data)    except SQLAlchemyError as e:        error=str(e.__dict__['orig'])        msg_display=error        show_msg(msg_display,'error') # send error message     except tk.TclError as e:        msg_display=e.args[0]        show_msg(msg_display,'error') # send error message     else:        msg_display="Number of records updated: " + str(id.rowcount)

Full code is here: How try except is used in applications

Python error exception list with messages

There are some common errors which usually raised during execution. A built-in known exception list is available in Python. We can use them and handle specific solution based on the type of exception.

AssertionError	When a condition goes false while using assert
AttributeError	When Nonexistence method is called
EOFError	When any input function reches end of the file
FloatingPointError	When any floating point opration fails
GeneratorExit	When close method is of generator is called
ImportError	When import module is not found
IndexError	When element at index is not found
KeyError	When accessed Key is not present in a dictionary
KeyboardInterrupt	When process is interrupted by Keyboard ( Ctrl + C )
MemoryError	When process runs out of memory
NameError	When accessed variable is not found
NotImplementedError	When abstract methods raise this exception
OSError	When Operating system raised error
OverflowError	When result is too large to handle
ReferenceError	when a weak reference proxy, created by the weakref.proxy() function
RuntimeError	error that doesn’t fall in any of the other categories
StopIteration	When next() does not have any element to return
SyntaxError	When syntax error is encountered by the compiler
IndentationError	When there is an Indentation Error
TabError	When inconsistence Indentation exist
SystemError	Interpreter generates internal error
SystemExit	When sys.exit() raise the error
TypeError	When When object has wrong data type
UnboundLocalError	When a local variable is referenced before assignment
UnicodeError	When Unicode encoding decoding error occur
UnicodeEncodeError	When can't encode Unicode character
UnicodeDecodeError	When can't decode Unicode character
UnicodeTranslateError	When Unicode error occur during translating.
ValueError	When an argument of incorrect value to use
ZeroDivisionError	When denominator of a division or modulo operation is zero

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