Try,Except,Else,Finally Blocks

• In python, it’s strongly encouraged to use try/except blocks to catch exceptions when we can do something about them.

try and except

• most commonly used

try:
	foobar
except NameError as err:
	print(err) # name 'foobar' is not defined
 
def get(d,key):
    try:
        return d[key]
    except KeyError:
        return None
 
""" When we use an error that is not the error """
try:
	foobar
except ValueError as err:
	print(err)
print("after the try") # Doesn't get printed
#Traceback (most recent call last):  
# File "/home/lkim/Desktop/python_practice/error_handling.py"  
#, line 2, in <module>  
#   foobar  
#NameError: name 'foobar' is not defined
 
""" NOT RECOMMENDED """
try:
	foobar
except:
	print("ERROR")
# we're catching EVERY error, so we're not correctly identifying "what" went wrong
# catch all the errors. if there's more errors, add more except cases

else and finally

• else
  • runs only if no exceptions are raised
  • if you return something in try, then else is never reached
• finally will run no matter what!

try: 
	num = int(input("Enter a number: "))
except:
    print("Not a number!")
else:
    print("in else!")
finally:
    print("in finally!")
 
# Enter a number: 4  
# in the else!  
# in the finally!
 
# Enter a number: d  
# Not a number!  
# in the finally!

A common structure: while loop

# guessing game
while True:
	try:
		num = int(input("Number: "))
	except:
		print("Not a number!")
	else:
		print("Good job!")
		break
	finally:
		# this gets printed even after break in else
		print("PRINTED NO MATTER WHAT!")