Python - Nested try Block

Nested try Block in Python

In a Python program, if there is anothertry-exceptconstruct either inside either atryblock or inside itsexceptblock, it is known as a nested-try block. This is needed when different blocks like outer and inner may cause different errors. To handle them, we need nested try blocks.

We start with an example having a single “try − except − finally” construct. If the statements inside try encounter exception, it is handled by except block. With or without exception occurred, the finally block is always executed.

Example 1

Here, thetryblock has “division by 0” situation, hence theexceptblock comes into play. It is equipped to handle the generic exception with Exception class.

a=10
b=0
try:
   print (a/b)
except Exception:
   print ("General Exception")
finally:
   print ("inside outer finally block")

It will produce the followingoutput−

General Exception
inside outer finally block

Example 2

Let us now see how to nest thetryconstructs. We put another “try − except − finally” blocks inside the existing try block. The except keyword for inner try now handles generic Exception, while we ask the except block of outer try to handle ZeroDivisionError.

Since exception doesn’t occur in the innertryblock, its corresponding generic Except isn’t called. The division by 0 situation is handled by outer except clause.

a=10
b=0
try:
   print (a/b)
   try:
      print ("This is inner try block")
   except Exception:
      print ("General exception")
   finally:
      print ("inside inner finally block")
      
except ZeroDivisionError:
   print ("Division by 0")
finally:
   print ("inside outer finally block")

It will produce the followingoutput−

Division by 0
inside outer finally block

Example 3

Now we reverse the situation. Out of the nestedtryblocks, the outer one doesn’t have any exception raised, but the statement causing division by 0 is inside inner try, and hence the exception handled by innerexceptblock. Obviously, theexceptpart corresponding to outertry: will not be called upon.

a=10
b=0
try:
   print ("This is outer try block")
   try:
      print (a/b)
   except ZeroDivisionError:
      print ("Division by 0")
   finally:
      print ("inside inner finally block")
      
except Exception:
   print ("General Exception")
finally:
   print ("inside outer finally block")

It will produce the followingoutput−

This is outer try block
Division by 0
inside inner finally block
inside outer finally block

In the end, let us discuss another situation which may occur in case of nested blocks. While there isn’t any exception in the outertry:, there isn’t a suitable except block to handle the one inside the innertry: block.

Example 4

In the following example, the innertry: faces “division by 0”, but its corresponding except: is looking for KeyError instead of ZeroDivisionError. Hence, the exception object is passed on to the except: block of the subsequent except statement matching with outer try: statement. There, the zeroDivisionError exception is trapped and handled.

a=10
b=0
try:
   print ("This is outer try block")
   try:
      print (a/b)
   except KeyError:
      print ("Key Error")
   finally:
      print ("inside inner finally block")
      
except ZeroDivisionError:
   print ("Division by 0")
finally:
   print ("inside outer finally block")

It will produce the followingoutput−

This is outer try block
inside inner finally block
Division by 0
inside outer finally block