Python - Nested try Block

Nested try Block in Python

In a Python program, if there is anothertry-except construct either inside either atry block or inside itsexcept block, 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, thetry block has "division by 0" situation, hence theexcept block comes into play. It is equipped to handle the generic exception with Exception class.

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

It will produce the followingoutput −

General Exceptioninside outer finally block

Example 2

Let us now see how to nest thetry constructs. 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 innertry block, its corresponding generic Except isn't called. The division by 0 situation is handled by outer except clause.

a=10b=0try:   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 0inside outer finally block

Example 3

Now we reverse the situation. Out of the nestedtry blocks, 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 innerexcept block. Obviously, theexcept part corresponding to outertry: will not be called upon.

a=10b=0try:   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 blockDivision by 0inside inner finally blockinside 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=10b=0try:   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 blockinside inner finally blockDivision by 0inside outer finally block