When an exception occurs in Python, it causes the current process to stop and passes it to the calling process until it is handled. If not handled, our program will crash. For example, if function
A calls function
B which in turn calls function
C and an exception occurs in function
C. If it is not handled in
C, the exception passes to
B and then to
A. If never handled, an error message is spit out and our program come to a sudden, unexpected halt.
Catching Exceptions in Python
In Python, exceptions can be handled using a
try statement. A critical operation which can raise exception is placed inside the
try clause and the code that handles exception is written in
except clause. It is up to us, what operations we perform once we have caught the exception. Here is a simple example.
# import module sys to get the type of exception import sys while True: try: x = int(input("Enter an integer: ")) r = 1/x break except: print("Oops!",sys.exc_info(),"occured.") print("Please try again.") print() print("The reciprocal of",x,"is",r)
Here is a sample run of this program.
Enter an integer: a Oops! <class 'ValueError'> occured. Please try again. Enter an integer: 1.3 Oops! <class 'ValueError'> occured. Please try again. Enter an integer: 0 Oops! <class 'ZeroDivisionError'> occured. Please try again. Enter an integer: 2 The reciprocal of 2 is 0.5
In this program, we loop until the user enters an integer that has a valid reciprocal. The portion that can cause exception is placed inside
try block. If no exception occurs,
except block is skipped and normal flow continues. But if any exception occurs, it is caught by the
except block. Here, we print the name of the exception using
ex_info() function inside
sys module and ask the user to try again. We can see that the values 'a' and '1.3' causes
ValueError and '0' causes
Catching Specific Exceptions in Python
In the above example, we did not mention any exception in the
except clause. This is not a good programming practice as it will catch all exceptions and handle every case in the same way. We can specify which exceptions an
except clause will catch. A
try clause can have any number of
except clause to handle them differently but only one will be executed in case an exception occurs. We can use a tuple of values to specify multiple exceptions in an
except clause. Here is an example pseudo code.
try: # do something pass except ValueError: # handle ValueError exception pass except (TypeError, ZeroDivisionError): # handle multiple exceptions # TypeError and ZeroDivisionError pass except: # handle all other exceptions pass
In Python programming, exceptions are raised when corresponding errors occur at run time, but we can forcefully raise it using the keyword
raise. We can also optionally pass in value to the exception to clarify why that exception was raised.
>>> raise KeyboardInterrupt Traceback (most recent call last): ... KeyboardInterrupt >>> raise MemoryError("This is an argument") Traceback (most recent call last): ... MemoryError: This is an argument >>> try: ... a = int(input("Enter a positive integer: ")) ... if a <= 0: ... raise ValueError("That is not a positive number!") ... except ValueError as ve: ... print(ve) ... Enter a positive integer: -2 That is not a positive number!
try statement in Python can have an optional
finally clause. This clause is executed no matter what, and is generally used to release external resources. For example, we may be connected to a remote data center through the network or working with a file or working with a Graphical User Interface (GUI). In all these circumstances, we must clean up the resource once used, whether it was successful or not. These actions (closing a file, GUI or disconnecting from network) are performed in the
finally clause to guarantee execution.
Here is an example to illustrate this.
try: f = open("test.txt",encoding = 'utf-8') # perform file operations finally: f.close()
This type of construct makes sure the file is closed even if an exception occurs.