Python Exception Handling Using try, except and finally statement
In this tutorial, you’ll learn how to handle exception in your Python program using try, except and finally statement with the help of an example.
Exceptions in Python
Python has many built-in exceptions that are raised when your program experiences a blunder (something in the program turns out badly).
When these exemptions happen, the Python translator stops the current procedure and passes it to the calling procedure until it is handled. If not taken care of, the program will crash.
For instance, let us consider a program where we have a function A that calls work B, which thusly calls work C. In the event that a special case happens in work C yet isn’t taken care of in C, the exemption goes to B and afterward to A.
If never handled, an error message is displayed and our program comes to a sudden unexpected halt.
Catching Exceptions in Python
In Python, exceptions can be handled using a try statement.
The critical operation which can raise an exception is placed inside the try clause. The code that handles the exceptions is written in the except clause.
We can thus choose what operations to perform once we have caught the exception. Here is a simple example.
# import module sys to get the type of exception import sys randomList = ['a', 0, 2] for entry in randomList: try: print("The entry is", entry) r = 1/int(entry) break except: print("Oops!", sys.exc_info(), "occurred.") print("Next entry.") print() print("The reciprocal of", entry, "is", r)
The entry is a Oops! <class 'ValueError'> occurred. Next entry. The entry is 0 Oops! <class 'ZeroDivisionError'> occured. Next entry. The entry is 2 The reciprocal of 2 is 0.5
In this program, we circle through the estimations of the randomList list. As recently referenced, the part that can cause an exception is set inside the try block.
If no exemption happens, the except block is skipped and a typical stream continues(for last worth). Be that as it may, if any special case happens, it is gotten by the aside from square (first and second values).
Here, we print the name of the exception using the exc_info() function inside the sys module. We can see that a causes ValueError and 0 causes ZeroDivisionError.
Since every exception in Python inherits from the base Exception class, we can also perform the above task in the following way:
# import module sys to get the type of exception import sys randomList = ['a', 0, 2] for entry in randomList: try: print("The entry is", entry) r = 1/int(entry) break except Exception as e: print("Oops!", e.__class__, "occurred.") print("Next entry.") print() print("The reciprocal of", entry, "is", r)
This program has the same output as the above program.
Catching Specific Exceptions in Python
In the above example, we didn’t make reference to a, particularly exception in the except clause.
This is anything but a decent programming practice as it will get all exemptions and handle each case similarly. We can determine which exceptions an except clause should catch.
A try clause can have any number of except clauses to handle different exceptions, however, just one will be executed on the off chance that an exemption happens.
We can use a tuple of qualities to indicate numerous special cases in an aside from condition. Here is an example of a 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
Raising Exceptions in Python
In Python programming, exceptions are raised when blunders happen at runtime. We can likewise physically raise exemptions utilizing the raise keyword.
We can alternatively pass esteems to the special case to explain why that exemption 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!
Python try with else clause
In some situations, you might want to run a certain block of code if the code block inside try ran without any errors. For these cases, you can use the optional else keyword with the try statement.
Note: Exceptions in the else clause are not handled by the preceding except clauses.
Let’s look at an example:
# program to print the reciprocal of even numbers try: num = int(input("Enter a number: ")) assert num % 2 == 0 except: print("Not an even number!") else: reciprocal = 1/num print(reciprocal)
If we pass an odd number:
Enter a number: 1 Not an even number!
If we pass an even number, the reciprocal is computed and displayed.
Enter a number: 4 0.25
However, if we pass 0, we get ZeroDivisionError as the code block inside else is not handled by preceding except.
Enter a number: 0 Traceback (most recent call last): File "<string>", line 7, in <module> reciprocal = 1/num ZeroDivisionError: division by zero
The 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 a Graphical User Interface (GUI).
In every one of these conditions, we should tidy up the asset before the program stops whether it effectively ran or not. These activities (closing a file, GUI, or disconnecting from network) are performed in the finally clause to guarantee the execution.
Here is an example of file operations to illustrate this.
try: f = open("test.txt",encoding = 'utf-8') # perform file operations finally: f.close()
This sort of build ensures that the file is closed even if an exemption happens during the program execution.
Please feel free to give your comment if you face any difficulty here.
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