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Python Exception Handling
Posted on Oct 31, 2019
Exception handling enables you handle errors gracefully and do something meaningful about it. Like display a message to user if intended file not found. Python handles exception using
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try: # write some code # that might throw exception except <ExceptionType>: # Exception handler, alert the user
As you can see in try block you need to write code that might throw an exception. When exception occurs code in the try block is skipped. If there exist a matching exception type in `except clause then it's handler is executed.
Let's take an example:
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try: f = open('somefile.txt', 'r') print(f.read()) f.close() except IOError: print('file not found')
The above code work as follows:
- First statement between
exceptblock are executed.
- If no exception occurs then code under
exceptclause will be skipped.
- If file don't exists then exception will be raised and the rest of the code in the
tryblock will be skipped
- When exceptions occurs, if the exception type matches exception name after
exceptkeyword, then the code in that
exceptclause is executed.
The above code is only capable of handling
IOError exception. To handle other kind of exception you need to add more
try statement can have more than once
except clause, It can also have optional
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try: <body> except <ExceptionType1>: <handler1> except <ExceptionTypeN>: <handlerN> except: <handlerExcept> else: <process_else> finally: <process_finally>
except clause is similar to
elif. When exception occurs, it is checked to match the exception type in
except clause. If match is found then handler for the matching case is executed. Also note that in last
ExceptionType is omitted. If exception does not match any exception type before the last
except clause, then the handler for last
except clause is executed.
Statements under the
else clause run only when no exception is raised.
Statements in the
finally clause will run every time no matter exception occurs or not.
Now let's take an example.
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try: num1, num2 = eval(input("Enter two numbers, separated by a comma : ")) result = num1 / num2 print("Result is", result) except ZeroDivisionError: print("Division by zero is error !!") except SyntaxError: print("Comma is missing. Enter numbers separated by comma like this 1, 2") except: print("Wrong input") else: print("No exceptions") finally: print("This will execute no matter what")
eval() function lets a python program run python code within itself,
eval() expects a string argument.
To learn more about the
eval() visit eval() in Python.
Raising exceptions #
To raise your exceptions from your own methods you need to use raise keyword like this
raise ExceptionClass("Your argument")
Let's take an example
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def enterage(age): if age < 0: raise ValueError("Only positive integers are allowed") if age % 2 == 0: print("age is even") else: print("age is odd") try: num = int(input("Enter your age: ")) enterage(num) except ValueError: print("Only positive integers are allowed") except: print("something is wrong")
Run the program and enter positive integer.
Enter your age: 12 age is even
Again run the program and enter a negative number.
Enter your age: -12 Only integers are allowed
Using Exception objects #
Now you know how to handle exception, in this section we will learn how to access exception object in exception handler code. You can use the following code to assign exception object to a variable.
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try: # this code is expected to throw exception except ExceptionType as ex: # code to handle exception
As you can see you can store exception object in variable
ex. Now you can use this object in exception handler code.
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try: number = eval(input("Enter a number: ")) print("The number entered is", number) except NameError as ex: print("Exception:", ex)
Run the program and enter a number.
Enter a number: 34 The number entered is 34
Again run the program and enter a string .
Enter a number: one Exception: name 'one' is not defined
Creating custom exception class #
You can create a custom exception class by extending
BaseException class or subclass of
As you can see from most of the exception classes in python extends from the
BaseException class. You can derive you own exception class from
BaseException class or from sublcass of
Create a new file called
NegativeAgeException.py and write the following code.
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class NegativeAgeException(RuntimeError): def __init__(self, age): super().__init__() self.age = age
Above code creates a new exception class named
NegativeAgeException, which consists of only constructor which call parent class constructor using
super().__init__() and sets the
Using custom exception class #
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def enterage(age): if age < 0: raise NegativeAgeException("Only positive integers are allowed") if age % 2 == 0: print("age is even") else: print("age is odd") try: num = int(input("Enter your age: ")) enterage(num) except NegativeAgeException: print("Only positive integers are allowed") except: print("something is wrong")
In the next post we will learn about Python Modules.
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