Python exception handling | Python theme month

Errors are errors that occur in a program, such as syntax errors.

It happens at compile time. Let’s look at an example.

if a<5
File "<interactive input>", line 1
    if a < 5
           ^
SyntaxError: invalid syntax
Copy the code

What is an exception?

Errors also occur at run time, and we call them exceptions. An exception is an event that occurs during program execution and interrupts the normal flow of program instructions.

Typically, a Python script throws an exception when it encounters an error condition that it cannot handle.

When a Python script throws an exception, it creates an exception object.

Typically, the script handles the exception immediately. If it does not, the program terminates and prints the trace to the error and its whereabouts.

>>> 1 / 0
Traceback (most recent call last):
 File "<string>", line 301.in run code
 File "<interactive input>", line 1.in <module>
ZeroDivisionError: division by zero
Copy the code

How to handle exceptions using try-except?

Here is the syntax for Python try-except-else blocks.

try: You do your operations here; .exceptExceptionI: Executes this block if there is ExceptionIexceptExceptionII: if there are ExceptionII, perform this piece...else: Executes this block if there is no exception.Copy the code

Take a look – 30 Python tutorials and tips

This is a list of valid use of Python try statements.

• Depending on requirements, a single try statement can have more than one except statement. In this case, the try block contains statements that can throw different types of exceptions.
• We can also add a generic except clause that handles all possible exception types.
• We can even include an else clause after the except clause. If the code in the try block does not throw an exception, the instructions in the else block are executed.

try: You do your operations here; .except: If there are any exceptions, this block...................... is executedelse: Executes this block if there is no exceptionCopy the code

Handle multiple exceptions with except

We can define more than one exception using the same except clause. This means that if the Python interpreter finds a matching exception, it will execute the code written under the except clause.

In short, when we define the except clause this way, we expect the same piece of code to throw different exceptions. In addition, we expect the same action to be taken in every case.

Please refer to the example below.

We can also enable Python exception handling with the try-finally statement.

Using the try block, we can optionally define a “finally” block. This clause allows us to define the statement we want to execute, regardless of whether the try block throws an exception.

This feature usually occurs when an external resource is released.

This is the code snippet of help.

try: You do your operations here; . This may be skipped due to any exceptionsfinally: This will always be done......................Copy the code

example

One key point is that we can define an “except” or “finally” clause for each try block. You can’t put these together. In addition, you should not use the “else” clause with the “finally” clause.

Let’s take an example to make it clearer.

try:
    fob = open('test'.'w')
    fob.write("Here's my test file to verify try-finally in exception handling.")
    print 'Try block execution'
finally:
    fob.close()
    print 'Finally block execution'
Copy the code

If no exception has occurred, you will see the following output.

>>tryPerform > >finallyperformCopy the code

Suppose we open a file in READ mode and then try to write to it. In this case, the following code will help you handle exceptions.

try:
    fob = open('test'.'r')
    try:
        fob.write("Here's my test file to verify try-finally in exception handling.")
        print 'Try block execution'
    finally:
        fob.close()
        print 'Finally block execution to close the file'
except IOError:
    print "Error: File not found or data read"
Copy the code

In this case, the interpreter throws an exception and displays the following output.

>>finallyBlock execution to close file >> Error: file not found or data readCopy the code

When some code raises an exception in the try block, execution is immediately passed to the “finally” block. After all statements in the finally block are executed, the exception is restored to the except block. But there must be a higher level next to a “try except” statement.

We can use the raise keyword to force an exception to be raised.

We can also choose to pass the value to the exception and specify why it happened.

>>> raise MemoryError
Traceback (most recent call last):
...
MemoryError
 
>>> raise MemoryError("This is an argument")
Traceback (most recent call last):
...
MemoryError: This is an argument
 
 
>>> try:
      a = int(input("Enter a positive integer value: "))
     if a <= 0:
            raise ValueError("This is not a positive number!!")
    except ValueError as ve:
      print(ve)
 
 
Following Output is displayed ifWe enter a negative number: Enter a positive integer: -5
 
This is not a positive number!!
Copy the code

Create custom exceptions in Python

>>> class UserDefinedError(Exception) :
.    pass
 
>>> raise UserDefinedError
Traceback (most recent call last):
...
__main__.UserDefinedError
 
>>> raise UserDefinedError("An error occurred")
Traceback (most recent call last):
...
__main__.UserDefinedError: An error occurred
Copy the code

In the code snippet above, you can see that we created a user-defined exception class, “UserDefinedError.” It uses the base Exception class as its parent. Therefore, the new user-defined exception class will raise an exception just like any other exception class, that is, by calling the “raise” statement with an optional error message.

Let’s take an example.

Summary – Python exception handling concepts

Most of the time, my goal is to find a topic that will help the reader in his or her work. That’s why we introduced this article to Exception handling in Python. If you enjoyed this article and are interested in seeing more of it, follow me here (Github/Gitee) for more information. Here is a summary of all my original and work source code

More on this at 🧵

• Python Multithreading tutorial | Python theme month
• Python Socket programming essentials | Python theme month
• 30 Python tutorials and tips | Python theme month
• Python statements, expressions, and indentation | Python theme month
• Python keywords, identifiers, and variables | Python theme month
• How to write comments and multi-line comments in Python | Python Theme Month
• Python data types — Basic to advanced learning | Python topic month

Recommended articles of the past:

• 20 Python Tips Everyone must Know | Python Theme month
• 100 Basic Python Interview Questions Part 1 (1-20) | Python topic month
• 100 Basic Python Interview Questions Part 2 (21-40) | Python topic month
• 100 Basic Python Interview Questions Part 3 (41-60) | Python topic month
• 100 Basic Python Interview Questions Part 4 (61-80) | Python topic month
• 100 Basic Python Interview Questions Part 5 (81-100) | Python topic month

If you do learn something new from this post, like it, bookmark it and share it with your friends. 🤗 Finally, don’t forget ❤ or 📑 for support