Python Exception Handling¶
Exception handling in Python involves intercepting errors during the execution of a program and taking corrective or alternative actions rather than letting the program crash. This is done using try-except blocks, among other tools.
Basic Exception Handling¶
The basic structure for handling exceptions in Python uses try and except statements:
try:
# Code block where exceptions can occur
result = 10 / 0
except ZeroDivisionError:
# Code that runs if an exception occurs
print("You can't divide by zero!")
You can't divide by zero!
Handling Multiple Exceptions¶
A single try block can handle multiple exceptions, which can be useful for reacting to different error types appropriately:
try:
value = int(input("Enter a number: "))
print(10 / value)
except ValueError:
print("Please enter a valid integer.")
except ZeroDivisionError:
print("Zero is not a valid value.")
3.3333333333333335
Generic Exception Handling¶
If you're not sure what type of error might occur, or you want to catch all possible errors, you can use a generic exception handler. However, this is generally discouraged unless absolutely necessary, as it can make debugging difficult:
try:
# risky code
some_complex_logic = '10' + 10
except Exception as e:
print("An error occurred:", e)
An error occurred: can only concatenate str (not "int") to str
The else Clause¶
You can use an else clause in your try-except block. The code inside the else block runs if the try block did not raise an exception:
try:
print("Trying to divide...")
result = 10 / 2
except ZeroDivisionError:
print("Divided by zero!")
else:
print("Division successful:", result)
Trying to divide... Division successful: 5.0
The finally Clause¶
The finally clause will execute as the last task in the try-except block. It runs whether or not an exception is raised and is often used for clean-up actions:
try:
file = open('file.txt', 'r')
file_content = file.read()
except FileNotFoundError:
print("File not found.")
finally:
file.close()
print("File closed.")
File not found.
--------------------------------------------------------------------------- NameError Traceback (most recent call last) Cell In[6], line 7 5 print("File not found.") 6 finally: ----> 7 file.close() 8 print("File closed.") NameError: name 'file' is not defined
Raising Exceptions¶
You can also raise exceptions intentionally with the raise keyword. This is often done to enforce certain conditions within functions:
def check_age(age):
if age < 18:
raise ValueError("Access denied, you are too young.")
print("Access granted.")
try:
check_age(15)
except ValueError as e:
print(e)
Access denied, you are too young.
Exception Handling in Real Scenarios¶
Exception handling is particularly useful in data input/output operations, network operations, and handling unexpected behaviors in data-driven applications. Properly managing exceptions can prevent your applications from crashing and provide meaningful error messages to the user or logs.
Best Practices¶
- Catch specific exceptions: Whenever possible, catch more specific exceptions rather than a general Exception class.
- Minimize try block scope: Keep your try block as small as possible, enclosing only the code that might generate the exception.
- Use meaningful error messages: Provide clear, actionable error messages that can help fix the issue.