Learn Errors and Exceptions: Handling Problems During Execution

When you write Python code, not all problems are detected at the same point in the program’s life cycle. Understanding the difference between syntax errors and exceptions is critical for writing robust programs. A syntax error occurs when your code does not follow the rules of Python’s grammar. These errors are found during the parsing phase, before the interpreter even starts to execute your code. For example, missing a colon after a function definition or using an invalid variable name will cause a syntax error. The interpreter stops immediately and reports the problem.

On the other hand, an exception is an error that occurs during the execution phase—after your code has been successfully parsed and compiled to bytecode. Exceptions are raised when the interpreter encounters an unexpected situation, such as dividing by zero or trying to access a file that does not exist. Unlike syntax errors, exceptions can be handled by your code, allowing your program to recover or respond gracefully.


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# Example: Handling an exception with try-except

try:
    result = 10 / 0
except ZeroDivisionError as e:
    print("Caught an exception:", e)
else:
    print("No exception occurred")
finally:
    print("Execution continues after try-except block")

When an exception is raised during execution, Python begins a process called exception propagation. The interpreter looks for a matching except block in the current function. If none is found, it unwinds the call stack—moving up to the function that called the current one, and so on—searching for a handler. This process continues until a handler is found or until there are no more frames left on the stack. If no handler is found, the interpreter prints a traceback and terminates the program.

This mechanism allows you to handle errors at the appropriate level in your program. For example, you might want a low-level function to raise an exception and handle it in a higher-level function that has more context about what should happen next.


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# Example: Exception propagation through nested calls

def inner():
    # This will raise a ValueError
    int("not a number")

def middle():
    inner()

def outer():
    try:
        middle()
    except ValueError as e:
        print("Exception caught in outer:", e)

outer()

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Section 1. Chapter 4

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              12345678910
            
# Example: Handling an exception with try-except

try:
    result = 10 / 0
except ZeroDivisionError as e:
    print("Caught an exception:", e)
else:
    print("No exception occurred")
finally:
    print("Execution continues after try-except block")


              12345678910111213141516
            
# Example: Exception propagation through nested calls

def inner():
    # This will raise a ValueError
    int("not a number")

def middle():
    inner()

def outer():
    try:
        middle()
    except ValueError as e:
        print("Exception caught in outer:", e)

outer()

Everything was clear?

Thanks for your feedback!

Section 1. Chapter 4

Errors and Exceptions: Handling Problems During Execution

1. What is the difference between a syntax error and an exception?

2. How does Python handle exceptions during execution?

3. What happens if an exception is not caught?

Errors and Exceptions: Handling Problems During Execution

1. What is the difference between a syntax error and an exception?

2. How does Python handle exceptions during execution?

3. What happens if an exception is not caught?