exceptions-and-error-handling.md

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Errors and Exception Handling

How are errors/exceptions treated?

PHP

Errors in PHP can be detected with a try block and a catch block. The code in the try block will be attempted and if any errors are encountered, the code in the catch block will execute. If there were no errors in the try code execution, the code in the catch block will be skipped.

Exceptions can be thrown manually with the throw keyword. Since exceptions are classes in PHP, you can still access all their inherited methods and properties.

<?php
    function foo($bar) {
        if($bar == "baz") {
            throw new Exception("Bar is equal to baz!");
        }
        return true;
    }

    try {
        foo("baz");
    } catch(Exception $e) {
        print "An exception was caught: ".$e->getMessage();
    }
?>

Will yield:

An exception was caught: Bar is equal to baz!

Custom exceptions can also be defined, since exceptions are objects:

<?php
    class myError extends Exception {
        public function reportError() {
            return "There was an error!";
        }
    }
?>

Python

Python differentiates between syntax errors and exceptions. Syntax errors raise at execution time and as one would expect only raise when there is invalid structure to the Python code.

Exceptions are raised when an error is encountered at some point during code execution. Exceptions can be caught using the except keyword. in a try block. If the code in a try block encounters any error, the code in the except block will be executed. Otherwise, the code in the except block will be skipped.

The try block can match a specific exception or catch all exceptions. If an exception is raised that is not handled by the try block, the exception will raise uncaught and terminate the program execution.

>>> 10/0 // An uncaught exception
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

try:
    10/0
except ZeroDivisionError: // Catch only ZeroDivisionError exceptions
    print("Divided by zero!")

try:
    10/0
except: // Catch any exception thrown
    print("Some exception was thrown!")

Exceptions can be manually thrown as well:

>>>raise NameError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError

Python has many built-in exceptions, but you can also make your own by subclassing the Exception class:

class myError(Exception):
    pass

>>>raise myError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  __main__.myError