| .. _tut-errors: |
| |
| ********************* |
| Errors and Exceptions |
| ********************* |
| |
| Until now error messages haven't been more than mentioned, but if you have tried |
| out the examples you have probably seen some. There are (at least) two |
| distinguishable kinds of errors: *syntax errors* and *exceptions*. |
| |
| |
| .. _tut-syntaxerrors: |
| |
| Syntax Errors |
| ============= |
| |
| Syntax errors, also known as parsing errors, are perhaps the most common kind of |
| complaint you get while you are still learning Python:: |
| |
| >>> while True print('Hello world') |
| File "<stdin>", line 1, in ? |
| while True print('Hello world') |
| ^ |
| SyntaxError: invalid syntax |
| |
| The parser repeats the offending line and displays a little 'arrow' pointing at |
| the earliest point in the line where the error was detected. The error is |
| caused by (or at least detected at) the token *preceding* the arrow: in the |
| example, the error is detected at the function :func:`print`, since a colon |
| (``':'``) is missing before it. File name and line number are printed so you |
| know where to look in case the input came from a script. |
| |
| |
| .. _tut-exceptions: |
| |
| Exceptions |
| ========== |
| |
| Even if a statement or expression is syntactically correct, it may cause an |
| error when an attempt is made to execute it. Errors detected during execution |
| are called *exceptions* and are not unconditionally fatal: you will soon learn |
| how to handle them in Python programs. Most exceptions are not handled by |
| programs, however, and result in error messages as shown here:: |
| |
| >>> 10 * (1/0) |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in ? |
| ZeroDivisionError: int division or modulo by zero |
| >>> 4 + spam*3 |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in ? |
| NameError: name 'spam' is not defined |
| >>> '2' + 2 |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in ? |
| TypeError: Can't convert 'int' object to str implicitly |
| |
| The last line of the error message indicates what happened. Exceptions come in |
| different types, and the type is printed as part of the message: the types in |
| the example are :exc:`ZeroDivisionError`, :exc:`NameError` and :exc:`TypeError`. |
| The string printed as the exception type is the name of the built-in exception |
| that occurred. This is true for all built-in exceptions, but need not be true |
| for user-defined exceptions (although it is a useful convention). Standard |
| exception names are built-in identifiers (not reserved keywords). |
| |
| The rest of the line provides detail based on the type of exception and what |
| caused it. |
| |
| The preceding part of the error message shows the context where the exception |
| happened, in the form of a stack traceback. In general it contains a stack |
| traceback listing source lines; however, it will not display lines read from |
| standard input. |
| |
| :ref:`bltin-exceptions` lists the built-in exceptions and their meanings. |
| |
| |
| .. _tut-handling: |
| |
| Handling Exceptions |
| =================== |
| |
| It is possible to write programs that handle selected exceptions. Look at the |
| following example, which asks the user for input until a valid integer has been |
| entered, but allows the user to interrupt the program (using :kbd:`Control-C` or |
| whatever the operating system supports); note that a user-generated interruption |
| is signalled by raising the :exc:`KeyboardInterrupt` exception. :: |
| |
| >>> while True: |
| ... try: |
| ... x = int(input("Please enter a number: ")) |
| ... break |
| ... except ValueError: |
| ... print("Oops! That was no valid number. Try again...") |
| ... |
| |
| The :keyword:`try` statement works as follows. |
| |
| * First, the *try clause* (the statement(s) between the :keyword:`try` and |
| :keyword:`except` keywords) is executed. |
| |
| * If no exception occurs, the *except clause* is skipped and execution of the |
| :keyword:`try` statement is finished. |
| |
| * If an exception occurs during execution of the try clause, the rest of the |
| clause is skipped. Then if its type matches the exception named after the |
| :keyword:`except` keyword, the except clause is executed, and then execution |
| continues after the :keyword:`try` statement. |
| |
| * If an exception occurs which does not match the exception named in the except |
| clause, it is passed on to outer :keyword:`try` statements; if no handler is |
| found, it is an *unhandled exception* and execution stops with a message as |
| shown above. |
| |
| A :keyword:`try` statement may have more than one except clause, to specify |
| handlers for different exceptions. At most one handler will be executed. |
| Handlers only handle exceptions that occur in the corresponding try clause, not |
| in other handlers of the same :keyword:`try` statement. An except clause may |
| name multiple exceptions as a parenthesized tuple, for example:: |
| |
| ... except (RuntimeError, TypeError, NameError): |
| ... pass |
| |
| The last except clause may omit the exception name(s), to serve as a wildcard. |
| Use this with extreme caution, since it is easy to mask a real programming error |
| in this way! It can also be used to print an error message and then re-raise |
| the exception (allowing a caller to handle the exception as well):: |
| |
| import sys |
| |
| try: |
| f = open('myfile.txt') |
| s = f.readline() |
| i = int(s.strip()) |
| except IOError as err: |
| print("I/O error: {0}".format(err)) |
| except ValueError: |
| print("Could not convert data to an integer.") |
| except: |
| print("Unexpected error:", sys.exc_info()[0]) |
| raise |
| |
| The :keyword:`try` ... :keyword:`except` statement has an optional *else |
| clause*, which, when present, must follow all except clauses. It is useful for |
| code that must be executed if the try clause does not raise an exception. For |
| example:: |
| |
| for arg in sys.argv[1:]: |
| try: |
| f = open(arg, 'r') |
| except IOError: |
| print('cannot open', arg) |
| else: |
| print(arg, 'has', len(f.readlines()), 'lines') |
| f.close() |
| |
| The use of the :keyword:`else` clause is better than adding additional code to |
| the :keyword:`try` clause because it avoids accidentally catching an exception |
| that wasn't raised by the code being protected by the :keyword:`try` ... |
| :keyword:`except` statement. |
| |
| When an exception occurs, it may have an associated value, also known as the |
| exception's *argument*. The presence and type of the argument depend on the |
| exception type. |
| |
| The except clause may specify a variable after the exception name. The |
| variable is bound to an exception instance with the arguments stored in |
| ``instance.args``. For convenience, the exception instance defines |
| :meth:`__str__` so the arguments can be printed directly without having to |
| reference ``.args``. One may also instantiate an exception first before |
| raising it and add any attributes to it as desired. :: |
| |
| >>> try: |
| ... raise Exception('spam', 'eggs') |
| ... except Exception as inst: |
| ... print(type(inst)) # the exception instance |
| ... print(inst.args) # arguments stored in .args |
| ... print(inst) # __str__ allows args to be printed directly, |
| ... # but may be overridden in exception subclasses |
| ... x, y = inst.args # unpack args |
| ... print('x =', x) |
| ... print('y =', y) |
| ... |
| <class 'Exception'> |
| ('spam', 'eggs') |
| ('spam', 'eggs') |
| x = spam |
| y = eggs |
| |
| If an exception has arguments, they are printed as the last part ('detail') of |
| the message for unhandled exceptions. |
| |
| Exception handlers don't just handle exceptions if they occur immediately in the |
| try clause, but also if they occur inside functions that are called (even |
| indirectly) in the try clause. For example:: |
| |
| >>> def this_fails(): |
| ... x = 1/0 |
| ... |
| >>> try: |
| ... this_fails() |
| ... except ZeroDivisionError as err: |
| ... print('Handling run-time error:', err) |
| ... |
| Handling run-time error: int division or modulo by zero |
| |
| |
| .. _tut-raising: |
| |
| Raising Exceptions |
| ================== |
| |
| The :keyword:`raise` statement allows the programmer to force a specified |
| exception to occur. For example:: |
| |
| >>> raise NameError('HiThere') |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in ? |
| NameError: HiThere |
| |
| The sole argument to :keyword:`raise` indicates the exception to be raised. |
| This must be either an exception instance or an exception class (a class that |
| derives from :class:`Exception`). |
| |
| If you need to determine whether an exception was raised but don't intend to |
| handle it, a simpler form of the :keyword:`raise` statement allows you to |
| re-raise the exception:: |
| |
| >>> try: |
| ... raise NameError('HiThere') |
| ... except NameError: |
| ... print('An exception flew by!') |
| ... raise |
| ... |
| An exception flew by! |
| Traceback (most recent call last): |
| File "<stdin>", line 2, in ? |
| NameError: HiThere |
| |
| |
| .. _tut-userexceptions: |
| |
| User-defined Exceptions |
| ======================= |
| |
| Programs may name their own exceptions by creating a new exception class. |
| Exceptions should typically be derived from the :exc:`Exception` class, either |
| directly or indirectly. For example:: |
| |
| >>> class MyError(Exception): |
| ... def __init__(self, value): |
| ... self.value = value |
| ... def __str__(self): |
| ... return repr(self.value) |
| ... |
| >>> try: |
| ... raise MyError(2*2) |
| ... except MyError as e: |
| ... print('My exception occurred, value:', e.value) |
| ... |
| My exception occurred, value: 4 |
| >>> raise MyError('oops!') |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in ? |
| __main__.MyError: 'oops!' |
| |
| In this example, the default :meth:`__init__` of :class:`Exception` has been |
| overridden. The new behavior simply creates the *value* attribute. This |
| replaces the default behavior of creating the *args* attribute. |
| |
| Exception classes can be defined which do anything any other class can do, but |
| are usually kept simple, often only offering a number of attributes that allow |
| information about the error to be extracted by handlers for the exception. When |
| creating a module that can raise several distinct errors, a common practice is |
| to create a base class for exceptions defined by that module, and subclass that |
| to create specific exception classes for different error conditions:: |
| |
| class Error(Exception): |
| """Base class for exceptions in this module.""" |
| pass |
| |
| class InputError(Error): |
| """Exception raised for errors in the input. |
| |
| Attributes: |
| expression -- input expression in which the error occurred |
| message -- explanation of the error |
| """ |
| |
| def __init__(self, expression, message): |
| self.expression = expression |
| self.message = message |
| |
| class TransitionError(Error): |
| """Raised when an operation attempts a state transition that's not |
| allowed. |
| |
| Attributes: |
| previous -- state at beginning of transition |
| next -- attempted new state |
| message -- explanation of why the specific transition is not allowed |
| """ |
| |
| def __init__(self, previous, next, message): |
| self.previous = previous |
| self.next = next |
| self.message = message |
| |
| Most exceptions are defined with names that end in "Error," similar to the |
| naming of the standard exceptions. |
| |
| Many standard modules define their own exceptions to report errors that may |
| occur in functions they define. More information on classes is presented in |
| chapter :ref:`tut-classes`. |
| |
| |
| .. _tut-cleanup: |
| |
| Defining Clean-up Actions |
| ========================= |
| |
| The :keyword:`try` statement has another optional clause which is intended to |
| define clean-up actions that must be executed under all circumstances. For |
| example:: |
| |
| >>> try: |
| ... raise KeyboardInterrupt |
| ... finally: |
| ... print('Goodbye, world!') |
| ... |
| Goodbye, world! |
| Traceback (most recent call last): |
| File "<stdin>", line 2, in ? |
| KeyboardInterrupt |
| |
| A *finally clause* is always executed before leaving the :keyword:`try` |
| statement, whether an exception has occurred or not. When an exception has |
| occurred in the :keyword:`try` clause and has not been handled by an |
| :keyword:`except` clause (or it has occurred in a :keyword:`except` or |
| :keyword:`else` clause), it is re-raised after the :keyword:`finally` clause has |
| been executed. The :keyword:`finally` clause is also executed "on the way out" |
| when any other clause of the :keyword:`try` statement is left via a |
| :keyword:`break`, :keyword:`continue` or :keyword:`return` statement. A more |
| complicated example:: |
| |
| >>> def divide(x, y): |
| ... try: |
| ... result = x / y |
| ... except ZeroDivisionError: |
| ... print("division by zero!") |
| ... else: |
| ... print("result is", result) |
| ... finally: |
| ... print("executing finally clause") |
| ... |
| >>> divide(2, 1) |
| result is 2.0 |
| executing finally clause |
| >>> divide(2, 0) |
| division by zero! |
| executing finally clause |
| >>> divide("2", "1") |
| executing finally clause |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in ? |
| File "<stdin>", line 3, in divide |
| TypeError: unsupported operand type(s) for /: 'str' and 'str' |
| |
| As you can see, the :keyword:`finally` clause is executed in any event. The |
| :exc:`TypeError` raised by dividing two strings is not handled by the |
| :keyword:`except` clause and therefore re-raised after the :keyword:`finally` |
| clause has been executed. |
| |
| In real world applications, the :keyword:`finally` clause is useful for |
| releasing external resources (such as files or network connections), regardless |
| of whether the use of the resource was successful. |
| |
| |
| .. _tut-cleanup-with: |
| |
| Predefined Clean-up Actions |
| =========================== |
| |
| Some objects define standard clean-up actions to be undertaken when the object |
| is no longer needed, regardless of whether or not the operation using the object |
| succeeded or failed. Look at the following example, which tries to open a file |
| and print its contents to the screen. :: |
| |
| for line in open("myfile.txt"): |
| print(line) |
| |
| The problem with this code is that it leaves the file open for an indeterminate |
| amount of time after this part of the code has finished executing. |
| This is not an issue in simple scripts, but can be a problem for larger |
| applications. The :keyword:`with` statement allows objects like files to be |
| used in a way that ensures they are always cleaned up promptly and correctly. :: |
| |
| with open("myfile.txt") as f: |
| for line in f: |
| print(line) |
| |
| After the statement is executed, the file *f* is always closed, even if a |
| problem was encountered while processing the lines. Objects which, like files, |
| provide predefined clean-up actions will indicate this in their documentation. |
| |
| |