8. Błędy i wyjątki

Do tej pory wiadomości o błędach były tylko wspomniane, ale jeśli próbowałaś przykładów to pewnie udało Ci się na nie natknąć. Występują (przynajmniej) dwa charakterystyczne typy błędów: błędy składni (syntax errors) oraz wyjątki (exceptions).

8.1. Błędy składni

Błędy składni, inaczej zwane błędami parsowania, to najczęstrze skargi jakie otrzymasz w swoim kierunku gdy wciąż uczysz się Pythona

>>> while True print('Hello world')
  File "<stdin>", line 1
    while True print('Hello world')
                   ^
SyntaxError: invalid syntax

Parser powtarza błędną linię i wyświetla małą „strzałkę” wskazującą najwcześniejszy punkt linii, w którym wykryto błąd. Błąd jest spowodowany (lub przynajmniej wykryty) przez token poprzedzający strzałkę: w przykładzie błąd jest wykryty w funkcji print(), ponieważ brakuje przed nim dwukropka (':'). Nazwa pliku i numer linii są drukowane, abyś wiedział(a), gdzie szukać, w przypadku, gdy dane wejściowe pochodzą ze skryptu.

8.2. Wyjątki

Nawet jeśli instrukcja lub wyrażenie jest poprawne składniowo, może ona wywołać błąd podczas próby jej wykonania. Błędy zauważone podczas wykonania programu są nazywane wyjątkami (exceptions) i nie zawsze są niedopuszczalne: już niedługo nauczysz w jaki sposób je obsługiwać. Większość wyjątków nie jest jednak obsługiwana przez program przez co wyświetlane są informacje o błędzie jak pokazano poniżej:

>>> 10 * (1/0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>> 4 + spam*3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "int") to str

Ostatni wiersz komunikatu o błędzie wskazuje, co się stało. Wyjątki występują w różnych typach, a typ jest drukowany jako część komunikatu: typy w przykładzie to ZeroDivisionError, NameError i TypeError. Ciąg wydrukowany jako typ wyjątku jest nazwą wbudowanego wyjątku, który wystąpił. Jest to prawdą dla wszystkich wbudowanych wyjątków, ale nie musi być prawdą dla wyjątków zdefiniowanych przez użytkownika (choć jest to przydatna konwencja). Standardowe nazwy wyjątków są wbudowanymi identyfikatorami (nie zarezerwowanymi słowami kluczowymi).

Pozostała część linii dostarcza szczegółów na temat typu wyjątku oraz informacji, co go spowodowało.

Wcześniejsza część komunikatu o błędzie pokazuje kontekst, w którym wystąpił wyjątek, w postaci śladu stosu. Ogólnie rzecz biorąc, zawiera on ślad stosu z listą linii źródłowych; jednak nie wyświetli linii odczytanych ze standardowego wejścia.

Built-in Exceptions wymienia wbudowane wyjątki i ich znaczenie.

8.3. Obsługa wyjątków

Możliwe jest pisanie programów, które obsługują wybrane wyjątki. Spójrzmy na poniższy przykład, który prosi użytkownika o wprowadzenie danych, dopóki nie zostanie wprowadzona poprawna liczba całkowita, ale pozwala użytkownikowi na przerwanie programu (przy użyciu Control-C lub czegokolwiek innego obsługiwanego przez system operacyjny); zauważ, że przerwanie wygenerowane przez użytkownika jest sygnalizowane przez podniesienie wyjątku KeyboardInterrupt.

>>> while True:
...     try:
...         x = int(input("Please enter a number: "))
...         break
...     except ValueError:
...         print("Oops!  That was no valid number.  Try again...")
...

Instrukcja try działa następująco.

• W pierwszej kolejności wykonywane są instrukcje pod klauzulą try - pomiędzy słowami kluczowymi try i except.

• Jeżeli nie wystąpi żaden wyjątek, klauzula except jest pomijana i zostaje zakończone wykonywanie instrukcji try.

• Jeśli wyjątek wystąpi podczas wykonywania klauzuli try, reszta klauzuli jest pomijana. Następnie, jeśli jego typ pasuje do wyjątku nazwanego po słowie kluczowym except, wykonywana jest klauzula except, a następnie wykonanie jest kontynuowane po bloku try/except.

• If an exception occurs which does not match the exception named in the except clause, it is passed on to outer try statements; if no handler is found, it is an unhandled exception and execution stops with an error message.

Instrukcja try może mieć więcej niż jedną klauzulę except, aby określić programy obsługi dla różnych wyjątków. Wykonany zostanie co najwyżej jeden handler. Obsługiwane są tylko wyjątki, które występują w odpowiadających im klauzulach try, a nie w kodzie obsługi tej samej instrukcji try. Klauzula except może określać wiele wyjątków krotką w nawiasach, na przykład:

... except (RuntimeError, TypeError, NameError):
...     pass

A class in an except clause is compatible with an exception if it is the same class or a base class thereof (but not the other way around — an except clause listing a derived class is not compatible with a base class). For example, the following code will print B, C, D in that order:

class B(Exception):
    pass

class C(B):
    pass

class D(C):
    pass

for cls in [B, C, D]:
    try:
        raise cls()
    except D:
        print("D")
    except C:
        print("C")
    except B:
        print("B")

Note that if the except clauses were reversed (with except B first), it would have printed B, B, B — the first matching except clause is triggered.

When an exception occurs, it may have associated values, also known as the exception’s arguments. The presence and types of the arguments depend on the exception type.

The except clause may specify a variable after the exception name. The variable is bound to the exception instance which typically has an args attribute that stores the arguments. For convenience, builtin exception types define __str__() to print all the arguments without explicitly accessing .args.

>>> try:
...     raise Exception('spam', 'eggs')
... except Exception as inst:
...     print(type(inst))    # the exception type
...     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

The exception’s __str__() output is printed as the last part («detail») of the message for unhandled exceptions.

BaseException is the common base class of all exceptions. One of its subclasses, Exception, is the base class of all the non-fatal exceptions. Exceptions which are not subclasses of Exception are not typically handled, because they are used to indicate that the program should terminate. They include SystemExit which is raised by sys.exit() and KeyboardInterrupt which is raised when a user wishes to interrupt the program.

Exception can be used as a wildcard that catches (almost) everything. However, it is good practice to be as specific as possible with the types of exceptions that we intend to handle, and to allow any unexpected exceptions to propagate on.

The most common pattern for handling Exception is to print or log the exception and then re-raise it (allowing a caller to handle the exception as well):

import sys

try:
    f = open('myfile.txt')
    s = f.readline()
    i = int(s.strip())
except OSError as err:
    print("OS error:", err)
except ValueError:
    print("Could not convert data to an integer.")
except Exception as err:
    print(f"Unexpected {err=}, {type(err)=}")
    raise

The try … 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 OSError:
        print('cannot open', arg)
    else:
        print(arg, 'has', len(f.readlines()), 'lines')
        f.close()

The use of the else clause is better than adding additional code to the try clause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the try … except statement.

Exception handlers do not handle only exceptions that occur immediately in the try clause, but also those that 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: division by zero

8.4. Rzucanie wyjątków

Instrukcja raise pozwala programiście wymusić wystąpienie żądanego wyjątku. Na przykład:

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

The sole argument to raise indicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from BaseException, such as Exception or one of its subclasses). If an exception class is passed, it will be implicitly instantiated by calling its constructor with no arguments:

raise ValueError  # shorthand for 'raise ValueError()'

If you need to determine whether an exception was raised but don’t intend to handle it, a simpler form of the 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 <module>
NameError: HiThere

8.5. Łańcuch wyjątków

If an unhandled exception occurs inside an except section, it will have the exception being handled attached to it and included in the error message:

>>> try:
...     open("database.sqlite")
... except OSError:
...     raise RuntimeError("unable to handle error")
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
FileNotFoundError: [Errno 2] No such file or directory: 'database.sqlite'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: unable to handle error

To indicate that an exception is a direct consequence of another, the raise statement allows an optional from clause:

# exc must be exception instance or None.
raise RuntimeError from exc

This can be useful when you are transforming exceptions. For example:

>>> def func():
...     raise ConnectionError
...
>>> try:
...     func()
... except ConnectionError as exc:
...     raise RuntimeError('Failed to open database') from exc
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 2, in func
ConnectionError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: Failed to open database

It also allows disabling automatic exception chaining using the from None idiom:

>>> try:
...     open('database.sqlite')
... except OSError:
...     raise RuntimeError from None
...
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError

For more information about chaining mechanics, see Built-in Exceptions.

8.6. Wyjątki zdefiniowane przez użytkownika

Programs may name their own exceptions by creating a new exception class (see Klasy for more about Python classes). Exceptions should typically be derived from the Exception class, either directly or indirectly.

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.

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.

8.7. Defining Clean-up Actions

The 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 <module>
KeyboardInterrupt

If a finally clause is present, the finally clause will execute as the last task before the try statement completes. The finally clause runs whether or not the try statement produces an exception. The following points discuss more complex cases when an exception occurs:

• If an exception occurs during execution of the try clause, the exception may be handled by an except clause. If the exception is not handled by an except clause, the exception is re-raised after the finally clause has been executed.

• An exception could occur during execution of an except or else clause. Again, the exception is re-raised after the finally clause has been executed.

• If the finally clause executes a break, continue or return statement, exceptions are not re-raised.

• If the try statement reaches a break, continue or return statement, the finally clause will execute just prior to the break, continue or return statement’s execution.

• If a finally clause includes a return statement, the returned value will be the one from the finally clause’s return statement, not the value from the try clause’s return statement.

Na przykład:

>>> def bool_return():
...     try:
...         return True
...     finally:
...         return False
...
>>> bool_return()
False

Bardziej skomplikowany przykład:

>>> 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 <module>
  File "<stdin>", line 3, in divide
TypeError: unsupported operand type(s) for /: 'str' and 'str'

As you can see, the finally clause is executed in any event. The TypeError raised by dividing two strings is not handled by the except clause and therefore re-raised after the finally clause has been executed.

In real world applications, the finally clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.

8.8. 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, end="")

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 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, end="")

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.

8.9. Raising and Handling Multiple Unrelated Exceptions

There are situations where it is necessary to report several exceptions that have occurred. This is often the case in concurrency frameworks, when several tasks may have failed in parallel, but there are also other use cases where it is desirable to continue execution and collect multiple errors rather than raise the first exception.

The builtin ExceptionGroup wraps a list of exception instances so that they can be raised together. It is an exception itself, so it can be caught like any other exception.

>>> def f():
...     excs = [OSError('error 1'), SystemError('error 2')]
...     raise ExceptionGroup('there were problems', excs)
...
>>> f()
  + Exception Group Traceback (most recent call last):
  |   File "<stdin>", line 1, in <module>
  |   File "<stdin>", line 3, in f
  | ExceptionGroup: there were problems
  +-+---------------- 1 ----------------
    | OSError: error 1
    +---------------- 2 ----------------
    | SystemError: error 2
    +------------------------------------
>>> try:
...     f()
... except Exception as e:
...     print(f'caught {type(e)}: e')
...
caught <class 'ExceptionGroup'>: e
>>>

By using except* instead of except, we can selectively handle only the exceptions in the group that match a certain type. In the following example, which shows a nested exception group, each except* clause extracts from the group exceptions of a certain type while letting all other exceptions propagate to other clauses and eventually to be reraised.

>>> def f():
...     raise ExceptionGroup(
...         "group1",
...         [
...             OSError(1),
...             SystemError(2),
...             ExceptionGroup(
...                 "group2",
...                 [
...                     OSError(3),
...                     RecursionError(4)
...                 ]
...             )
...         ]
...     )
...
>>> try:
...     f()
... except* OSError as e:
...     print("There were OSErrors")
... except* SystemError as e:
...     print("There were SystemErrors")
...
There were OSErrors
There were SystemErrors
  + Exception Group Traceback (most recent call last):
  |   File "<stdin>", line 2, in <module>
  |   File "<stdin>", line 2, in f
  | ExceptionGroup: group1
  +-+---------------- 1 ----------------
    | ExceptionGroup: group2
    +-+---------------- 1 ----------------
      | RecursionError: 4
      +------------------------------------
>>>

Note that the exceptions nested in an exception group must be instances, not types. This is because in practice the exceptions would typically be ones that have already been raised and caught by the program, along the following pattern:

>>> excs = []
... for test in tests:
...     try:
...         test.run()
...     except Exception as e:
...         excs.append(e)
...
>>> if excs:
...    raise ExceptionGroup("Test Failures", excs)
...

8.10. Enriching Exceptions with Notes

When an exception is created in order to be raised, it is usually initialized with information that describes the error that has occurred. There are cases where it is useful to add information after the exception was caught. For this purpose, exceptions have a method add_note(note) that accepts a string and adds it to the exception’s notes list. The standard traceback rendering includes all notes, in the order they were added, after the exception.

>>> try:
...     raise TypeError('bad type')
... except Exception as e:
...     e.add_note('Add some information')
...     e.add_note('Add some more information')
...     raise
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
TypeError: bad type
Add some information
Add some more information
>>>

For example, when collecting exceptions into an exception group, we may want to add context information for the individual errors. In the following each exception in the group has a note indicating when this error has occurred.

>>> def f():
...     raise OSError('operation failed')
...
>>> excs = []
>>> for i in range(3):
...     try:
...         f()
...     except Exception as e:
...         e.add_note(f'Happened in Iteration {i+1}')
...         excs.append(e)
...
>>> raise ExceptionGroup('We have some problems', excs)
  + Exception Group Traceback (most recent call last):
  |   File "<stdin>", line 1, in <module>
  | ExceptionGroup: We have some problems (3 sub-exceptions)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "<stdin>", line 3, in <module>
    |   File "<stdin>", line 2, in f
    | OSError: operation failed
    | Happened in Iteration 1
    +---------------- 2 ----------------
    | Traceback (most recent call last):
    |   File "<stdin>", line 3, in <module>
    |   File "<stdin>", line 2, in f
    | OSError: operation failed
    | Happened in Iteration 2
    +---------------- 3 ----------------
    | Traceback (most recent call last):
    |   File "<stdin>", line 3, in <module>
    |   File "<stdin>", line 2, in f
    | OSError: operation failed
    | Happened in Iteration 3
    +------------------------------------
>>>