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
iexcept
.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 kluczowymexcept
, 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 anexcept
clause. If the exception is not handled by anexcept
clause, the exception is re-raised after thefinally
clause has been executed.An exception could occur during execution of an
except
orelse
clause. Again, the exception is re-raised after thefinally
clause has been executed.If the
finally
clause executes abreak
,continue
orreturn
statement, exceptions are not re-raised.If the
try
statement reaches abreak
,continue
orreturn
statement, thefinally
clause will execute just prior to thebreak
,continue
orreturn
statement’s execution.If a
finally
clause includes areturn
statement, the returned value will be the one from thefinally
clause’sreturn
statement, not the value from thetry
clause’sreturn
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.
8.10. 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
+------------------------------------
>>>