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. Błedy 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
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 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.
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
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 ZeroDivisionError
, NameError
and 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).
Pozostała część linii dostarcza szczegółów na temat typu wyjątku oraz informacji co go spowodowało.
The preceding part of the error message shows the context where the exception occurred, 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.
Built-in Exceptions lists the built-in exceptions and their meanings.
8.3. Obsługa wyjątków¶
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 Control-C or
whatever the operating system supports); note that a user-generated interruption
is signalled by raising the 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 try
statement works as follows.
First, the try clause (the statement(s) between the
try
andexcept
keywords) is executed.If no exception occurs, the except clause is skipped and execution of the
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
except
keyword, the except clause is executed, and then execution continues after thetry
statement.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 a message as shown above.
A 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 try
statement. An except clause may
name multiple exceptions as a parenthesized tuple, for example:
... 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.
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 OSError as err:
print("OS error: {0}".format(err))
except ValueError:
print("Could not convert data to an integer.")
except:
print("Unexpected error:", sys.exc_info()[0])
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.
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
__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: division by zero
8.4. Rzucanie Wyjątków¶
The 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 <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 Exception
). 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¶
The raise
statement allows an optional from
which enables
chaining exceptions. For example:
# 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 IOError
...
>>> try:
... func()
... except IOError 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
OSError
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
Exception chaining happens automatically when an exception is raised inside an
except
or finally
section. Exception chaining can be
disabled by using 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. More information on classes is presented in chapter Klasy.
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.
Dla przykładu:
>>> def bool_return():
... try:
... return True
... finally:
... return False
...
>>> bool_return()
False
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 <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.