8. خطاها و استثناها

تا اینجا پیام‌های خطا فقط به صورت کوتاه ذکر شده‌اند، اما اگر مثال‌ها را امتحان کرده باشید احتمالاً برخی از آن‌ها را دیده‌اید. حداقل دو نوع متمایز از خطا وجود دارد: خطاهای دستوری «syntax errors» و استثناها «exceptions».

8.1. خطاهای دستوری

خطاهای دستوری، که به‌عنوان خطاهای تجزیه‌گر «parsing errors» نیز شناخته می‌شوند، شاید رایج‌ترین نوع خطاهایی باشند که هنگام یادگیری پایتون با آن‌ها مواجه می‌شوید:

>>> 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. استثناء ها

حتی اگر یک دستور یا عبارت از نظر دستوری «syntactically» درست باشد، ممکن است هنگام اجرای آن خطایی رخ دهد. خطاهایی که هنگام اجرا شناسایی می‌شوند «استثنا» «exceptions» نام دارند و الزاماً باعث توقف برنامه نمی‌شوند؛ به‌زودی خواهید آموخت چگونه آن‌ها را در برنامه‌های پایتون مدیریت کنید. با این حال، بیشتر استثناها در برنامه‌ها مدیریت نمی‌شوند و به پیام‌های خطا ختم می‌شوند، مانند چیزی که در اینجا نمایش داده شده است:

>>> 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

آخرین خط از پیام خطا نشان می دهد که چه اتفاقی افتاده است. استثناها دارای انواع مختلفی هستند و نوع آنها به عنوان بخشی از پیام چاپ می شود: انواع موجود در مثال عبارتند از ZeroDivisionError، NameError و TypeError. رشته ای که به عنوان نوع استثنا چاپ می شود، نام استثنای داخلی است که رخ داده است. این موضوع برای تمام استثناهای داخلی صدق می کند، اما برای استثناهای تعریف شده توسط کاربر لزوماً صحیح نیست (گرچه این یک قاعدهٔ مفید محسوب می‌شود). نام های استثنای استاندارد شناسه های داخلی هستند (نه کلمات کلیدی رزرو شده).

بخش باقی‌ماندهٔ خط، جزئیاتی را بسته به نوع استثنا «exception» و علت وقوع آن ارائه می‌دهد.

بخش قبلی پیام خطا، زمینه‌ای را نشان می‌دهد که در آن استثنا رخ داده است، به‌صورت یک ردگیری پشته «stack traceback». به‌طور کلی، این بخش شامل ردگیری پشته‌ای است که خطوطی از کد منبع را فهرست می‌کند؛ با این حال، خطوطی که از ورودی استاندارد «standard input» خوانده شده‌اند در آن نمایش داده نمی‌شوند.

Built-in Exceptions لیستی از استثناهای داخلی و معانی آن ها را ارائه می دهد.

8.3. مدیریت استثناها

امکان نوشتن برنامه‌هایی وجود دارد که برخی از استثناها را مدیریت می‌کنند. به مثال زیر نگاه کنید که از کاربر ورودی می‌خواهد تا زمانی که یک عدد صحیح معتبر وارد شود، اما به کاربر اجازه می‌دهد برنامه را قطع کند (با فشردن Control-C یا هر روشی که سیستم‌عامل پشتیبانی می‌کند)؛ توجه داشته باشید که قطع برنامه توسط کاربر با ایجاد استثنای KeyboardInterrupt مشخص می‌شود.

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

دستور try به شکل زیر عمل می‌کند.

• ابتدا بخش try clause (دستوری که بین کلیدواژه های try و except قرار دارد) اجرا می شود.

• اگر هیچ استثنایی رخ ندهد، بخش except نادیده گرفته می‌شود و اجرای دستور try به پایان می‌رسد.

• اگر در طول اجرای بخش try استثنایی رخ دهد، بقیه آن بخش نادیده گرفته می‌شود. سپس، اگر نوع استثنا با استثنایی که بعد از کلیدواژه except آمده مطابقت داشته باشد، بخش except اجرا می‌شود و پس از آن اجرای برنامه بعد از بلوک try/except ادامه می‌یابد.

• 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.

All exceptions inherit from BaseException, and so it can be used 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 BaseException as err:
    print(f"Unexpected {err=}, {type(err)=}")
    raise

Alternatively the last except clause may omit the exception name(s), however the exception value must then be retrieved from sys.exc_info()[1].

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. Raising Exceptions

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. Exception Chaining

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. User-defined Exceptions

Programs may name their own exceptions by creating a new exception class (see کلاس ها 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 کلاس ها.

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.

For example:

>>> 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.