# 8. Kesalahan errors dan Pengecualian exceptions

Sampai sekarang pesan kesalahan belum lebih dari yang disebutkan, tetapi jika Anda telah mencoba contohnya, Anda mungkin telah melihat beberapa. Ada (setidaknya) dua jenis kesalahan yang dapat dibedakan: syntax errors dan exceptions.

## 8.1. Kesalahan Sintaksis

Kesalahan sintaksis, juga dikenal sebagai kesalahan penguraian parsing, mungkin merupakan jenis keluhan paling umum yang Anda dapatkan saat Anda masih belajar Python:

    >>> 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 little arrows pointing at the place where the error was detected. Note that this is not always the place that needs to be fixed. In the example, the error is detected at the function `print()`, since a colon (`':'`) is missing just before it.

The file name (`<stdin>` in our example) and line number are printed so you know where to look in case the input came from a file.

## 8.2. Pengecualian

Bahkan jika suatu pernyataan atau ungkapan secara sintaksis benar, itu dapat menyebabkan kesalahan ketika suatu usaha dilakukan untuk mengeksekusinya. Kesalahan yang terdeteksi selama eksekusi disebut exceptions dan tidak fatal tanpa syarat: Anda akan segera belajar cara menanganinya dalam program Python. Namun, sebagian besar pengecualian tidak ditangani oleh program, dan menghasilkan pesan kesalahan seperti yang ditunjukkan di sini:

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

Baris terakhir dari pesan kesalahan menunjukkan apa yang terjadi. Pengecualian ada berbagai jenis yang berbeda, dan tipe dicetak sebagai bagian dari pesan: tipe dalam contoh adalah `ZeroDivisionError`, `NameError` dan `TypeError`. String yang dicetak sebagai jenis pengecualian adalah nama pengecualian bawaan yang terjadi. Ini berlaku untuk semua pengecualian bawaan, tetapi tidak harus sama untuk pengecualian yang dibuat pengguna (meskipun ini adalah konvensi yang bermanfaat). Nama pengecualian standar adalah pengidentifikasi bawaan (bukan kata kunci yang dipesan reserved keyword).

Sisa baris menyediakan detail berdasarkan jenis pengecualian dan apa yang menyebabkannya.

Bagian sebelumnya dari pesan kesalahan menunjukkan konteks di mana pengecualian terjadi, dalam bentuk tumpukan traceback Secara umum, ini berisi daftar baris sumber tumpukan traceback; namun, ini tidak akan menampilkan baris yang dibaca dari masukan standar.

Built-in Exceptions memberikan daftar pengecualian bawaan dan artinya.

## 8.3. Menangani Pengecualian

Dimungkinkan untuk menulis program yang menangani pengecualian yang dipilih. Lihatlah contoh berikut, yang meminta masukan dari pengguna sampai integer yang valid telah dimasukkan, tetapi memungkinkan pengguna untuk menghentikan program (menggunakan `Control`-`C` atau apa pun yang didukung sistem operasi); perhatikan bahwa gangguan yang dibuat pengguna ditandai dengan munculnya pengecualian `KeyboardInterrupt`.

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

Pernyataan `try` berfungsi sebagai berikut.

- Pertama, try clause (pernyataan(-pernyataan) di antara kata kunci `try` dan `except`) dieksekusi.

- Jika tidak ada pengecualian terjadi, except clause dilewati dan eksekusi pernyataan `try` selesai.

- 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 the try/except block.

- 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.

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 matches exceptions which are instances of the class itself or one of its derived classes (but not the other way around --- an except clause listing a derived class does not match instances of its base classes). 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()

Penggunaan klausa `else` lebih baik daripada menambahkan kode tambahan ke klausa `try` karena menghindari secara tidak sengaja menangkap pengecualian yang tidak dimunculkan oleh kode yang dilindungi oleh pernyataan `try` ... `except`.

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. Memunculkan Pengecualian

Pernyataan `raise` memungkinkan programmer untuk memaksa pengecualian yang ditentukan terjadi. Sebagai contoh:

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

Jika Anda perlu menentukan apakah pengecualian muncul tetapi tidak bermaksud menanganinya, bentuk yang lebih sederhana dari pernyataan `raise` memungkinkan Anda untuk memunculkan kembali pengecualian:

    >>> 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>
        raise NameError('HiThere')
    NameError: HiThere

## 8.5. Rantai Pengecualian

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>
        open("database.sqlite")
        ~~~~^^^^^^^^^^^^^^^^^^^
    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>
        raise RuntimeError("unable to handle error")
    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

Ini dapat berguna saat Anda mengubah pengecualian. Sebagai contoh:

    >>> 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>
        func()
        ~~~~^^
      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>
        raise RuntimeError('Failed to open database') from exc
    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>
        raise RuntimeError from None
    RuntimeError

Untuk informasi lebih lanjut tentang mekanisme rantai, lihat Built-in Exceptions.

## 8.6. Pengecualian yang Ditentukan Pengguna

Program dapat memberi nama pengecualian mereka sendiri dengan membuat kelas pengecualian baru (lihat Classes untuk informasi lebih lanjut tentang kelas Python). Pengecualian biasanya berasal dari kelas `Exception`, baik secara langsung atau tidak langsung.

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.

Sebagian besar pengecualian didefinisikan dengan nama yang diakhiri dengan "Error", mirip dengan penamaan pengecualian standar.

Many standard modules define their own exceptions to report errors that may occur in functions they define.

## 8.7. Mendefinisikan Tindakan Pembersihan

Pernyataan `try` memiliki klausa opsional lain yang dimaksudkan untuk menentukan tindakan pembersihan yang harus dijalankan dalam semua keadaan. Sebagai contoh:

    >>> try:
    ...     raise KeyboardInterrupt
    ... finally:
    ...     print('Goodbye, world!')
    ...
    Goodbye, world!
    Traceback (most recent call last):
      File "<stdin>", line 2, in <module>
        raise KeyboardInterrupt
    KeyboardInterrupt

Jika ada klausa `finally`, klausa untuk `finally` akan dijalankan sebagai tugas terakhir sebelum pernyataan untuk `try` selesai. Klausa untuk `finally` dapat berjalan bai atau tidak apabila pernyataan `try` menghasilkan suatu pengecualian. Poin-poin berikut membahas kasus yang lebih kompleks saat pengecualian terjadi:

- Jika pengecualian terjadi selama eksekusi klausa untuk `try`, maka pengecualian tersebut dapat ditangani oleh klausa `except`. Jika pengecualian tidak ditangani oleh klausa `except`, maka pengecualian dimunculkan kembali setelah klausa `finally` dieksekusi.

- Pengecualian dapat terjadi selama pelaksanaan klausa `except` atau `else`. Sekali lagi, pengecualian akan muncul kembali setelah klausa `finally` telah dieksekusi.

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

- Jika pernyataan klausa untuk `try` mencapai klausa `break`, `continue` atau `return` maka, pernyataan untuk klausa `finally` akan dieksekusi sebelum `break`, `continue` atau `return` dieksekusi.

- 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.

Sebagai contoh:

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

Contoh yang lebih rumit:

    >>> 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>
        divide("2", "1")
        ~~~~~~^^^^^^^^^^
      File "<stdin>", line 3, in divide
        result = x / y
                 ~~^~~
    TypeError: unsupported operand type(s) for /: 'str' and 'str'

Seperti yang Anda lihat, klausa `finally` dieksekusi dalam peristiwa apa pun. `TypeError` yang ditimbulkan dengan membagi dua string tidak ditangani oleh klausa `except` dan karenanya kembali muncul setelah klausa `finally` telah dieksekusi.

Dalam aplikasi dunia nyata, klausa `finally` berguna untuk melepaskan sumber daya eksternal (seperti berkas atau koneksi jaringan), terlepas dari apakah penggunaan sumber daya tersebut berhasil.

## 8.8. Tindakan Pembersihan yang Sudah Ditentukan

Beberapa objek mendefinisikan tindakan pembersihan standar yang harus dilakukan ketika objek tidak lagi diperlukan, terlepas dari apakah operasi menggunakan objek berhasil atau gagal. Lihatlah contoh berikut, yang mencoba membuka berkas dan mencetak isinya ke layar.

    for line in open("myfile.txt"):
        print(line, end="")

Masalah dengan kode ini adalah bahwa ia membiarkan berkas terbuka untuk jumlah waktu yang tidak ditentukan setelah bagian kode ini selesai dieksekusi. Ini bukan masalah dalam skrip sederhana, tetapi bisa menjadi masalah untuk aplikasi yang lebih besar. Pernyataan `with` memungkinkan objek seperti berkas digunakan dengan cara yang memastikan mereka selalu dibersihkan secepatnya dan dengan benar.

    with open("myfile.txt") as f:
        for line in f:
            print(line, end="")

Setelah pernyataan dieksekusi, file f selalu ditutup, bahkan jika ada masalah saat pemrosesan baris-baris. Objek yang, seperti berkas-berkas, memberikan tindakan pembersihan yang telah ditentukan, akan menunjukkan ini dalam dokumentasinya.

## 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>
      |     f()
      |     ~^^
      |   File "<stdin>", line 3, in f
      |     raise ExceptionGroup('there were problems', excs)
      | ExceptionGroup: there were problems (2 sub-exceptions)
      +-+---------------- 1 ----------------
        | OSError: error 1
        +---------------- 2 ----------------
        | SystemError: error 2
        +------------------------------------
    >>> try:
    ...     f()
    ... except Exception as e:
    ...     print(f'caught {type(e)}: {e}')
    ...
    caught <class 'ExceptionGroup'>: there were problems (2 sub-exceptions)
    >>>

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>
      |     f()
      |     ~^^
      |   File "<stdin>", line 2, in f
      |     raise ExceptionGroup(
      |     ...<12 lines>...
      |     )
      | ExceptionGroup: group1 (1 sub-exception)
      +-+---------------- 1 ----------------
        | ExceptionGroup: group2 (1 sub-exception)
        +-+---------------- 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>
        raise TypeError('bad type')
    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>
      |     raise ExceptionGroup('We have some problems', excs)
      | ExceptionGroup: We have some problems (3 sub-exceptions)
      +-+---------------- 1 ----------------
        | Traceback (most recent call last):
        |   File "<stdin>", line 3, in <module>
        |     f()
        |     ~^^
        |   File "<stdin>", line 2, in f
        |     raise OSError('operation failed')
        | OSError: operation failed
        | Happened in Iteration 1
        +---------------- 2 ----------------
        | Traceback (most recent call last):
        |   File "<stdin>", line 3, in <module>
        |     f()
        |     ~^^
        |   File "<stdin>", line 2, in f
        |     raise OSError('operation failed')
        | OSError: operation failed
        | Happened in Iteration 2
        +---------------- 3 ----------------
        | Traceback (most recent call last):
        |   File "<stdin>", line 3, in <module>
        |     f()
        |     ~^^
        |   File "<stdin>", line 2, in f
        |     raise OSError('operation failed')
        | OSError: operation failed
        | Happened in Iteration 3
        +------------------------------------
    >>>