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dev (3.14)3.13.03.123.113.103.93.83.73.63.53.43.33.23.13.02.72.6

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TABLE OF CONTENTS

 * 8. Errors and Exceptions
   * 8.1. Syntax Errors
   * 8.2. Exceptions
   * 8.3. Handling Exceptions
   * 8.4. Raising Exceptions
   * 8.5. Exception Chaining
   * 8.6. User-defined Exceptions
   * 8.7. Defining Clean-up Actions
   * 8.8. Predefined Clean-up Actions
   * 8.9. Raising and Handling Multiple Unrelated Exceptions
   * 8.10. Enriching Exceptions with Notes

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8. ERRORS AND EXCEPTIONS¶

Until now error messages haven’t been more than mentioned, but if you have tried
out the examples you have probably seen some. There are (at least) two
distinguishable kinds of errors: syntax errors and exceptions.


8.1. SYNTAX ERRORS¶

Syntax errors, also known as parsing errors, are perhaps the most common kind of
complaint you get while you are still learning 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 ‘arrow’s pointing at
the token in the line where the error was detected. The error may be caused by
the absence of a token before the indicated token. 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. EXCEPTIONS¶

Even if a statement or expression is syntactically correct, it may cause an
error when an attempt is made to execute it. Errors detected during execution
are called exceptions and are not unconditionally fatal: you will soon learn how
to handle them in Python programs. Most exceptions are not handled by programs,
however, and result in error messages as shown here:

>>>

>>> 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


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).

The rest of the line provides detail based on the type of exception and what
caused it.

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. HANDLING EXCEPTIONS¶

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 and except 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 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()


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. 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>
    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()'


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>
    raise NameError('HiThere')
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>
    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


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>
    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


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
Classes 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>
    raise KeyboardInterrupt
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>
    divide("2", "1")
    ~~~~~~^^^^^^^^^^
  File "<stdin>", line 3, in divide
    result = x / y
             ~~^~~
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>
  |     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'>: 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>
  |     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
    +------------------------------------
>>>





TABLE OF CONTENTS

 * 8. Errors and Exceptions
   * 8.1. Syntax Errors
   * 8.2. Exceptions
   * 8.3. Handling Exceptions
   * 8.4. Raising Exceptions
   * 8.5. Exception Chaining
   * 8.6. User-defined Exceptions
   * 8.7. Defining Clean-up Actions
   * 8.8. Predefined Clean-up Actions
   * 8.9. Raising and Handling Multiple Unrelated Exceptions
   * 8.10. Enriching Exceptions with Notes

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9. Classes


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