Inom Turdikulov8. Errors and Exceptions
There are (at least) two distinguishable kinds of errors (general types): ==syntax errors and exceptions==.
Syntax errors, also known as parsing errors.
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.
while True print('Hello world')
# File "<stdin>", line 1
# while True print('Hello world')
# ^^^^^
#SyntaxError: invalid syntaxEven 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. Most exceptions are not handled by programs, however, and result in error messages as shown here:
# The last line of the error message in exceptions 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 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.
# The rest of the line provides detail based on the type of exception and
# what caused it.
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 strBuilt-in Exceptions lists the built-in exceptions and their meanings.
Following example 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 (with/without exceptions occuring).
- First, the try clause (the statement(s) between the
tryandexceptkeywords) is executed. - If no exception occurs, the except clause is skipped and execution
of the
trystatement is finished. - If an exception occurs during execution of the
tryclause, the rest of the clause is skipped. Then, if its type matches the exception named after theexceptkeyword, 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
trystatements; if no handler is found, it is an unhandled exception and execution stops with an error message.
try:
try:
# print(a)
print(1 / 0)
except NameError:
print("This is a NameError from inner exception")
except ValueError:
print("This is a ZeroDivisionError from outer exception")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):
passA 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:
# 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.
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")
print("---")
# A class in an `except` clause matches exceptions which are instances of
# the class itself or one of its derived classes
try:
raise C()
except B:
print("C, but except is B")
print("---")
# An *except clause* listing a derived class does not match
# instances of its base classes
# This will raise an exception
try:
raise B()
except D:
print("D")
except C:
print("C")
print("---")
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 object.__str__ to print all the arguments
without explicitly accessing .args. :
try:
raise Exception('spam', 'eggs') # Exception with two arguments
except Exception as inst: # usually used as `e` or `err`
print(type(inst)) # <class 'Exception'>, the exception type
print(inst.args) # ('spam', 'eggs'), arguments stored in .args
print(inst) # ('spam', 'eggs'), __str__ allows args to be printed directly,
# but may be overridden in exception subclasses
x, y = inst.args # unpack args
print('x =', x) # x = spam
print('y =', y) # y = eggsWhat is the base class for all exceptions and difference between Exception
subclass?
BaseException is the common base class of all exceptions. One of its
subclasses, Exception (user-defined exceptions), 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.
Is possible to catch all exceptions with Exception?
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.
try:
1/0
except Exception as e:
print(f"Caught any exception type: {e}")How to use of Exception to print or log the exception?
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('/tmp/myfile.txt')
s = f.readline()
i = int(s.strip())
uknown_func()
except OSError as err:
print("OS error, we just notify you:", err)
except ValueError:
print("Could not convert data to an integer, we just notify you.")
except Exception as err:
print(f"Unexpected {err=}, {type(err)=}, program will exit.")
raiseThe 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:
# NOTE: A much cleaner way of doing this is using the with statement
import sys
argv = ["/tmp/myfile.txt"]
for arg in argv:
try:
f = open(arg, 'r')
except OSError:
# We catch here only file open errors, not read or close errors
print('cannot open', arg)
else:
# This code must be NOT catched by `except OSError`
#
# This won't catch an IOError from the read/write or close calls.
# Which is good, because then reason woudn't have been “Unable to create
# file on disk.” – it would have been a different error, one that your
# code wasn't prepared for. It's a good idea not to try to handle
# such errors.
print(arg, 'has', len(f.readlines()), 'lines')
f.close()When else block of try … except statement is better to use?
The use of the else clause is better than adding additional code to the try
clause because it avoids accidentally catching an exception that must be
trigger by the try … except block.
Can we catch exceptions in deeply nested code?
Yes. 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 zeroHow to manually raise an exception?
The raise statement allows the programmer to force a specified
exception to occur. For example:
raise NameError('HiThere') # raise argument is an exception instance/class
# Traceback (most recent call last):
# File "<stdin>", line 1, in <module>
# NameError: HiThereIf an exception class is passed to raise how it will be processed?
It will be implicitly instantiated by calling its constructor with no arguments:
raise ValueError # shorthand for 'raise ValueError()'What if you need to catch exception do something and then re-raise it?
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: HiThereException Chaining, what if an unhandled exception occurs inside an except
section?
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 errorHow 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>
# RuntimeErrorFor more information about chaining mechanics, see Built-in Exceptions
Programs may name their own exceptions by creating a new exception class
Exceptions should typically be derived from the ==Exception== class, either
directly or indirectly.
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.
try:
raise KeyboardInterrupt
finally:
raise Exception
print('Goodbye, world!')
# Goodbye, world!
# Traceback (most recent call last):
# File "<stdin>", line 2, in <module>
# KeyboardInterruptIf 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.
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. 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
print(bool_return()) # FalseA 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")
# No any exciptions
divide(2, 1)
# result is 2.0
# executing finally clause
# ZeroDivisionError
divide(2, 0)
# division by zero!
# executing finally clause
# 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.
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'When the finnaly clause (try) is useful in real world applications?
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.
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="")What 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.
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.
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>
# | File "<stdin>", line 3, in f
# | ExceptionGroup: there were problems
# +-+---------------- 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 these 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>
# | File "<stdin>", line 2, in f
# | ExceptionGroup: group1
# +-+---------------- 1 ----------------
# | ExceptionGroup: group2
# +-+---------------- 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)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
# >>>
# 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
# +------------------------------------
# >>>