- 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 “”, line 1
while True print(‘Hello world’)
^^^^^
SyntaxError: invalid syntax
The parser repeats the offending line and displays little arrows
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 “”, line 1, in
10 * (1/0)
~^~
ZeroDivisionError: division by zero
4 + spam3 Traceback (most recent call last): File “”, line 1, in4 + spam3
^^^^
NameError: name ‘spam’ is not defined
‘2’ + 2
Traceback (most recent call last):
File “”, line 1, in
‘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)
…
(‘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 “”, line 1, in
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 “”, line 2, in
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 “”, line 2, in
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 “”, line 4, in
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 “”, line 2, in
func()
~~~~^^
File “”, line 2, in func
ConnectionError
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File “”, line 4, in
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 “”, line 4, in
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 “”, line 2, in
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 “”, line 1, in
divide(“2”, “1”)~~^^^^^^^^^^
File “”, 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 “”, line 1, in
| f()
| ~^^
| File “”, 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: 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 “”, line 2, in
| f()
| ~^^
| File “”, 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 “”, line 2, in
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 “”, line 1, in
| raise ExceptionGroup(‘We have some problems’, excs)
| ExceptionGroup: We have some problems (3 sub-exceptions)
+-+—————- 1 —————-
| Traceback (most recent call last):
| File “”, line 3, in
| f()
| ~^^
| File “”, line 2, in f
| raise OSError(‘operation failed’)
| OSError: operation failed
| Happened in Iteration 1
+—————- 2 —————-
| Traceback (most recent call last):
| File “”, line 3, in
| f()
| ~^^
| File “”, line 2, in f
| raise OSError(‘operation failed’)
| OSError: operation failed
| Happened in Iteration 2
+—————- 3 —————-
| Traceback (most recent call last):
| File “”, line 3, in
| f()
| ~^^
| File “”, line 2, in f
| raise OSError(‘operation failed’)
| OSError: operation failed
| Happened in Iteration 3
+————————————
