Errors and Exceptions
No matter your skill as a programmer, you will eventually make a coding mistake. Such mistakes come in three basic flavors:
- Syntax errors: Errors where the code is not valid Python (generally easy to fix)
- Runtime errors: Errors where syntactically valid code fails to execute, perhaps due to invalid user input (sometimes easy to fix)
- Semantic errors: Errors in logic: code executes without a problem, but the result is not what you expect (often very difficult to track-down and fix)
Here we’re going to focus on how to deal cleanly with runtime errors. As we’ll see, Python handles runtime errors via its exception handling framework.
Runtime Errors¶
If you’ve done any coding in Python, you’ve likely come across runtime errors. They can happen in a lot of ways.
For example, if you try to reference an undefined variable:
Or if you try an operation that’s not defined:
Or you might be trying to compute a mathematically ill-defined result:
Or maybe you’re trying to access a sequence element that doesn’t exist:
Note that in each case, Python is kind enough to not simply indicate that an error happened, but to spit out a meaningful exception that includes information about what exactly went wrong, along with the exact line of code where the error happened. Having access to meaningful errors like this is immensely useful when trying to trace the root of problems in your code.
Catching Exceptions: try and except ¶
The main tool Python gives you for handling runtime exceptions is the try . except clause. Its basic structure is this:
Note that the second block here did not get executed: this is because the first block did not return an error. Let’s put a problematic statement in the try block and see what happens:
Here we see that when the error was raised in the try statement (in this case, a ZeroDivisionError ), the error was caught, and the except statement was executed.
One way this is often used is to check user input within a function or another piece of code. For example, we might wish to have a function that catches zero-division and returns some other value, perhaps a suitably large number like $10^<100>$:
There is a subtle problem with this code, though: what happens when another type of exception comes up? For example, this is probably not what we intended:
Dividing an integer and a string raises a TypeError , which our over-zealous code caught and assumed was a ZeroDivisionError ! For this reason, it’s nearly always a better idea to catch exceptions explicitly:
Built-in Exceptions¶
In Python, all exceptions must be instances of a class that derives from BaseException . In a try statement with an except clause that mentions a particular class, that clause also handles any exception classes derived from that class (but not exception classes from which it is derived). Two exception classes that are not related via subclassing are never equivalent, even if they have the same name.
The built-in exceptions listed below can be generated by the interpreter or built-in functions. Except where mentioned, they have an “associated value” indicating the detailed cause of the error. This may be a string or a tuple of several items of information (e.g., an error code and a string explaining the code). The associated value is usually passed as arguments to the exception class’s constructor.
User code can raise built-in exceptions. This can be used to test an exception handler or to report an error condition “just like” the situation in which the interpreter raises the same exception; but beware that there is nothing to prevent user code from raising an inappropriate error.
The built-in exception classes can be subclassed to define new exceptions; programmers are encouraged to derive new exceptions from the Exception class or one of its subclasses, and not from BaseException . More information on defining exceptions is available in the Python Tutorial under User-defined Exceptions .
Exception context¶
When raising a new exception while another exception is already being handled, the new exception’s __context__ attribute is automatically set to the handled exception. An exception may be handled when an except or finally clause, or a with statement, is used.
This implicit exception context can be supplemented with an explicit cause by using from with raise :
The expression following from must be an exception or None . It will be set as __cause__ on the raised exception. Setting __cause__ also implicitly sets the __suppress_context__ attribute to True , so that using raise new_exc from None effectively replaces the old exception with the new one for display purposes (e.g. converting KeyError to AttributeError ), while leaving the old exception available in __context__ for introspection when debugging.
The default traceback display code shows these chained exceptions in addition to the traceback for the exception itself. An explicitly chained exception in __cause__ is always shown when present. An implicitly chained exception in __context__ is shown only if __cause__ is None and __suppress_context__ is false.
In either case, the exception itself is always shown after any chained exceptions so that the final line of the traceback always shows the last exception that was raised.
Inheriting from built-in exceptions¶
User code can create subclasses that inherit from an exception type. It’s recommended to only subclass one exception type at a time to avoid any possible conflicts between how the bases handle the args attribute, as well as due to possible memory layout incompatibilities.
CPython implementation detail: Most built-in exceptions are implemented in C for efficiency, see: Objects/exceptions.c. Some have custom memory layouts which makes it impossible to create a subclass that inherits from multiple exception types. The memory layout of a type is an implementation detail and might change between Python versions, leading to new conflicts in the future. Therefore, it’s recommended to avoid subclassing multiple exception types altogether.
Base classes¶
The following exceptions are used mostly as base classes for other exceptions.
The base class for all built-in exceptions. It is not meant to be directly inherited by user-defined classes (for that, use Exception ). If str() is called on an instance of this class, the representation of the argument(s) to the instance are returned, or the empty string when there were no arguments.
The tuple of arguments given to the exception constructor. Some built-in exceptions (like OSError ) expect a certain number of arguments and assign a special meaning to the elements of this tuple, while others are usually called only with a single string giving an error message.
This method sets tb as the new traceback for the exception and returns the exception object. It was more commonly used before the exception chaining features of PEP 3134 became available. The following example shows how we can convert an instance of SomeException into an instance of OtherException while preserving the traceback. Once raised, the current frame is pushed onto the traceback of the OtherException , as would have happened to the traceback of the original SomeException had we allowed it to propagate to the caller.
Add the string note to the exception’s notes which appear in the standard traceback after the exception string. A TypeError is raised if note is not a string.
New in version 3.11.
A list of the notes of this exception, which were added with add_note() . This attribute is created when add_note() is called.
New in version 3.11.
All built-in, non-system-exiting exceptions are derived from this class. All user-defined exceptions should also be derived from this class.
The base class for those built-in exceptions that are raised for various arithmetic errors: OverflowError , ZeroDivisionError , FloatingPointError .
Raised when a buffer related operation cannot be performed.
The base class for the exceptions that are raised when a key or index used on a mapping or sequence is invalid: IndexError , KeyError . This can be raised directly by codecs.lookup() .
Concrete exceptions¶
The following exceptions are the exceptions that are usually raised.
Raised when an assert statement fails.
Raised when an attribute reference (see Attribute references ) or assignment fails. (When an object does not support attribute references or attribute assignments at all, TypeError is raised.)
The name and obj attributes can be set using keyword-only arguments to the constructor. When set they represent the name of the attribute that was attempted to be accessed and the object that was accessed for said attribute, respectively.
Changed in version 3.10: Added the name and obj attributes.
Raised when the input() function hits an end-of-file condition (EOF) without reading any data. (N.B.: the io.IOBase.read() and io.IOBase.readline() methods return an empty string when they hit EOF.)
Not currently used.
Raised when a generator or coroutine is closed; see generator.close() and coroutine.close() . It directly inherits from BaseException instead of Exception since it is technically not an error.
Raised when the import statement has troubles trying to load a module. Also raised when the “from list” in from . import has a name that cannot be found.
The name and path attributes can be set using keyword-only arguments to the constructor. When set they represent the name of the module that was attempted to be imported and the path to any file which triggered the exception, respectively.
Changed in version 3.3: Added the name and path attributes.
A subclass of ImportError which is raised by import when a module could not be located. It is also raised when None is found in sys.modules .
New in version 3.6.
Raised when a sequence subscript is out of range. (Slice indices are silently truncated to fall in the allowed range; if an index is not an integer, TypeError is raised.)
Raised when a mapping (dictionary) key is not found in the set of existing keys.
Raised when the user hits the interrupt key (normally Control — C or Delete ). During execution, a check for interrupts is made regularly. The exception inherits from BaseException so as to not be accidentally caught by code that catches Exception and thus prevent the interpreter from exiting.
Catching a KeyboardInterrupt requires special consideration. Because it can be raised at unpredictable points, it may, in some circumstances, leave the running program in an inconsistent state. It is generally best to allow KeyboardInterrupt to end the program as quickly as possible or avoid raising it entirely. (See Note on Signal Handlers and Exceptions .)
Raised when an operation runs out of memory but the situation may still be rescued (by deleting some objects). The associated value is a string indicating what kind of (internal) operation ran out of memory. Note that because of the underlying memory management architecture (C’s malloc() function), the interpreter may not always be able to completely recover from this situation; it nevertheless raises an exception so that a stack traceback can be printed, in case a run-away program was the cause.
Raised when a local or global name is not found. This applies only to unqualified names. The associated value is an error message that includes the name that could not be found.
The name attribute can be set using a keyword-only argument to the constructor. When set it represent the name of the variable that was attempted to be accessed.
Changed in version 3.10: Added the name attribute.
This exception is derived from RuntimeError . In user defined base classes, abstract methods should raise this exception when they require derived classes to override the method, or while the class is being developed to indicate that the real implementation still needs to be added.
It should not be used to indicate that an operator or method is not meant to be supported at all – in that case either leave the operator / method undefined or, if a subclass, set it to None .
NotImplementedError and NotImplemented are not interchangeable, even though they have similar names and purposes. See NotImplemented for details on when to use it.
This exception is raised when a system function returns a system-related error, including I/O failures such as “file not found” or “disk full” (not for illegal argument types or other incidental errors).
The second form of the constructor sets the corresponding attributes, described below. The attributes default to None if not specified. For backwards compatibility, if three arguments are passed, the args attribute contains only a 2-tuple of the first two constructor arguments.
The constructor often actually returns a subclass of OSError , as described in OS exceptions below. The particular subclass depends on the final errno value. This behaviour only occurs when constructing OSError directly or via an alias, and is not inherited when subclassing.
A numeric error code from the C variable errno .
Under Windows, this gives you the native Windows error code. The errno attribute is then an approximate translation, in POSIX terms, of that native error code.
Under Windows, if the winerror constructor argument is an integer, the errno attribute is determined from the Windows error code, and the errno argument is ignored. On other platforms, the winerror argument is ignored, and the winerror attribute does not exist.
The corresponding error message, as provided by the operating system. It is formatted by the C functions perror() under POSIX, and FormatMessage() under Windows.
For exceptions that involve a file system path (such as open() or os.unlink() ), filename is the file name passed to the function. For functions that involve two file system paths (such as os.rename() ), filename2 corresponds to the second file name passed to the function.
Changed in version 3.3: EnvironmentError , IOError , WindowsError , socket.error , select.error and mmap.error have been merged into OSError , and the constructor may return a subclass.
Changed in version 3.4: The filename attribute is now the original file name passed to the function, instead of the name encoded to or decoded from the filesystem encoding and error handler . Also, the filename2 constructor argument and attribute was added.
Raised when the result of an arithmetic operation is too large to be represented. This cannot occur for integers (which would rather raise MemoryError than give up). However, for historical reasons, OverflowError is sometimes raised for integers that are outside a required range. Because of the lack of standardization of floating point exception handling in C, most floating point operations are not checked.
This exception is derived from RuntimeError . It is raised when the interpreter detects that the maximum recursion depth (see sys.getrecursionlimit() ) is exceeded.
New in version 3.5: Previously, a plain RuntimeError was raised.
This exception is raised when a weak reference proxy, created by the weakref.proxy() function, is used to access an attribute of the referent after it has been garbage collected. For more information on weak references, see the weakref module.
Raised when an error is detected that doesn’t fall in any of the other categories. The associated value is a string indicating what precisely went wrong.
Raised by built-in function next() and an iterator ‘s __next__() method to signal that there are no further items produced by the iterator.
The exception object has a single attribute value , which is given as an argument when constructing the exception, and defaults to None .
When a generator or coroutine function returns, a new StopIteration instance is raised, and the value returned by the function is used as the value parameter to the constructor of the exception.
If a generator code directly or indirectly raises StopIteration , it is converted into a RuntimeError (retaining the StopIteration as the new exception’s cause).
Changed in version 3.3: Added value attribute and the ability for generator functions to use it to return a value.
Changed in version 3.5: Introduced the RuntimeError transformation via from __future__ import generator_stop , see PEP 479.
Changed in version 3.7: Enable PEP 479 for all code by default: a StopIteration error raised in a generator is transformed into a RuntimeError .
Must be raised by __anext__() method of an asynchronous iterator object to stop the iteration.
New in version 3.5.
Raised when the parser encounters a syntax error. This may occur in an import statement, in a call to the built-in functions compile() , exec() , or eval() , or when reading the initial script or standard input (also interactively).
The str() of the exception instance returns only the error message. Details is a tuple whose members are also available as separate attributes.
The name of the file the syntax error occurred in.
Which line number in the file the error occurred in. This is 1-indexed: the first line in the file has a lineno of 1.
The column in the line where the error occurred. This is 1-indexed: the first character in the line has an offset of 1.
The source code text involved in the error.
Which line number in the file the error occurred ends in. This is 1-indexed: the first line in the file has a lineno of 1.
The column in the end line where the error occurred finishes. This is 1-indexed: the first character in the line has an offset of 1.
For errors in f-string fields, the message is prefixed by “f-string: ” and the offsets are offsets in a text constructed from the replacement expression. For example, compiling f’Bad field’ results in this args attribute: (‘f-string: …’, (‘’, 1, 2, ‘(a b)n’, 1, 5)).
Changed in version 3.10: Added the end_lineno and end_offset attributes.
Base class for syntax errors related to incorrect indentation. This is a subclass of SyntaxError .
Raised when indentation contains an inconsistent use of tabs and spaces. This is a subclass of IndentationError .
Raised when the interpreter finds an internal error, but the situation does not look so serious to cause it to abandon all hope. The associated value is a string indicating what went wrong (in low-level terms).
You should report this to the author or maintainer of your Python interpreter. Be sure to report the version of the Python interpreter ( sys.version ; it is also printed at the start of an interactive Python session), the exact error message (the exception’s associated value) and if possible the source of the program that triggered the error.
This exception is raised by the sys.exit() function. It inherits from BaseException instead of Exception so that it is not accidentally caught by code that catches Exception . This allows the exception to properly propagate up and cause the interpreter to exit. When it is not handled, the Python interpreter exits; no stack traceback is printed. The constructor accepts the same optional argument passed to sys.exit() . If the value is an integer, it specifies the system exit status (passed to C’s exit() function); if it is None , the exit status is zero; if it has another type (such as a string), the object’s value is printed and the exit status is one.
A call to sys.exit() is translated into an exception so that clean-up handlers ( finally clauses of try statements) can be executed, and so that a debugger can execute a script without running the risk of losing control. The os._exit() function can be used if it is absolutely positively necessary to exit immediately (for example, in the child process after a call to os.fork() ).
The exit status or error message that is passed to the constructor. (Defaults to None .)
Raised when an operation or function is applied to an object of inappropriate type. The associated value is a string giving details about the type mismatch.
This exception may be raised by user code to indicate that an attempted operation on an object is not supported, and is not meant to be. If an object is meant to support a given operation but has not yet provided an implementation, NotImplementedError is the proper exception to raise.
Passing arguments of the wrong type (e.g. passing a list when an int is expected) should result in a TypeError , but passing arguments with the wrong value (e.g. a number outside expected boundaries) should result in a ValueError .
Raised when a reference is made to a local variable in a function or method, but no value has been bound to that variable. This is a subclass of NameError .
Raised when a Unicode-related encoding or decoding error occurs. It is a subclass of ValueError .
UnicodeError has attributes that describe the encoding or decoding error. For example, err.object[err.start:err.end] gives the particular invalid input that the codec failed on.
The name of the encoding that raised the error.
A string describing the specific codec error.
The object the codec was attempting to encode or decode.
The first index of invalid data in object .
The index after the last invalid data in object .
Raised when a Unicode-related error occurs during encoding. It is a subclass of UnicodeError .
Raised when a Unicode-related error occurs during decoding. It is a subclass of UnicodeError .
Raised when a Unicode-related error occurs during translating. It is a subclass of UnicodeError .
Raised when an operation or function receives an argument that has the right type but an inappropriate value, and the situation is not described by a more precise exception such as IndexError .
Raised when the second argument of a division or modulo operation is zero. The associated value is a string indicating the type of the operands and the operation.
The following exceptions are kept for compatibility with previous versions; starting from Python 3.3, they are aliases of OSError .
exception EnvironmentError ¶ exception IOError ¶ exception WindowsError ¶
Only available on Windows.
OS exceptions¶
The following exceptions are subclasses of OSError , they get raised depending on the system error code.
Raised when an operation would block on an object (e.g. socket) set for non-blocking operation. Corresponds to errno EAGAIN , EALREADY , EWOULDBLOCK and EINPROGRESS .
In addition to those of OSError , BlockingIOError can have one more attribute:
An integer containing the number of characters written to the stream before it blocked. This attribute is available when using the buffered I/O classes from the io module.
Raised when an operation on a child process failed. Corresponds to errno ECHILD .
A base class for connection-related issues.
A subclass of ConnectionError , raised when trying to write on a pipe while the other end has been closed, or trying to write on a socket which has been shutdown for writing. Corresponds to errno EPIPE and ESHUTDOWN .
A subclass of ConnectionError , raised when a connection attempt is aborted by the peer. Corresponds to errno ECONNABORTED .
A subclass of ConnectionError , raised when a connection attempt is refused by the peer. Corresponds to errno ECONNREFUSED .
A subclass of ConnectionError , raised when a connection is reset by the peer. Corresponds to errno ECONNRESET .
Raised when trying to create a file or directory which already exists. Corresponds to errno EEXIST .
Raised when a file or directory is requested but doesn’t exist. Corresponds to errno ENOENT .
Raised when a system call is interrupted by an incoming signal. Corresponds to errno EINTR .
Changed in version 3.5: Python now retries system calls when a syscall is interrupted by a signal, except if the signal handler raises an exception (see PEP 475 for the rationale), instead of raising InterruptedError .
Raised when a file operation (such as os.remove() ) is requested on a directory. Corresponds to errno EISDIR .
Raised when a directory operation (such as os.listdir() ) is requested on something which is not a directory. On most POSIX platforms, it may also be raised if an operation attempts to open or traverse a non-directory file as if it were a directory. Corresponds to errno ENOTDIR .
Raised when trying to run an operation without the adequate access rights — for example filesystem permissions. Corresponds to errno EACCES , EPERM , and ENOTCAPABLE .
Changed in version 3.11.1: WASI’s ENOTCAPABLE is now mapped to PermissionError .
Raised when a given process doesn’t exist. Corresponds to errno ESRCH .
Raised when a system function timed out at the system level. Corresponds to errno ETIMEDOUT .
New in version 3.3: All the above OSError subclasses were added.
PEP 3151 — Reworking the OS and IO exception hierarchy
Warnings¶
The following exceptions are used as warning categories; see the Warning Categories documentation for more details.
Base class for warning categories.
Base class for warnings generated by user code.
Base class for warnings about deprecated features when those warnings are intended for other Python developers.
Ignored by the default warning filters, except in the __main__ module ( PEP 565). Enabling the Python Development Mode shows this warning.
The deprecation policy is described in PEP 387.
Base class for warnings about features which are obsolete and expected to be deprecated in the future, but are not deprecated at the moment.
This class is rarely used as emitting a warning about a possible upcoming deprecation is unusual, and DeprecationWarning is preferred for already active deprecations.
Ignored by the default warning filters. Enabling the Python Development Mode shows this warning.
The deprecation policy is described in PEP 387.
Base class for warnings about dubious syntax.
Base class for warnings about dubious runtime behavior.
Base class for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python.
Base class for warnings about probable mistakes in module imports.
Ignored by the default warning filters. Enabling the Python Development Mode shows this warning.
Base class for warnings related to Unicode.
Base class for warnings related to encodings.
New in version 3.10.
Base class for warnings related to bytes and bytearray .
Base class for warnings related to resource usage.
Ignored by the default warning filters. Enabling the Python Development Mode shows this warning.
New in version 3.2.
Exception groups¶
The following are used when it is necessary to raise multiple unrelated exceptions. They are part of the exception hierarchy so they can be handled with except like all other exceptions. In addition, they are recognised by except* , which matches their subgroups based on the types of the contained exceptions.
exception ExceptionGroup ( msg , excs ) ¶ exception BaseExceptionGroup ( msg , excs ) ¶
Both of these exception types wrap the exceptions in the sequence excs . The msg parameter must be a string. The difference between the two classes is that BaseExceptionGroup extends BaseException and it can wrap any exception, while ExceptionGroup extends Exception and it can only wrap subclasses of Exception . This design is so that except Exception catches an ExceptionGroup but not BaseExceptionGroup .
The BaseExceptionGroup constructor returns an ExceptionGroup rather than a BaseExceptionGroup if all contained exceptions are Exception instances, so it can be used to make the selection automatic. The ExceptionGroup constructor, on the other hand, raises a TypeError if any contained exception is not an Exception subclass.
The msg argument to the constructor. This is a read-only attribute.
A tuple of the exceptions in the excs sequence given to the constructor. This is a read-only attribute.
Returns an exception group that contains only the exceptions from the current group that match condition, or None if the result is empty.
The condition can be either a function that accepts an exception and returns true for those that should be in the subgroup, or it can be an exception type or a tuple of exception types, which is used to check for a match using the same check that is used in an except clause.
The nesting structure of the current exception is preserved in the result, as are the values of its message , __traceback__ , __cause__ , __context__ and __notes__ fields. Empty nested groups are omitted from the result.
The condition is checked for all exceptions in the nested exception group, including the top-level and any nested exception groups. If the condition is true for such an exception group, it is included in the result in full.
Like subgroup() , but returns the pair (match, rest) where match is subgroup(condition) and rest is the remaining non-matching part.
Returns an exception group with the same message , but which wraps the exceptions in excs .
This method is used by subgroup() and split() . A subclass needs to override it in order to make subgroup() and split() return instances of the subclass rather than ExceptionGroup .
subgroup() and split() copy the __traceback__ , __cause__ , __context__ and __notes__ fields from the original exception group to the one returned by derive() , so these fields do not need to be updated by derive() .
Note that BaseExceptionGroup defines __new__() , so subclasses that need a different constructor signature need to override that rather than __init__() . For example, the following defines an exception group subclass which accepts an exit_code and and constructs the group’s message from it.
Like ExceptionGroup , any subclass of BaseExceptionGroup which is also a subclass of Exception can only wrap instances of Exception .
How to Fix Runtime Errors in Python
A runtime error is a type of error that occurs during program execution. The Python interpreter executes a script if it is syntactically correct. However, if it encounters an issue at runtime, which is not detected when the script is parsed, script execution may halt unexpectedly.
What Causes Runtime Errors
Some of the most common examples of runtime errors in Python are:
-
.
- Using an undefined variable or function name.
- Performing an operation on incompatible types.
- Accessing a list element, dictionary key or object attribute that does not exist.
- Accessing a file that does not exist.
Python Runtime Error Examples
Here’s a few examples of runtime errors in Python:
Division by zero
If a number is divided by zero in Python, a runtime error is raised:
In the above example, a number is attempted to be divided by zero. Running the above code raises a ZeroDivisionError :
Using an undefined variable or function name
A runtime error is raised if an attempt is made to access an identifier, such as a variable or function name, that is not declared previously:
In the above example, an undefined identifier myString is attempted to be accessed. Running the above code raises a NameError :
Performing an operation on incompatible types
If an operation, such as addition, multiplication etc., is performed between incompatible data types, a runtime error is raised:
In the above example, a string is attempted to be concatenated with a number. Since these types are incompatible, a TypeError is raised when the above code is executed:
Accessing a non-existent list element, dictionary key or object attribute
If an attempt is made to access a non-existent index or element in a list, dictionary or object, a runtime error is raised.
In the above example, an attempt is made to access an item in a list using an out-of-range index, which raises an IndexError :
Accessing a file that does not exist
If a file that does not exist is attempted to be accessed, a runtime error is raised:
In the above example, a non-existent file myFile.txt is attempted to be opened in read-only mode, which raises a FileNotFoundError :
How to Fix Runtime Errors in Python
To fix runtime errors in Python, the following steps can be taken:
- Identify the error message and note the specific problem being reported.
- Check the code for logical, mathematical or typographical errors.
- Ensure all identifiers are defined properly before being used.
- Make sure the correct data types are being used and are being used correctly.
- Verify that list items, dictionary keys, and other objects are being accessed using valid indices or keys.
- If necessary, consult the documentation for the relevant library or module.
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Exception Handling in Python
Exception : An exception if an error occurs during the execution of the program, which disrupts the normal flow of the program’s instructions.
Exception Handling: Exception handling does not mean that repairing an exception, we have to define an alternative way to continue the program normally. The method of defining an alternative way is nothing but an exception handling.
The Main Objective of the Exception Handling:
- It is highly recommended to perform exception handling.
- The main objective is to terminate the program in a graceful way/ normal termination of the application so that we should not miss anything or should not block our resources.
There are two types of Exceptions that Occur during the Execution of the program
Syntax Error in Python Exception Handling
The Errors which occur due to invalid syntax in the program are called syntax Errors; the programmer is responsible for correcting those errors. Once all the syntax errors are corrected, then only the program execution will start.
Runtime Error in Python Exception Handling
Runtime errors are also known as exceptions. While executing the program, at the runtime if something goes wrong because of end-user input or due to memory problem or programming logic, etc..then we will encounter runtime errors.
The Exception handling concept is applicable only for Runtime errors and not for Syntax errors.
Default Exception Handling in Python
Every exception in python is an object, and for every exception type, the corresponding class is available.
Whenever an exception occurs, the python virtual machine will create the corresponding exception object and will check for the handling code, if the corresponding handling code is available, then the code will be run normally.
If the handling code is not available, then the python interpreter terminates the program abnormally and prints corresponding exception information to the console, and the rest of the program won’t be executed.
To prevent this abnormal termination, we should handle exceptions explicitly, of course, by using try-except blocks.
Pythons Exception Hierarchy
Every exception in python is class, all the exception classes are child classes of a base class exception either directly or indirectly, and hence BaseException acts as a root for python exception hierarchy.
Being a programmer most of the time, we need to concentrate/handle the exception and its child classes.
The Image of Pythons Exception Hierarchy
Customized Exception Handling in python
The main objective of exception handling is to perform graceful termination of an application, and hence it is highly recommended to handle exceptions.
The code which may raise an exception is called risky code, and we have to take that code inside the try block and the corresponding handling code we have to take inside the except block.
So if any exceptions arise inside the try block immediately, the exception block will execute, and the alternative code will execute, and the program terminates normally.
The syntax for the customized exception handling is:
Example: Without Try Except Block
In the below code, the exception is raised in the second statement, and hence there is no except block which provides the alternate code to terminate the program normally, then python will throw a ZeroDivisionError, and the program is terminated abnormally.
Example: With Try-Except Block
In the below program an except block is present and hence, once the error occurs at the second statement, then the control will automatically go to except block which executes the alternate code, and the program will terminate normally by executing the third statement/ rest of the program.
The output is:
Control Flow in Try-Except Block in Python’s Exception Handling
Within the try block if anywhere an exception raised then the rest of the try block won’t be executed even though if we handled that exception, hence inside the try block we have to take only risky code and the length of the try block should be as less as possible.
The output is: Even After providing the except block, the third statement in the try block won’t be executed.
In addition to the try block, there may be a chance of raising exceptions inside the except and finally blocks also.
In any statement which is not a part of the try block and raising an exception, then it is always an abnormal termination.
How to print Exception Information to the Console
If you want to print the exception information inside the except blocks, like Name of the exception, Type of the exception or Description of the exception then you can enter the information before the handling code.
An Exception object got created with a reference variable, and the reference variable can be anything.
The syntax to print Exception Information to the console is:
The output is:
We can print the type of the exception by using the corresponding class object as follow:
The output is:
If you want to print only the exception class Name then:
The output is:
If you want to print the description of the exception, then :
The output is:
The following example demonstrates handling both parent class exception and child class exceptions by using BaseException as the class object.
The output of the parent class:
The output of handling child class exception:
Try Block with Multiple Exception Block in python
The Way of Handling an Exception is varied from exception to exception, hence for every possible exception type, We have to take separate except block. The exception block will execute until to get the matched exception block.
Try block with multiple exceptions is possible and recommended to use.
If the try block with multiple blocks is available, based on the available exception the corresponding except block will be executed.
The following example will demonstrate the try block with multiple except block
The output is: When we give 10 as the first value and second value as 5
The output, when we give the first value as 10 and second value as 0
The output, when we provide the first value as 10 and second value as Two
The following example demonstrates how the except block will execute from top to bottom
The output is:
The same code, if write the arithmetic error first and next to zero division error then, the arithmetic error exception will execute because the zero division error is the child class of arithmetic error.
The output is:
Single Exception block that can handle multiple exceptions in python
If a handling code is the same for multiple exceptions then instead of taking different except blocks, we can take single except block that can handle all the exceptions.
The parenthesis is mandatory and this group of exceptions internally considered as Tuple.
The following example demonstrates the single exception block handling multiple exceptions
The output is: When the input is 10 and 5
The output is: When the input is 10 and 5 , then the zerodivisionerror exception will execute
The output is: When the input is 10 and Two , then the Valueerror exception will execute
Default Except Block in python
We can use default except block to handle any exceptions; In the default except block generally, we can print exception information to the console.
The following example demonstrates the default except block
The output is: The ZeroDivisionError block will execute when input is 30 and 0
The output is: The Except block will execute when the zerodivisionerror does not match.
In the above code if we remove zerodivisionerror block also the except block can handle it
The output is:
Various possible combinations of except block
- except ZeroDivisionError:
- except (ZeroDivisionError):
- except ZeroDivisionError as msg:
- except (ZeroDivisionError) as msg
- except (ZeroDivisionError, ValueError):
- except (ZeroDivisionError, ValueError) as msg:
- except:
If except block is defined for only one exception, then parenthesis is optional, and if except block is defined for more than one except block, then parenthesis is mandatory.
If we use parenthesis then as keyword must be outside the parenthesis.
Invalid Combinations of except Block
- except (ZeroDivisionError as msg)
- except ZeroDivisionError, ValueError:
- except (ZeroDivisionError, ValueError as msg):
Finally, Block in python
The finally keyword is used in a try. except blocks. It defines a block of code to run when the try. except. else block is final.
It is not recommended to place a cleanup code inside the try block because there is no guarantee for the execution of every statement inside the try block.
And also, it is not recommended to place a cleanup code inside the except block, because if there is no exception then except block won’t be executed.
We required someplace to define a cleanup code that executes irrespective of whether the exception raised or not raised and whether the exception is handled or not handled, the best place is nothing but a finally block.
The main purpose of the finally code is to maintain a cleanup code.
Case 1: If No Exception
If there is no exception then try and finally block will execute
Case 2: If the exception raised and handled
If an exception got raised and handled by exception block, then try, except and finally blocks got executed.
Case 3:If exception raised but not handled
If the exception raised but not handled then first try and finally block will execute, and then zeroDivisionError will execute.
OS.__exit(0) vs Finally Block in python
There is only one situation where finally block won’t be executed, i.e., whenever we are using OS._exit(0).
Whenever we are using the OS_exit(0), then the python virtual machine itself will be shut down and in this particular case finally, the block won’t be executed.
Here Zero represents the status Code.
Zero means normal termination.
Non-zero means abnormal termination
The python virtual machine internally uses this status code.
Whether it is zero or non zero, there is no difference in the result of the program.
The following example demonstrates the OS._exit(0)
Difference between Finally Block and Destructor
Control Flow in try-except-finally blocks
The following cases demonstrate the control flow in try-except-finally blocks
Case 1: If there is no exception
If there is no exception occur, then, try block will execute and finally block will also execute, excluding the except and then the remaining code will execute normally.
The output will be : statement-1,2,3,5,6 and normal termination
Case 2: If an exception raised at the statement-2 and corresponding except block matched, then the control automatically goes to except block and then finally block will execute, also remaining blocks will execute normally.
The output will be: statement-1,4,5,6 and normal termination.
Case 3:If an exception raised at the statement-2 and the corresponding except block not matched; then the finally block will execute, and the abnormal termination will happen.
The output will be: statement-1, normal termination.
Case 4: If an exception raised at the statement-4 if an exception raised at the exception block, then finally block will execute and the abnormal termination will happen.
The output will be: statement-5 and abnormal termination.
Case 5:If an exception raised at statements 5 or 6 then it is always abnormal termination.
Nested try-except-finally Blocks
We can take try-except-finally blocks inside the try or except or finally. Hence nesting of try-except-finally blocks is possible.
The general risky code we have to take inside the outer try block and too much risky code we have to take inside the inner try block.
Inside the inner try block if an exception is raised, then the inner except block is responsible to handle it; if it is unable to handle, then the outer except block is responsible for handling it.
The following example demonstrates the nested try-except-finally block
There is no exception in the above program and hence the inner except and outer except block won’t be executed, except that all the blocks will execute.
The output is:
If an exception is raised in the inner try block and the inner except block will match then the inner exception block, and inner finally block will execute.
And the exception is handled by the inner exception itself, and hence the outer exception block won’t execute, but the outer finally block will execute.
The output is:
In the below example, an exception is raised in the inner try block but the corresponding except block not matched, and hence the inner finally block will execute and then the outer except block will execute and then the outer finally block.
The output is:
If an exception is raised in the outer try block, then the corresponding except block will be outer except block, and hence the outer except block and the outer finally will be executed.
The output is:
else Block with try-except-finally in python’s exception Handling
if-else==>if condition is false then only else will be executed
for-else==>If loop executed without a break, then only else block will execute.
while-else==>if loop executed without a break then only else will execute.
The following example demonstrates, if the condition is false, then only the else part will execute.
The output is:
The following example demonstrates the loops with else block
The output is:
The following example demonstrates While executing a loop if the break statement is encountered, then the else part is not going to execute.
The output is:
The following example demonstrates the; when there is no exception in the try block then else part will execute
The output is:
The following example demonstrates the execution of else block with try-except-finally when an exception occurs in the try block then the else part will not execute.
The output is:
The following example demonstrates without except block the else part is not valid and the python will throw a syntax error
The output is:
The following example demonstrates the else block with try-except-finally block
The output is: If the file is available in my system then
The output is: If the file is not present in the system.
The Various Possible Combinations of try-except-else-finally Block
Whenever we are writing try block, compulsory we should write except or finally block, Because the try block without except or finally is invalid.
Whenever we are writing except block, compulsory try block should be there, because the except without try block is always invalid.
Whenever we are writing finally block compulsory try block should be there, because the finally block without try is always invalid.
Whenever we are writing else block compulsory except block should be there, because else without except is always invalid.
We can write multiple except blocks for the same try block with, but we cannot write various else block and finally block.
The try-except-else-finally order is important.
We can write try-except-else-finally block inside try-except-else-finally block
The following table explains the valid syntax for the try-except-else-finally block.