Python Tutorial

Introduction Python Features Python Applications Python System requirements Python Installation Python Examples Python Basics Python Indentation Python Variables Python Data Types Python IDE Python Keywords Python Operators Python Comments Python Pass Statement

Python Conditional Statements

Python if Statement Python elif Statement Python If-else statement Python Switch Case

Python Loops

Python for loop Python while loop Python Break Statement Python Continue Statement Python Goto Statement

Python Arrays

Python Array Python Matrix

Python Strings

Python Strings Python Regex

Python Built-in Data Structure

Python Lists Python Tuples Python Lists vs Tuples Python Dictionary Python Sets

Python Functions

Python Function Python min() function Python max() function Python User-define Functions Python Built-in Functions Python Recursion Anonymous/Lambda Function in Python apply() function in python Python lambda() Function

Python File Handling

Python File Handling Python Read CSV Python Write CSV Python Read Excel Python Write Excel Python Read Text File Python Write Text File Read JSON File in Python

Python Exception Handling

Python Exception Handling Python Errors and exceptions Python Assert

Python OOPs Concept

OOPs Concepts in Python Classes & Objects in Python Inheritance in Python Polymorphism in Python Python Encapsulation Python Constructor Python Super function Python Static Method Static Variables in Python Abstraction in Python

Python Iterators

Iterators in Python Yield Statement In Python Python Yield vs Return

Python Generators

Python Generator

Python Decorators

Python Decorator

Python Functions and Methods

Python Built-in Functions Python String Methods Python List Methods Python Dictionary Methods Python Tuple Methods Python Set Methods

Python Modules

Python Modules Python Datetime Module Python Math Module Python Import Module Python Time ModulePython Random Module Python Calendar Module CSV Module in Python Python Subprocess Module

Python MySQL

Python MySQL Python MySQL Client Update Operation Delete Operation Database Connection Creating new Database using Python MySQL Creating Tables Performing Transactions

Python MongoDB

Python MongoDB

Python SQLite

Python SQLite

Python Data Structure Implementation

Python Stack Python Queue Python Linked List Python Hash Table Python Graph

Python Advance Topics

Speech Recognition in Python Face Recognition in Python Python Linear regression Python Rest API Python Command Line Arguments Python JSON Python Subprocess Python Virtual Environment Type Casting in Python Python Collections Python Attributes Python Commands Python Data Visualization Python Debugger Python DefaultDict Python Enumerate

Python 2

What is Python 2

Python 3

Anaconda in Python 3 Anaconda python 3 installation for windows 10 List Comprehension in Python3

How to

How to Parse JSON in Python How to Pass a list as an Argument in Python How to Install Numpy in PyCharm How to set up a proxy using selenium in python How to create a login page in python How to make API calls in Python How to run Python code from the command prompt How to read data from com port in python How to Read html page in python How to Substring a String in Python How to Iterate through a Dictionary in Python How to convert integer to float in Python How to reverse a string in Python How to take input in Python How to install Python in Windows How to install Python in Ubuntu How to install PIP in Python How to call a function in Python How to download Python How to comment multiple lines in Python How to create a file in Python How to create a list in Python How to declare array in Python How to clear screen in Python How to convert string to list in Python How to take multiple inputs in Python How to write a program in Python How to compare two strings in Python How to create a dictionary in Python How to create an array in Python How to update Python How to compare two lists in Python How to concatenate two strings in Python How to print pattern in Python How to check data type in python How to slice a list in python How to implement classifiers in Python How To Print Colored Text in Python How to open a file in python How to Open a file in python with Path How to run a Python file in CMD How to change the names of Columns in Python How to Concat two Dataframes in Python How to Iterate a List in Python How to learn python Online How to Make an App with Python How to develop a game in python How to print in same line in python How to create a class in python How to find square root in python How to import numy in python How to import pandas in python How to uninstall python How to upgrade PIP in python How to append a string in python How to comment out a block of code in Python How to change a value of a tuple in Python How to append an Array in Python How to Configure Python Interpreter in Eclipse How to plot a Histogram in Python How to Import Files in Python How to Download all Modules in Python How to get Time in seconds in Python How to Practice Python Programming How to plot multiple linear regression in Python How to set font for Text in Python How to Convert WhatsApp Chat Data into a Word Cloud using Python How to Install Tweepy in Python How to Write a Configuration file in Python How to Install Scikit-Learn How to add 2 lists in Python How to assign values to variables in Python and other languages How to build an Auto Clicker using Python How to check if the dictionary is empty in Python How to check the version of the Python Interpreter How to convert Float to Int in Python How to Convert Int to String in Python How to Define a Function in Python How to Install Pandas in Python How to Plot Graphs Using Python How to Program in Python on Raspberry pi How to Reverse a number in Python How to Sort a String in Python How to build a Virtual Assistant Using Python How to Fix an EOF Error in Python How to make a firewall in Python How to clear screen in Python How to Create User Defined Exceptions in Python How to determine if a binary tree is height-balanced How to Import Kaggle Datasets Directly into Google Colab How to Install Python in Kali Linux

Sorting

Python Sort List Sort Dictionary in Python Python sort() function Python Bubble Sort

Programs

Factorial Program in Python Prime Number Program in Python Fibonacci Series Program in Python Leap Year Program in Python Palindrome Program in Python Check Palindrome In Python Calculator Program in Python Armstrong Number Program in Python Python Program to add two numbers Anagram Program in Python Number Pattern Programs in Python Even Odd Program in Python GCD Program in Python Python Exit Program Python Program to check Leap Year Operator Overloading in Python Pointers in Python Python Not Equal Operator Raise Exception in Python Salary of Python Developers in India

Questions

What is a Script in Python What is the re.sub() function in Python After Python What Should I Learn What Does the Percent Sign (%) Mean in Python What is online python free IDE What is Python online compiler What are the Purposes of Python What is Python compiler GDB What is Ipython shell What does base case mean in recursion What does the if __name__ == "__main__" do in Python What is Sleeping Time in Python What is Collaborative Filtering in ML, Python What is the Python Global Interpreter Lock What is None Literal in Python What is the Output of the bool in Python Is Python Case-sensitive when Dealing with Identifiers Is Python Case Sensitive Is Python Object Oriented Programming language

Differences

Difference between Perl and Python Difference between python list and tuple Difference between Input() and raw_input() functions in Python Difference between Python 2 and Python 3 Difference Between Yield And Return In Python Important Difference between Python 2.x and Python 3.x with Example Difference between Package and Module in Python Difference between Expression and Statement in Python Difference between For Loop and While Loop in Python Difference between Module and Package in Python Difference between Sort and Sorted in Python Difference between Overloading and Overriding in Python Python vs HTML Python vs R Python vs Java Python vs PHP While vs For Loop in Python Python Iterator vs Iterable Set Vs List Python

Python Kivy

Kivy Architecture Kivy Buttons Kivy Layouts Kivy Widgets What is Kivy in Python

Python Tkinter

Application to get live USD/INR rate Using Tkinter in Python Application to Search Installed Application using Tkinter in Python Compound Interest GUI Calculator using Tkinter in Python Create a Table Using Tkinter in Python Create First GUI Application using Tkinter in Python File Explorer using Tkinter in Python GUI Calendar using Tkinter in Python GUI to extract lyrics from a song Using Tkinter in Python GUI to Shut down, Restart and Logout from the PC using Tkinter in Python Loan calculator using Tkinter in Python Make Notepad using Tkinter in Python Rank Based Percentile GUI Calculator using Tkinter in Python Screen Rotation app Using Tkinter in Python Spell Corrector GUI using Tkinter in Python Standard GUI Unit Converter using Tkinter in Python Text detection using Tkinter in Python To Do GUI Application using Tkinter in Python Weight Conversion GUI using Tkinter in Python Age calculator using Tkinter Create a Digital Clock Using Tkinter Create a GUI Marksheet using Tkinter Simple GUI Calculator Using Tkinter Simple Registration form using Tkinter Weight Conversion GUI Using Tkinter Color Game Using Tkinter in Python File Explorer in Python using Tkinter Making of Notepad Using Tkinter Python Simple FLAMES Game Using Tkinter Python ToDo GUI Application Using Tkinter Python

Python PyQt5

Compound Interest GUI Calculator using PyQt5 in Python Create Table Using PyQt5 in Python Create the First GUI Application using PyQt5 in Python GUI Calendar using PyQt5 in Python Loan Calculator using PyQt5 in Python Rank Based Percentile GUI Calculator using PyQt5 in Python Simple GUI calculator using PyQt5 in Python Standard GUI Unit Converter using PyQt5 in Python

Misc

Introduction to Scratch programming SKLearn Clustering SKLearn Linear Module Standard Scaler in SKLearn Python Time Library SKLearn Model Selection Standard Scaler in SKLearn Accuracy_score Function in Sklearn Append key Value to Dictionary in Python Cross Entropy in Python Cursor in Python Data Class in Python Imread Python Parameter Passing in Python Program of Cumulative Sum in Python Python Program for Linear Search Python Program to Generate a Random String Read numpy array in Python Scrimba python Sklearn linear Model in Python Scraping data in python Accessing Key-value in Dictionary in Python Find Median of List in Python Linear Regression using Sklearn with Example Problem-solving with algorithm and data structures using Python Python 2.7 data structures Python Variable Scope with Local & Non-local Examples Arguments and parameters in Python Assertion error in python Programs for Printing Pyramid Patterns in Python _name_ in Python Amazon rekognition using python Anaconda python 3.7 download for windows 10 64-bit Android apps for coding in python Augmented reality in python Best app for python Not supported between instances of str and int in python Python comment symbol Python Complex Class Python IDE names Selection Sort Using Python Hypothesis Testing in Python Idle python download for Windows Insertion Sort using Python Merge Sort using Python Python - Binomial Distribution Python Logistic Regression with Sklearn & Scikit Python Random shuffle() method Python variance() function Removing the First Character from the String in Python Adding item to a python dictionary Best books for NLP with Python Best Database for Python Count Number of Keys in Dictionary Python Cross Validation in Sklearn Drop() Function in Python EDA in Python Excel Automation with Python Python Program to Find the gcd of Two Numbers Python Web Development projects Adding a key-value pair to dictionary in Python Python Euclidean Distance Python Filter List Python Fit Transform Python e-book free download Python email utils Python range() Function Python random.seed() function Python PPTX Python Pickle Python Seaborn Python Coroutine Python EOL Python Infinity Python math.cos and math.acos function Python Project Ideas Based On Django Reverse a String in Python Reverse a Number in Python Python Word Tokenizer Python Trigonometric Functions Python try catch exception GUI Calculator in Python Implementing geometric shapes into the game in python Installing Packages in Python Python Try Except Python Sending Email Socket Programming in Python Python CGI Programming Python Data Structures Python abstract class Python Compiler Python K-Means Clustering NSE Tools In Python Operator Module In Python Palindrome In Python Permutations in Python Pillow Python introduction and setup Python Functionalities of Pillow Module Python Argmin Python whois Python JSON Schema Python lock Return Statement In Python Reverse a sentence In Python tell() function in Python Why learn Python? 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Python Raise an Exception

Syntax errors

Perhaps the most frequent type of Python-related complaint is syntax mistakes, also referred to as parsing errors:

while True print('Hello world')
  File "<stdin>", line 1
    while True print('Hello world')
 SyntaxError: invalid syntax

The problematic line is repeated by the parser, which also shows a tiny 'arrow' indicating to the line's initial spot where the mistake was found. The token before the arrow is where the error occurs, or at the very least, where it is noticed. In the example, the method print() is where the error is noticed because a colon (':') is absent.

If the data originated from a script, the file name and line number are printed so you know where to search.

Exceptions

Even if a sentence or formula is syntactically accurate, trying to implement it might result in an error. You will shortly learn how to manage exceptions, which are errors discovered during program execution but are not always fatal. The majority of exceptions, however, are not managed by programs and produce the following error messages:

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 str

What occurred is described in the error message's final sentence. The kinds of exceptions in the sample are ZeroDivisionError, NameError, and TypeError, and the type is displayed along with the message.

The identity of the occurrence of the built-in exception is contained in the string displayed as the exception type. Although it need not apply to user-defined exceptions, this is true for all built-in exceptions (although it is a useful convention). Integrated IDs are standard exception names (not reserved keywords).

Based on the sort of exception and what triggered it, the remainder of the line gives specific information.

In the form of a stack traceback, the error message's previous section demonstrates the environment in which the exception happened. Generally speaking, it includes a stack traceback showing the source lines; however, lines received from standard input are not displayed.

The built-in exclusions and their descriptions are listed in Built-in Exceptions.

The BaseException class must be a child of ExceptionType(). It typically belongs to the exception class as a subtype. It should be noted that the ExceptionType class does not necessarily have to be explicitly shared by it. It might tangentially descended from a subtype of the exception class.

The __init__ function of the BaseException class takes a *args parameter. It implies that when creating an exception, you can send any number of arguments to the exception object.

The raise line is used to report a ValueError exception in the example that follows. It gives the ValueError __init__ function three arguments:

Dealing with Exceptions

Programs that manage particular errors can be written. Consider the example below, which prompts for input until a legitimate integer is entered, but enables the user to halt the program using Control-C or any other method that the operating system supports. Take notice that a user-generated interruption is indicated 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...")

This is how the try sentence functions.

The statements between the try and except clauses, known as the try clause, are first performed.

If there is no exception, the except clause is omitted, and the try statement's processing is complete.

The remainder of the sentence is ignored if an exception arises while the try clause is being executed. The except clause is then performed, and processing moves on from the try/except block if its type fits the exception specified after the except keyword.

If an exception that isn't covered by the except clause occurs, it's passed to the outer try lines; if no controller is found, it's an unhandled exception, and execution halts with the notification shown above.

To define handlers for various exceptions, a try sentence may contain more than one except clause. There will only ever be one manager run. In contrast to other handlers of the same try line, handlers only deal with exceptions that appear in the associated try clause. An except clause may enumerate multiple instances as a parenthesized tuple, as in the accompanying illustration:

Exceptions include RuntimeError, TypeError, and NameError:
... pass

The same class or subclass of the class listed in the except clause is consistent with the exception; the other way around, a derived class listed in the except clause is incompatible with a parent class. For instance, the code below prints B, C, and 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")

You should be aware that if the except clauses are inverted, with except B appearing first, the first matching except clause is triggered.

An exception may have accompanying values, also referred to as the exception's inputs, when it happens. Depending on the exception type, the arguments' presence and types may vary.

After the exception name in the except clause, a variable may be specified. The args property, which is usually present on exception instances and saves the arguments, is bound to the variable.

Built-in exception types include a function called __str__() that allows printing of all parameters without directly accessing the file.args.

try:
    raise Exception('spam', 'eggs')
except Exception as inst:
    print(type(inst))    # the exception instance
    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

For unhandled exceptions, the final section of the message (the "detail") is displayed with the result of the __str__() function.

The basic class shared by all exceptions is called BaseException. The parent class for all non-fatal exceptions is one of its subtypes, Exception. Because they are used to signal that the program should end, exceptions that are not instances of Exception are usually not addressed.

They include KeyboardInterrupt, which is raised when a user wants to pause the application, and SystemExit, which is handled by sys.exit().

As a variable that captures (almost) everything, exception can be used. Nevertheless, it is best practice to be as precise as possible when describing the exception types we plan to manage and to permit any unexpected exceptions to continue to occur.

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

An optional else clause that must come after all except clauses is contained in the attempt... except sentence. It is beneficial for code that needs to run if the attempt phrase doesn't throw an exception. For instance:

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()

Because it prevents unintentionally catching an exception that wasn't generated by the code being secured by the try... except line, using the else clause is preferable to adding more code to the try clause.

Exception handlers deal with all exceptions, including those that arise inside functions that are called (even tangentially) in the try clause, not just those that appear explicitly in the try clause. For instance:

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

Raising Exceptions

The raise line enables the coder to compel the occurrence of a specific exception. As an illustration, report NameError('HiThere')

Traceback (last contact is the most recent):

Line 1 of file "stdin" in module

NameError: HiThere

The only defense that should be made identifies the defense of the exception. Either an exception object or an exception class must be present here (a class that derives from BaseException, such as Exception or one of its subclasses).

By calling the constructor of the exception class with no parameters, it will be automatically created if an exception class is passed:

alias for "raise ValueError()" is raise ValueError.

A more straightforward version of the raise line enables you to re-raise the exception if you need to ascertain whether an exception was raised but don't want to manage it:

>>>
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: HiThere

Exception Chaining

The exception that is being managed will be attached to the unhandled exception and included in the error message if it happens inside an except section:

>>>
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'

Another exception happened while the previous exception was being handled:

Traceback (last contact is the most recent):

Line 4 of file "stdin" in module

Unable to resolve error: RuntimeError

The raise statement enables an optional from phrase to specify whether one exception is a direct result of another: # exc must be exception instance or None.

exc report RuntimeError

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 prior exception served as a clear trigger for the subsequent exception:

Traceback (last contact is the most recent):

Line 4 of file "stdin" in module

RuntimeError: db opening failed

Additionally, automatic exception chaining may be stopped 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>
RuntimeError

Individual Exclusions

By creating a new exception class, programs are able to create their own exceptions (see Classes for more about Python classes). Typically, exceptions should originate from the Exception class, either directly or tangentially.

However, exception classes are typically kept basic, frequently only providing a few attributes that enable information about the mistake to be extracted by handlers for the exception. It is possible to build exception classes that can perform any action that a class can.

Similar to how the names of the standard exceptions are named, the majority of exceptions are specified with names that finish in "Error".

Many common modules define their own exceptions to signify potential flaws in the methods they describe.

Specifying Cleanup Procedures

Another optional phrase in the try statement is designed to specify cleanup procedures that must always be followedTry this, for instance: raise KeyboardInterrupt

In conclusion, put "Goodbye, universe!"

Hello, universe!

KeyboardInterrupt Traceback (last instruction is the most recent):

Line 2 of file "stdin" in module

If there is a finally clause, that task must be completed before the try sentence is complete.

Whether or not the try line results in an exception, the finally section is executed. The situations where an exception happens are covered in more detail in the following points:

An except clause may be used to manage an exception that happens while the try clause is being executed. After the finally clause has been performed, the exception is raised again if it is not managed by an except clause.

When an except or else phrase is being used, an exception could happen. Once more, after the eventually clause has been used, the exception is triggered.

Exceptions are not raised again if the finally sentence performs a break, continue, or return line.

The finally sentence will run just before the break, continue, or return line is executed if the try statement encounters one of those statements.

If there is a return statement in an eventually clause, that value will be returned rather than the value from the return statement in the attempt clause.

For instance:
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

putting ultimately phrase to use

Traceback (last contact is the most recent):

Line 1 of file "stdin" in module

TypeError: invalid argument type(s) for /:'str' and'str' in file "stdin>", line 3

You can see that the final adage is consistently used. Because the except clause does not address the TypeError caused by dividing two strings, it is raised again after the eventually clause has been performed.

The ultimately phrase is useful in real-world applications for giving up external resources, regardless of whether the use of the resource was successful (such as files or network links).

Standard Cleansing Techniques

When an object is no longer needed, some objects define standard cleanup procedures to be followed, regardless of whether the process using the object was successful or unsuccessful.

Consider the example that attempts to open a file and print its data to the screen in the next section.

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

The issue with this code is that it keeps the file open after this section of the code has completed running for an illogically long period of time. Simple scripts don't have this difficulty, but more complex apps might. Using things like files in a way that ensures prompt and thorough cleanup is possible thanks to the with expression.

with line in f: and open("myfile.txt")

line; end; display

The file f is always closed after the command has been executed, even if there was a problem processing the lines.

Objects that offer preset clean-up steps, like files, will make this clear in their literature.

Creating and Managing a Number of Separate Cases

Certain circumstances call for the reporting of multiple exceptions that have occurred. There are other use cases where it is preferable to continue processing and gather multiple failures rather than report the first exception, as is frequently the case in concurrency frameworks when multiple tasks may have failed in tandem.

A collection of exception objects is wrapped by the built-in ExceptionGroup so they can be raised collectively. It can be captured like any other exception because it is an exception in and of itself.

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

Instead of using except, we can manage only the exceptions in the group that fit a specific type by using except*. Each except* clause in the example that follows, which illustrates a stacked exception group, separates out from the group exceptions of a particular class while allowing all other exceptions to spread to other clauses and ultimately be raised again.

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
      +------------------------------------

Keep in mind that cases, not classes, must make up an exception group. This is due to the fact that in reality, the application would usually only raise and catch exceptions that fit 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)

Adding Comments to Exception

Typically, information describing the mistake that has happened is initialized when an exception is generated in order to be raised. In some circumstances, adding details after the exception was detected is helpful.

The add note(note) method on exceptions adds a string to the array of notes for the exception. All comments, in the order they were made, are included in the usual traceback depiction 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
>>>

For instance, when categorizing exceptions into exception groups, we might want to include context information for the particular errors. The notes that follow each exception in the group include the time this mistake happened.

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
    +------------------------------------