In Python, the try-except block is used to handle exceptions and errors gracefully, ensuring that your program can continue running even when something goes wrong. This tutorial will cover the basics of using the try-except block, its syntax, and best practices.
Exception handling allows you to manage errors in your code by capturing exceptions and taking appropriate actions instead of letting the program crash. An exception is an error that occurs during the execution of a program, and handling these exceptions ensures your program can respond to unexpected situations.
The try-except block in Python is used to catch and handle exceptions. The code that might cause an exception is placed inside the try block, and the code to handle the exception is placed inside the except block.
Syntax
Following is the basic syntax of the try-except block in Python −
try:# Code that might cause an exception
risky_code()except SomeException as e:# Code that runs if an exception occurs
handle_exception(e)
Example
In this example, if you enter a non-numeric value, a ValueError will be raised. If you enter zero, a ZeroDivisionError will be raised. The except blocks handle these exceptions and prints appropriate error messages −
try:
number =int(input("Enter a number: "))
result =10/ number
print(f"Result: {result}")except ZeroDivisionError as e:print("Error: Cannot divide by zero.")except ValueError as e:print("Error: Invalid input. Please enter a valid number.")
Handling Multiple Exceptions
In Python, you can handle multiple types of exceptions using multiple except blocks within a single try-except statement. This allows your code to respond differently to different types of errors that may occur during execution.
Syntax
Following is the basic syntax for handling multiple exceptions in Python −
try:# Code that might raise exceptions
risky_code()except FirstExceptionType:# Handle the first type of exception
handle_first_exception()except SecondExceptionType:# Handle the second type of exception
handle_second_exception()# Add more except blocks as needed for other exception types
Example
In the following example −
If you enter zero as the divisor, a “ZeroDivisionError” will be raised, and the corresponding except ZeroDivisionError block will handle it by printing an error message.
If you enter a non-numeric input for either the dividend or the divisor, a “ValueError” will be raised, and the except ValueError block will handle it by printing a different error message.
try:
dividend =int(input("Enter the dividend: "))
divisor =int(input("Enter the divisor: "))
result = dividend / divisor
print(f"Result of division: {result}")except ZeroDivisionError:print("Error: Cannot divide by zero.")except ValueError:print("Error: Invalid input. Please enter valid integers.")
Using Else Clause with Try-Except Block
In Python, the else clause can be used in conjunction with the try-except block to specify code that should run only if no exceptions occur in the try block. This provides a way to differentiate between the main code that may raise exceptions and additional code that should only execute under normal conditions.
Syntax
Following is the basic syntax of the else clause in Python −
try:# Code that might raise exceptions
risky_code()except SomeExceptionType:# Handle the exception
handle_exception()else:# Code that runs if no exceptions occurred
no_exceptions_code()
Example
In the following example −
If you enter a non-integer input, a ValueError will be raised, and the corresponding except ValueError block will handle it.
If you enter zero as the denominator, a ZeroDivisionError will be raised, and the corresponding except ZeroDivisionError block will handle it.
If the division is successful (i.e., no exceptions are raised), the else block will execute and print the result of the division.
try:
numerator =int(input("Enter the numerator: "))
denominator =int(input("Enter the denominator: "))
result = numerator / denominator
except ValueError:print("Error: Invalid input. Please enter valid integers.")except ZeroDivisionError:print("Error: Cannot divide by zero.")else:print(f"Result of division: {result}")
The Finally Clause
The finally clause provides a mechanism to guarantee that specific code will be executed, regardless of whether an exception is raised or not. This is useful for performing cleanup actions such as closing files or network connections, releasing locks, or freeing up resources.
Syntax
Following is the basic syntax of the finally clause in Python −
try:# Code that might raise exceptions
risky_code()except SomeExceptionType:# Handle the exception
handle_exception()else:# Code that runs if no exceptions occurred
no_exceptions_code()finally:# Code that always runs, regardless of exceptions
cleanup_code()
Example
In this example −
If the file “example.txt” exists, its content is read and printed, and the else block confirms the successful operation.
If the file is not found (FileNotFoundError), an appropriate error message is printed in the except block.
The finally block ensures that the file is closed (file.close()) regardless of whether the file operation succeeds or an exception occurs.
try:file=open("example.txt","r")
content =file.read()print(content)except FileNotFoundError:print("Error: The file was not found.")else:print("File read operation successful.")finally:if'file'inlocals():file.close()print("File operation is complete.")
Exception handling in Python refers to managing runtime errors that may occur during the execution of a program. In Python, exceptions are raised when errors or unexpected situations arise during program execution, such as division by zero, trying to access a file that does not exist, or attempting to perform an operation on incompatible data types.
Python provides two very important features to handle any unexpected error in your Python programs and to add debugging capabilities in them −
Exception Handling − This would be covered in this tutorial. Here is a list of standard Exceptions available in Python: Standard Exceptions.
An assertion is a sanity-check that you can turn on or turn off when you are done with your testing of the program.
The easiest way to think of an assertion is to liken it to a raise-if statement (or to be more accurate, a raise-if-not statement). An expression is tested, and if the result comes up false, an exception is raised.
Assertions are carried out by the assert statement, the newest keyword to Python, introduced in version 1.5.
Programmers often place assertions at the start of a function to check for valid input, and after a function call to check for valid output.
The assert Statement
When it encounters an assert statement, Python evaluates the accompanying expression, which is hopefully true. If the expression is false, Python raises an AssertionError exception.
The syntax for assert is −
assert Expression[, Arguments]
If the assertion fails, Python uses ArgumentExpression as the argument for the AssertionError. AssertionError exceptions can be caught and handled like any other exception using the try-except statement, but if not handled, they will terminate the program and produce a trace back.
Example
Here is a function that converts a temperature from degrees Kelvin to degrees Fahrenheit. Since zero degrees Kelvin is as cold as it gets, the function bails out if it sees a negative temperature −
defKelvinToFahrenheit(Temperature):assert(Temperature >=0),"Colder than absolute zero!"return((Temperature-273)*1.8)+32print(KelvinToFahrenheit(273))print(int(KelvinToFahrenheit(505.78)))print(KelvinToFahrenheit(-5))
When the above code is executed, it produces the following result −
32.0
451
Traceback (most recent call last):
File "test.py", line 9, in <module>
print (KelvinToFahrenheit(-5))
File "test.py", line 4, in KelvinToFahrenheit
assert (Temperature >= 0),"Colder than absolute zero!"
AssertionError: Colder than absolute zero!
What is Exception?
An exception is an event, which occurs during the execution of a program that disrupts the normal flow of the program’s instructions. In general, when a Python script encounters a situation that it cannot cope with, it raises an exception. An exception is a Python object that represents an error.
When a Python script raises an exception, it must either handle the exception immediately otherwise it terminates and quits.
Handling an Exception in Python
If you have some suspicious code that may raise an exception, you can defend your program by placing the suspicious code in a try: block. After the try: block, include an except: statement, followed by a block of code which handles the problem as elegantly as possible.
The try: block contains statements which are susceptible for exception
If exception occurs, the program jumps to the except: block.
If no exception in the try: block, the except: block is skipped.
Syntax
Here is the simple syntax of try…except…else blocks −
try:
You do your operations here
......................except ExceptionI:
If there is ExceptionI, then execute this block.except ExceptionII:
If there is ExceptionII, then execute this block.......................else:
If there is no exception then execute this block.
Here are few important points about the above-mentioned syntax −
A single try statement can have multiple except statements. This is useful when the try block contains statements that may throw different types of exceptions.
You can also provide a generic except clause, which handles any exception.
After the except clause(s), you can include an else clause. The code in the else block executes if the code in the try: block does not raise an exception.
The else block is a good place for code that does not need the try: block’s protection.
Example
This example opens a file, writes content in the file and comes out gracefully because there is no problem at all.
try:
fh =open("testfile","w")
fh.write("This is my test file for exception handling!!")except IOError:print("Error: can\'t find file or read data")else:print("Written content in the file successfully")
fh.close()
It will produce the following output −
Written content in the file successfully
However, change the mode parameter in open() function to “w”. If the testfile is not already present, the program encounters IOError in except block, and prints following error message −
Error: can't find file or read data
Example
This example tries to open a file where you do not have write permission, so it raises an exception −
try:
fh =open("testfile","r")
fh.write("This is my test file for exception handling!!")except IOError:print("Error: can\'t find file or read data")else:print("Written content in the file successfully")
This produces the following result −
Error: can't find file or read data
The except Clause with No Exceptions
You can also use the except statement with no exceptions defined as follows −
try:
You do your operations here;......................except:
If there isany exception, then execute this block.......................else:
If there is no exception then execute this block.
This kind of a try-except statement catches all the exceptions that occur. Using this kind of try-except statement is not considered a good programming practice though, because it catches all exceptions but does not make the programmer identify the root cause of the problem that may occur.
The except Clause with Multiple Exceptions
You can also use the same except statement to handle multiple exceptions as follows −
try:
You do your operations here;......................except(Exception1[, Exception2[,...ExceptionN]]]):
If there isany exception from the given exception list,
then execute this block.......................else:
If there is no exception then execute this block.
The try-finally Clause
You can use a finally: block along with a try: block. The finally block is a place to put any code that must execute, whether the try-block raised an exception or not. The syntax of the try-finally statement is this −
try:
You do your operations here;......................
Due to any exception, this may be skipped.finally:
This would always be executed.......................
You cannot use else clause as well along with a finally clause.
Example
try:
fh =open("testfile","w")
fh.write("This is my test file for exception handling!!")finally:print("Error: can\'t find file or read data")
If you do not have permission to open the file in writing mode, then this will produce the following result −
Error: can't find file or read data
Same example can be written more cleanly as follows −
try:
fh =open("testfile","w")try:
fh.write("This is my test file for exception handling!!")finally:print("Going to close the file")
fh.close()except IOError:print("Error: can\'t find file or read data")
When an exception is thrown in the try block, the execution immediately passes to the finally block. After all the statements in the finally block are executed, the exception is raised again and is handled in the except statements if present in the next higher layer of the try-except statement.
Argument of an Exception
An exception can have an argument, which is a value that gives additional information about the problem. The contents of the argument vary by exception. You capture an exception’s argument by supplying a variable in the except clause as follows −
try:
You do your operations here;......................except ExceptionType, Argument:
You can print value of Argument here...
If you write the code to handle a single exception, you can have a variable follow the name of the exception in the except statement. If you are trapping multiple exceptions, you can have a variable follow the tuple of the exception.
This variable receives the value of the exception mostly containing the cause of the exception. The variable can receive a single value or multiple values in the form of a tuple. This tuple usually contains the error string, the error number, and an error location.
Example
Following is an example for a single exception −
# Define a function here.deftemp_convert(var):try:returnint(var)except ValueError as Argument:print("The argument does not contain numbers\n", Argument)# Call above function here.
temp_convert("xyz")
This produces the following result −
The argument does not contain numbers
invalid literal for int() with base 10: 'xyz'
Raising an Exceptions
You can raise exceptions in several ways by using the raise statement. The general syntax for the raise statement is as follows.
Syntax
raise [Exception [, args [, traceback]]]
Here, Exception is the type of exception (for example, NameError) and argument is a value for the exception argument. The argument is optional; if not supplied, the exception argument is None.
The final argument, trace back, is also optional (and rarely used in practice), and if present, is the traceback object used for the exception.
Example
An exception can be a string, a class or an object. Most of the exceptions that the Python core raises are classes, with an argument that is an instance of the class. Defining new exceptions is quite easy and can be done as follows −
deffunctionName( level ):if level <1:raise"Invalid level!", level
# The code below to this would not be executed# if we raise the exception
Note: In order to catch an exception, an “except” clause must refer to the same exception thrown either class object or simple string. For example, to capture above exception, we must write the except clause as follows −
try:
Business Logic here...except"Invalid level!":
Exception handling here...else:
Rest of the code here...
User-Defined Exceptions
Python also allows you to create your own exceptions by deriving classes from the standard built-in exceptions.
Here is an example related to RuntimeError. Here, a class is created that is subclassed from RuntimeError. This is useful when you need to display more specific information when an exception is caught.
In the try block, the user-defined exception is raised and caught in the except block. The variable e is used to create an instance of the class Networkerror.
Here is a list of Standard Exceptions available in Python −
Sr.No.
Exception Name & Description
1
ExceptionBase class for all exceptions
2
StopIterationRaised when the next() method of an iterator does not point to any object.
3
SystemExitRaised by the sys.exit() function.
4
StandardErrorBase class for all built-in exceptions except StopIteration and SystemExit.
5
ArithmeticErrorBase class for all errors that occur for numeric calculation.
6
OverflowErrorRaised when a calculation exceeds maximum limit for a numeric type.
7
FloatingPointErrorRaised when a floating point calculation fails.
8
ZeroDivisionErrorRaised when division or modulo by zero takes place for all numeric types.
9
AssertionErrorRaised in case of failure of the Assert statement.
10
AttributeErrorRaised in case of failure of attribute reference or assignment.
11
EOFErrorRaised when there is no input from either the raw_input() or input() function and the end of file is reached.
12
ImportErrorRaised when an import statement fails.
13
KeyboardInterruptRaised when the user interrupts program execution, usually by pressing Ctrl+c.
14
LookupErrorBase class for all lookup errors.
15
IndexErrorRaised when an index is not found in a sequence.
16
KeyErrorRaised when the specified key is not found in the dictionary.
17
NameErrorRaised when an identifier is not found in the local or global namespace.
18
UnboundLocalErrorRaised when trying to access a local variable in a function or method but no value has been assigned to it.
19
EnvironmentErrorBase class for all exceptions that occur outside the Python environment.
20
IOErrorRaised when an input/ output operation fails, such as the print statement or the open() function when trying to open a file that does not exist.
21
IOErrorRaised for operating system-related errors.
22
SyntaxErrorRaised when there is an error in Python syntax.
23
IndentationErrorRaised when indentation is not specified properly.
24
SystemErrorRaised when the interpreter finds an internal problem, but when this error is encountered the Python interpreter does not exit.
25
SystemExitRaised when Python interpreter is quit by using the sys.exit() function. If not handled in the code, causes the interpreter to exit.
26
TypeErrorRaised when an operation or function is attempted that is invalid for the specified data type.
27
ValueErrorRaised when the built-in function for a data type has the valid type of arguments, but the arguments have invalid values specified.
28
RuntimeErrorRaised when a generated error does not fall into any category.
29
NotImplementedErrorRaised when an abstract method that needs to be implemented in an inherited class is not actually implemented.
In Python, syntax errors are among the most common errors encountered by programmers, especially those who are new to the language. This tutorial will help you understand what syntax errors are, how to identify them, and how to fix them.
What is a Syntax Error?
A syntax error in Python (or any programming language) is an error that occurs when the code does not follow the syntax rules of the language. Syntax errors are detected by the interpreter or compiler at the time of parsing the code, and they prevent the code from being executed.
These errors occur because the written code does not conform to the grammatical rules of Python, making it impossible for the interpreter to understand and execute the commands.
Common Causes of Syntax Errors
Following are the common causes of syntax errors −Missing colons (:) after control flow statements (e.g., if, for, while) − Colons are used to define the beginning of an indented block, such as in functions, loops, and conditionals.
# Error: Missing colon (:) after the if statementifTrueprint("This will cause a syntax error")
Incorrect indentation − Python uses indentation to define the structure of code blocks. Incorrect indentation can lead to syntax errors.
# Error: The print statement is not correctly indenteddefexample_function():print("This will cause a syntax error")
Misspelled keywords or incorrect use of keywords.
# Error: 'print' is misspelled as 'prnt'
prnt("Hello, World!")
Unmatched parentheses, brackets, or braces − Python requires that all opening parentheses (, square brackets [, and curly braces { have corresponding closing characters ), ], and }.
# Error: The closing parenthesis is missing.print("This will cause a syntax error"
How to Identify Syntax Errors
Identifying syntax errors in Python can sometimes be easy, especially when you get a clear error message from the interpreter. However, other times, it can be a bit tricky. Here are several ways to help you identify and resolve syntax errors effectively −
Reading Error Messages
When you run a Python script, the interpreter will stop execution and display an error message if it encounters a syntax error. Understanding how to read these error messages is very important.
Example Error Message
File "script.py", line 1print("Hello, World!"^
SyntaxError: EOL while scanning string literal
This error message can be broken down into parts −
File “script.py”: Indicates the file where the error occurred.
line 1: Indicates the line number in the file where the interpreter detected the error.
print(“Hello, World!”: Shows the line of code with the error.
^: Points to the location in the line where the error was detected.
Using an Integrated Development Environment (IDE)
IDEs are helpful in identifying syntax errors as they often provide real-time feedback. Here are some features of IDEs that helps in identifying syntax errors −
Syntax Highlighting: IDEs highlight code syntax in different colors. If a part of the code is incorrectly colored, it may indicate a syntax error.
Linting: Tools like pylint or flake8 check your code for errors and stylistic issues.
Error Underlining: Many IDEs underline syntax errors with a red squiggly line.
Tooltips and Error Messages: Hovering over the underlined code often provides a tooltip with a description of the error.
Popular IDEs with these features include PyCharm, Visual Studio Code, and Jupyter Notebook.
Running Code in Small Chunks
If you have a large script, it can be useful to run the code in smaller chunks. This can help isolate the part of the code causing the syntax error.
For example, if you have a script with multiple functions and you get a syntax error, try running each function independently to narrow down where the error might be.
Using Version Control
Version control systems like Git can help you track changes to your code. If you encounter a syntax error, you can compare the current version of the code with previous versions to see what changes might have introduced the error.
Fixing Syntax Errors
Fixing syntax errors in Python involves understanding the error message provided by the interpreter, identifying the exact issue in the code, and then making the necessary corrections. Here is a detailed guide on how to systematically approach and fix syntax errors −
Read the Error Message Carefully
Pythons error messages are quite informative. They indicate the file name, line number, and the type of syntax error −
Example Error Message
Assume we have written a print statement as shown below −
print("Hello, World!"
The following message indicates that there is a syntax error on line 1, showing that somewhere in the code, a parenthesis was left unclosed, which leads to a syntax error.
File "/home/cg/root/66634a37734ad/main.py", line 1print("Hello, World!"^
SyntaxError:'(' was never closed
To fix this error, you need to ensure that every opening parenthesis has a corresponding closing parenthesis. Here is the corrected code −
print("Hello, World!")
Locate the Error
To locate the error, you need to go to the line number mentioned in the error message. Additionally, check not only the indicated line but also the lines around it, as sometimes the issue might stem from previous lines.
Understand the Nature of the Error
To understand the nature of the error, you need to identify what type of syntax error it is (e.g., missing parenthesis, incorrect indentation, missing colon, etc.). Also, refer to common syntax errors and their patterns.