One has pretty much control on which information from the traceback to be displayed/logged when catching exceptions.
The code
with open("not_existing_file.txt", 'r') as text:
pass
would produce the following traceback:
Traceback (most recent call last):
File "exception_checks.py", line 19, in <module>
with open("not_existing_file.txt", 'r') as text:
FileNotFoundError: [Errno 2] No such file or directory: 'not_existing_file.txt'
Print/Log the full traceback
As others already mentioned, you can catch the whole traceback by using the traceback module:
import traceback
try:
with open("not_existing_file.txt", 'r') as text:
pass
except Exception as exception:
traceback.print_exc()
This will produce the following output:
Traceback (most recent call last):
File "exception_checks.py", line 19, in <module>
with open("not_existing_file.txt", 'r') as text:
FileNotFoundError: [Errno 2] No such file or directory: 'not_existing_file.txt'
You can achieve the same by using logging:
try:
with open("not_existing_file.txt", 'r') as text:
pass
except Exception as exception:
logger.error(exception, exc_info=True)
Output:
__main__: 2020-05-27 12:10:47-ERROR- [Errno 2] No such file or directory: 'not_existing_file.txt'
Traceback (most recent call last):
File "exception_checks.py", line 27, in <module>
with open("not_existing_file.txt", 'r') as text:
FileNotFoundError: [Errno 2] No such file or directory: 'not_existing_file.txt'
Print/log error name/message only
You might not be interested in the whole traceback, but only in the most important information, such as Exception name and Exception message, use:
try:
with open("not_existing_file.txt", 'r') as text:
pass
except Exception as exception:
print("Exception: {}".format(type(exception).__name__))
print("Exception message: {}".format(exception))
Output:
Exception: FileNotFoundError
Exception message: [Errno 2] No such file or directory: 'not_existing_file.txt'
One has pretty much control on which information from the traceback to be displayed/logged when catching exceptions.
The code
with open("not_existing_file.txt", 'r') as text:
pass
would produce the following traceback:
Traceback (most recent call last):
File "exception_checks.py", line 19, in <module>
with open("not_existing_file.txt", 'r') as text:
FileNotFoundError: [Errno 2] No such file or directory: 'not_existing_file.txt'
Print/Log the full traceback
As others already mentioned, you can catch the whole traceback by using the traceback module:
import traceback
try:
with open("not_existing_file.txt", 'r') as text:
pass
except Exception as exception:
traceback.print_exc()
This will produce the following output:
Traceback (most recent call last):
File "exception_checks.py", line 19, in <module>
with open("not_existing_file.txt", 'r') as text:
FileNotFoundError: [Errno 2] No such file or directory: 'not_existing_file.txt'
You can achieve the same by using logging:
try:
with open("not_existing_file.txt", 'r') as text:
pass
except Exception as exception:
logger.error(exception, exc_info=True)
Output:
__main__: 2020-05-27 12:10:47-ERROR- [Errno 2] No such file or directory: 'not_existing_file.txt'
Traceback (most recent call last):
File "exception_checks.py", line 27, in <module>
with open("not_existing_file.txt", 'r') as text:
FileNotFoundError: [Errno 2] No such file or directory: 'not_existing_file.txt'
Print/log error name/message only
You might not be interested in the whole traceback, but only in the most important information, such as Exception name and Exception message, use:
try:
with open("not_existing_file.txt", 'r') as text:
pass
except Exception as exception:
print("Exception: {}".format(type(exception).__name__))
print("Exception message: {}".format(exception))
Output:
Exception: FileNotFoundError
Exception message: [Errno 2] No such file or directory: 'not_existing_file.txt'
Every programming language has its way of handling exceptions and errors, and Python is no exception.
Python comes with a built-in try…except syntax with which you can handle errors and stop them from interrupting the running of your program.
In this article, you’ll learn how to use that try…except syntax to handle exceptions in your code so they don’t stop your program from running.
What We’ll Cover
- What is an Exception?
- The
try…exceptSyntax - How to Handle Exceptions with
try…except - How to Print an Exception with
try…except - How to Print the Exception Name
- Conclusion
What is an Exception?
In Python, an exception is an error object. It is an error that occurs during the execution of your program and stops it from running – subsequently displaying an error message.
When an exception occurs, Python creates an exception object which contains the type of the error and the line it affects.
Python has many built-in exceptions such as IndexError, NameError, TypeError, ValueError, ZeroDivisionError KeyError, and many more.
The try…except Syntax
Instead of allowing these exceptions to stop your program from running, you can put the code you want to run in a try block and handle the exception in the except block.
The basic syntax of try…except looks like this:
try:
# code to run
except:
# handle error
How to Handle Exceptions with try…except
You can handle each of the exceptions mentioned in this article with try…except. In fact, you can handle all the exceptions in Python with try…except.
For example, if you have a large program and you don’t know whether an identifier exists or not, you can execute what you want to do with the identifier in a try block and handle a possible error in the except block:
try:
print("Here's variable x:", x)
except:
print("An error occured") # An error occured
You can see that the except ran because there’s no variable called x in the code.
Keep reading. Because I will show you how to make those errors look better by showing you how to handle exceptions gracefully.
But what if you want to print the exact exception that occurred? You can do this by assigning the Exception to a variable right in front of the except keyword.
When you do this and print the Exception to the terminal, it is the value of the Exception that you get.
This is how I printed the ZeroDivisionError exception to the terminal:
try:
res = 190 / 0
except Exception as error:
# handle the exception
print("An exception occurred:", error) # An exception occurred: division by zero
And this is how I printed the NameError exception too:
try:
print("Here's variable x:", x)
except Exception as error:
print("An error occurred:", error) # An error occurred: name 'x' is not defined
You can follow this pattern to print any exception to the terminal.
How to Print the Exception Name
What if you want to get the exact exception name and print it to the terminal? That’s possible too. All you need to do is use the type() function to get the type of the exception and then use the __name__ attribute to get the name of the exception.
This is how I modified the ZeroDivisionError example to print the exact exception:
try:
res = 190 / 0
except Exception as error:
# handle the exception
print("An exception occurred:", type(error).__name__) # An exception occurred: ZeroDivisionError
And this is how I modified the other example to print the NameError example:
try:
print("Here's variable x:", x)
except Exception as error:
print("An error occurred:", type(error).__name__) # An error occurred: NameError
Normally, when you encounter an Exception such as NameError and ZeroDivisionError, for example, you get the error in the terminal this way:
You can combine the type() function and that error variable to make the exception look better:
try:
print("Here's variable x:", x)
except Exception as error:
print("An error occurred:", type(error).__name__, "–", error) # An error occurred: NameError – name 'x' is not defined
try:
res = 190 / 0
except Exception as error:
# handle the exception
print("An exception occurred:", type(error).__name__, "–", error) # An exception occurred: ZeroDivisionError – division by zero
Conclusion
As shown in this article, the try…except syntax is a great way to handle errors and prevent your program from stopping during execution.
You can even print that Exception to the terminal by assigning the error to a variable, and get the exact type of the Exception with the type() function.
Happy coding!
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Sometimes, a Python script comes across an unusual situation that it can’t handle, and the program gets terminated or crashes. In this article, we’ll learn How to catch and print exception messages in Python which can help us better understand and debug such errors.
Step-by-Step Guide
When working with Python code, it’s crucial to handle errors effectively. You can achieve this by using ‘try-except’ statements or Python’s logging module, particularly ‘logger.exception()’. Here’s a step-by-step guide on how to catch and print exception messages in Python:
Using ‘try-except’:
1) Enclose the code you want to monitor for exceptions within a ‘try’ block.
2) Catch the exception using ‘except Exception as e’, where ‘e’ captures the exception object.
3) To print the exception message, simply use ‘print(e)’ or process it further.
Using ‘logger.exception()’:
1) Utilize Python’s logging module for a more structured approach.
2) Set up a logger instance using ‘import logging’ and ‘logger = logging.getLogger()’.
3) In your code, use ‘try’ and ‘except’ blocks to catch exceptions.
4) To log the exception message, use ‘logger.exception(“An exception occurred:”)’.
By following these methods, you can anticipate and manage errors in your Python code effectively, providing valuable insights into what went wrong during program execution.
Common Examples of Exception Messages
The most common example of an exception is a “FileNotFoundError” which is raised when you’re importing a file, but it doesn’t exist. Similarly, dividing a number by zero gives a “ZeroDivisionError” and displays a system-generated error message. All these run-time errors are known as exceptions. These exceptions should be caught and reported to prevent the program from being terminated.
In Python, exceptions are handled with the (try… except) statement. The statements which handle the exceptions are placed in the except block whereas the try clause includes the expressions which can raise an exception. Consider an example in which you take a list of integers as input from the user.
# Creating an empty list
new_list =[]
n = int(input("Enter number of elements : "))
for i in range(n):
item = int(input())
# Add the item in the list
new_list.append(item)
print(new_list)The program shown above takes integers as input and creates a list of these integers. If the user enters any character, the program will crash and generate the following output.
Output:
Enter number of elements : 7
23
45
34
65
2a
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-1-ac783af2c9a3> in <module>()
3 n = int(input("Enter number of elements : "))
4 for i in range(n):
----> 5 item = int(input())
6 # Add the item in the list
7 new_list.append(item)
ValueError: invalid literal for int() with base 10: '2a'Different Methods to catch Exception messages
1) Using the Try and Except block
The first method to catch and print the exception messages in Python is by using except and try statements. If the user enters anything except the integer, we want the program to skip that input and move to the next value. In this way, our program will not crash and will catch and print the exception message. This can be done using try and except statements. Inside the try clause, we’ll take input from the user and append it to “new_list” variable. If the user has entered any input except integers mistakenly, the except block will print “Invalid entry” and move towards the next value. In this way, the program continues to run and skip the invalid entries.
# Creating an empty list
new_list =[]
n = int(input("Enter number of elements : "))
for i in range(n):
try:
item = int(input())
# Add the item in the list
new_list.append(item)
except:
print("Invalid Input!")
print("Next entry.")
print("The list entered by user is: ", new_list)Output:
Enter number of elements : 7
65
43
23
4df
Invalid Input!
Next entry.
76
54
90
The list entered by user is: [65, 43, 23, 76, 54, 90]There are various methods to catch and report these exceptions using try and except block. Some of them are listed below along with examples.
Catching and Reporting/Print exceptions messages in Python
This is the second method to catch and print the exception messages in Python. With the help of the print function, you can capture, get and print an exception message in Python. Consider an example in which you have a list containing elements of different data types. You want to divide all the integers by any number. This number on division with the string datatypes will raise “TypeError” and the program will terminate if the exceptions are not handled. The example shown below describes how to handle this problem by capturing the exception using the try-except block and reporting it using the print command.
list_arr=[76,65,87,"5f","7k",78,69]
for elem in list_arr:
try:
print("Result: ", elem/9)
except Exception as e:
print("Exception occurred for value '"+ elem + "': "+ repr(e))Result: 8.444444444444445
Result: 7.222222222222222
Result: 9.666666666666666
Exception occurred for value '5f': TypeError("unsupported operand type(s) for /: 'str' and 'int'")
Exception occurred for value '7k': TypeError("unsupported operand type(s) for /: 'str' and 'int'")
Result: 8.666666666666666
Result: 7.666666666666667print an Error message using try and logger.exception
Another method is to use logger.exception() which produces an error message as well as the log trace, which contains information such as the code line number at which the exception occurred and the time the exception occurred. This logger.exception() method should be included within the except statement; otherwise, it will not function properly.
import logging
logger=logging.getLogger()
num1=int(input("Enter the number 1:"))
num2=int(input("Enter the number 2:"))
try:
print("Result: ", num1/num2)
except Exception as e:
logger.exception("Exception Occured while code Execution: "+ str(e))Enter the number 1:82
Enter the number 2:4
Result: 20.5Suppose a user enters 0 in the 2nd number, then this will raise a “ZeroDivisionError” as shown below.
Enter the number 1:9
Enter the number 2:0
Exception Occured while code Execution: division by zero
Traceback (most recent call last):
File "<ipython-input-27-00694f615c2f>", line 11, in <module>
print("Result: ", num1/num2)
ZeroDivisionError: division by zeroSimilarly, if you’ve two lists consisting of integers and you want to create a list consisting of results obtained by dividing list1 with list2. Suppose you don’t know whether the two lists consist of integers or not.
import logging
logger=logging.getLogger()
list1=[45, 32, 76, 43, 0, 76]
list2=[24, "world", 5, 0, 4, 6]
Result=[]
for i in range(len(list1)):
try:
Result.append(list1[i]/list2[i])
except Exception as e:
logger.exception("Exception Occured while code Execution: "+ str(e))
print(Result)In this example, “world” in the 2nd index of list2 is a string and 32 on division with a string would raise an exception. But, we have handled this exception using try and except block. The logger.exception() command prints the error along with the line at which it occurred and then moves toward the next index. Similarly, all the values are computed and stored in another list which is then displayed at the end of the code.
Exception Occured while code Execution: unsupported operand type(s) for /: 'int' and 'str'
Traceback (most recent call last):
File "<ipython-input-1-5a40f7f6c621>", line 8, in <module>
Result.append(list1[i]/list2[i])
TypeError: unsupported operand type(s) for /: 'int' and 'str'
Exception Occured while code Execution: division by zero
Traceback (most recent call last):
File "<ipython-input-1-5a40f7f6c621>", line 8, in <module>
Result.append(list1[i]/list2[i])
ZeroDivisionError: division by zero
[1.875, 15.2, 0.0, 12.666666666666666]The logger module has another function “logger.error()” which returns only an error message. The following example demonstrates how the logger.error() function may be used to capture exception messages in Python. In this example, we have just replaced logger.exception in the above example with logger.error() function
import logging
logger=logging.getLogger()
list1=[45, 32,76,43,0, 76]
list2=[24, "world", 5, 0, 4, 6]
Result=[]
for i in range(len(list1)):
try:
Result.append(list1[i]/list2[i])
except Exception as e:
logger.error("Exception Occured while code Execution: "+ str(e))
print(Result)Output:
Exception Occured while code Execution: unsupported operand type(s) for /: 'int' and 'str'
Exception Occured while code Execution: division by zero
[1.875, 15.2, 0.0, 12.666666666666666]Catching and printing Specific Exception messages
The previous section was all about how to catch and print exceptions. But, how will you catch a specific exception such as Valueerror, ZeroDivisionError, ImportError, etc? There are two cases if you want to catch one specific exception or multiple specific exceptions. The following example shows how to catch a specific exception.
a = 'hello'
b = 4
try:
print(a + b)
except TypeError as typo:
print(typo)Output:
can only concatenate str (not "int") to strSimilarly, if you want to print the result of “a/b” also and the user enters 0 as an input in variable “b”, then the same example would not be able to deal with ZeroDivisionError. Therefore we have to use multiple Except clauses as shown below.
a = 6
b = 0
try:
print(a + b)
print(a/b)
except TypeError as typo:
print(typo)
except ZeroDivisionError as zer:
print(zer)The same code is now able to handle multiple exceptions.
6
division by zeroTo summarize, all of the methods described above are effective and efficient. You may use any of the methods listed above to catch and print the exception messages in Python depending on your preferences and level of comfort with the method. If you’ve any queries regarding this article, please let us know in the comment section. Your feedback matters a lot to us.
In this article, let us learn about printing error messages from Exceptions with the help of 5 specifically chosen examples.
I have divided this article into 2 major sections
- Printing custom error messages and
- Printing a specific part of the default error message. By “default error message“, I mean the error message that you typically get in the command line if you did not catch a given exception)
Depending on which of the 2 options above you are looking for, you can jump to the respective section of the article using the table of content below.
So, let’s begin!
Printing Custom Error messages
There are 3 ways to print custom error messages in Python. Let us start with the simplest of the 3, which is using a print() statement.
Option#1: Using a simple print() statement
The first and easiest option is to print error messages using a simple print() statement as shown in the example below.
try:
#Some Problematic code that can produce Exceptions
x = 5/0
except Exception as e:
print('A problem has occurred from the Problematic code: ', e)Running this code will give the output below.
A problem has occurred from the Problematic code: division by zeroHere the line “x = 5/0″ in Example 1 above raised a “ZeroDivisionError” which was caught by our except clause and the print() statement printed the default error message which is “division by zero” to the standard output.
One thing to note here is the line “except Exception as e“. This line of code’s function is to catch all possible exceptions, whichever occurs first as an “Exception” object. This object is stored in the variable “e” (line 4), which returns the string ‘division by zero‘ when used with the print() statement (line 5).
To summarize if you wish to print out the default error message along with a custom message use Option#1.
This is the simplest way to print error messages in python. But this option of putting your custom messages into print statements might not work in cases where you might be handling a list of exceptions using a single except clause. If you are not exactly sure how to catch a list of exceptions using a single except clause, I suggest reading my other article in the link below.
Python: 3 Ways to Catch Multiple Exceptions in a single “except” clause
There I have explained the 3 ways through which you can catch a list of exceptions along with tips on when is the right situation to catch each of these exceptions.
Now that we have learned how to print the default string which comes with an exception object, let us next learn how to customize the message that e carried (the string ‘division by zero‘) and replace that with our own custom error message.
Option#2: Using Custom Exception classes to get customized error messages
In Python, you can define your own custom exception classes by inheriting from another Exception class as shown in the code below.
class MyOwnException(Exception):
def __str__(self):
return 'My Own Exception has occurred'
def __repr__(self):
return str(type(self))
try:
raise MyOwnException
except MyOwnException as e:
print(e)
print(repr(e))How to choose the exception class to inherit from?
In the above example, I have inherited from the Exception class in python, but the recommended practice is to choose a class that closely resembles your use-case.
For example, say you are trying to work with a string type object and you are given a list type object instead, here you should inherit your custom exception from TypeError since this Exception type closely resembles your use case which is “the variable is not of expected type”.
If you are looking for getting an appropriate Exception class to inherit from, I recommend having a look at all the built-in exceptions from the official python page here. For the sake of keeping this example simple, I have chosen the higher-level exception type named “Exception” class to inherit from.
In the code below, we are collecting values from the user and to tell the user that there is an error in the value entered we are using the ValueError class.
class EnteredGarbageError(ValueError):
def __str__(self):
return 'You did not select an option provided!'
try:
options = ['A', 'B', 'C']
x = input('Type A or B or C: ')
if x not in options:
raise EnteredGarbageError
else:
print ('You have chosen: ', x)
except EnteredGarbageError as err:
print(err)Now that we understand how to choose a class to inherit from, let us next have a look at how to customize the default error messages that these classes return.
How to customize the error message in our custom exception class?
To help us achieve our purpose here which is to print some custom error messages, all objects in python come with 2 methods named __str__ and __repr__. This is pronounced “dunder-str” and “dunder-repr” where “dunder” is short for “double underscore”.
Dunder-str method:
The method __str__ returns a string and this is what the built-in print() function calls whenever we pass it an object to print.
print(object1)In the line above, python will call the __str__ method of the object and prints out the string returned by that method.
Let us have a look at what python’s official documentation over at python.org has to say about the str method.
In simpler words, the str method returns a human-readable string for logging purposes, and when this information is passed to the built-in function print(), the string it returns gets printed.
So since our implementation of str returns the string “My Own Exception has occurred” this string got printed on the first line of the exception message.
Dunder-repr method:
__repr__ is another method available in all objects in python.
Where it differs from the dunder-str method is the fact that while the __str__ is used for getting a “friendly message”, the __repr__ method is used for getting, a more of a, “formal message”. You can think of str as a text you got from your friends and repr as a notice you got from a legal representative!
The below screenshot from python’s official documentation explains the use of __repr__ method.
Again, in simpler words, repr is typically used to print some “formal” or “official” information about an object in Python
In our Example 2 above, the repr method returned the class name using the built-in type() function.
Next, let us see another variation where we can print different error messages using a single Exception class without making a custom class.
Option#3: Custom Error messages from the raise statement
try:
raise Exception('I wish to print this message')
except Exception as error:
print(error)Lucky for us, python has made this process incredibly simple! Just pass in the message as an argument to the type of exception you wish to raise and this will print that custom message instead!
In the above code, we are throwing an exception of type “Exception” by calling its constructor and giving the custom message as an argument, which then overrides the default __str__ method to return the string passed in.
If you wish to learn more about raise statement, I suggest reading my other article in the link below
Python: Manually throw/raise an Exception using the “raise” statement
where I have explained 3 ways you can use the raise statement in python and when to use each.
But when to use option 2 and when to use option 3?
On the surface, Option#3 of passing in the custom message may look like it made option#2 of using custom classes useless. But the main reason to use Option#2 is the fact that Option#2 can be used to override more than just the __str__ method.
Let’s next move on to section 2 of this article and look at how to choose a specific part of the default error message (the error printed on the console when you don’t catch an exception) and use that to make our own error messages
Choosing Parts of Default Error Messages to print
To understand what I mean by “Default Error Message” let us see an example
raise ValueError("This is an ValueError")This line when run, will print the following error message
Traceback (most recent call last):
File "<ipython-input-24-57127e33a735>", line 1, in <module>
raise ValueError("This is an ValueError")
ValueError: This is an ValueErrorThis error message contains 3 Parts
- Exception Type (ValueError)
- Error message (This is an ValueError)
- and the stack trace (the 1st few lines showing us where exactly in the program the exception has occurred)
For visual learners out there we have also made an interesting video on that topic!
The information needed
- to extract and use each of the individual pieces of information listed above and
- when to use what piece of information
is already covered with the help of several examples in my previous article in the link below
Python Exceptions: Getting and Handling Error Messages as strings.
And with that I will end this article!
If you are looking for another interesting read, try the article in the link below.
Exceptions in Python: Everything You Need To Know!
The above article covers all the basics of Exception handling in Python like
- when and how to ignore exceptions
- when and how to retry the problematic code that produced the exception and
- when and how to log the errors
I hope you enjoyed reading this article and got some value from it.
Feel free to share it with your friends and colleagues!