Python print error and exit

Recommended Ways to Print To Stderr in Python

In this article, we will discuss how you can print to stderr in Python. Stderr is a standard error stream that we use when we want to print error messages. Therefore if you want to output or print error messages, you can print them to stderr. There are many ways you can print to stderr, and we will discuss those methods in this article one after the other.

Using Logging Module

The logging module in Python keeps a log of messages. Further, we can use this module to print to stderr in Python by configuring some settings of the module. To print to stderr using logging module eventually you will need to use some functions of logging module they are as follows:

• ‘logging.basicConfig()’ function , this function takes the format of the message as an input parameter.
• ‘logging.getLogger()’ this function is used for returning objects from logging module.
• ‘log.warning()’ this function takes the warning message text and prints it to the standard output.

However, you need to use these functions in combination with each other to print your desired error message. Let’s understand with the help of an example.

import logging logging.basicConfig(format = '%(messages)s') log = logging.getLogger() log.warning('ERROR!')

You can also use an alternative way by using the sys module and logging module both and in addition to that by using the ‘print()’ function and setting the parameter in the ‘print()’ function.

import logging import sys logging.basicConfig(format = '%(messages)s') log = logging.getLogger() print(log, file = sys.stderr)

Print To Stderr Using Sys Module

The sys module provides many functions, and one of them is ‘sys.stderr.write()’, which we can use to print error messages. To print to stderr in Python using the ‘sys.stderr.write()’ function, you have to follow its syntax usage. To understand the syntax and usage of this function, let us see an example.

import sys sys.stderr.write("ERROR!")

As you can see, using the sys module to print to standard error is pretty simple and straightforward.

And to learn more about libraries in Python, check this post.

Print To Stderr Using Print() Function

We can also use the ‘print()’ function to print to stderr in Python. Although for printing to standard error using the ‘print()’ function, first we need to set this function’s parameter. To change the parameter, we will use the sys module, which offers many functions. Eventually, we will use sys.stderr to change the parameter of the ‘print()’ function.

import sys print("ERROR", file = sys.stderr)

You can also ‘print()’ function to print string variables to stderr in Python. In order to do that, first, you need to assign a variable to a string you want to print to standard error and then print the following variable.

import sys x = "ERROR MESSAGE" print(x, file = sys.stderr)

Further, you can also print a string which you take from user input and print to stderr in Python.

import sys x = input("Enter the string you want to print to stderr: ") print(x, file = sys)

Enter the string you want to print to stderr: ERROR MESSAGE ERROR MESSAGE

In addition to this, you can also print a variable containing a list to standard error. Let us take an example to furthermore understand how.

import sys x = ["This", "Is", "Error"] print(x, file = sys.stderr)

Print To Stderr Using Special Function

You can also create a function that prints to standard error. The advantage of creating a special function is that if you want to print to standard error frequently, you will not need to use the ‘print()’ and ‘write()’ functions again and again. Nonetheless, the special function will do that for you. In order to understand better, let us use an example.

import sys def error(error_message): print(error_message, file=sys.stderr)

As you can see in this special function, too, we use the ‘print()’ function and sys module and its corresponding ‘sys.stderr’ function. We call the function to print to stderr in Python. Further, you can also use this function to print strings and lists by calling this function.

Similarly, we can use this function to print a variable that contains the string. Again let us take an example to understand.

Further you can also use this function to print a variable that contains list. The function will print that list to stderr in Python.

x = ["ERROR MESSAGE!", "THIS IS EXAMPLE"] error(x)

["ERROR MESSAGE!", "THIS IS EXAMPLE"]

And even furthermore, you can use this function to print to stderr in Python the inputs you take from the user. To do the following, you first need to store the input in a variable and then print that variable through the function we have made.

x = input("Enter the message you want to print to stderr: ") error(x)

Enter the message you want to print to stderr: ERROR MESSAGE! ERROR MESSAGE!

Write Stderr To File

We can also write standard errors to a file in Python. In order to do the following, we need to again use the sys module. Firstly we will assign the path of the file to a variable and then use the ‘sys.stderr’ function to open that path and the ‘print()’ function to write the error message or string in that file. Let us see an example of this method.

import sys pathvariable = 'path/to/yourfile.txt' sys.stderr = open(path, 'w') print("ERROR MESSAGE")

in the above code, we use the ‘open()’ function and give ‘w’ as a parameter to write in that file.

Exit After Printing To Stderr

Sometimes you might want to exit the program after printing to stderr. In order to do that, you need to import the sys module and use the ‘sys.exit()’ function after printing to stderr in Python. Eventually, let’s understand better with the help of an example.

import sys print("ERROR", file = sys.stderr) sys.exit(1)

Although there is also an alternative method you can use to exit the program after printing to stderr in Python.

import sys print("ERROR MESSAGE", file = sys.stderr) raise SystemExit(1)

In the latter method, we raise an exception called ‘SystemExit.’ When we raise this exception, the top-level interpreter in the compiler senses this special class exception and follows the exit routine of this exception. If you notice, we have written ‘(1)’ as a parameter in this special class exception. The reason for this is that ‘SystemExit’ takes integers as a parameter for exiting the code. Therefore we use an integer as a parameter to exit the code.

FAQs

Stderr is a standard error stream in Python that we can use to display error messages.

Printing to stderr means that you can display error messages on the terminal or the output terminal.

You can use various methods discussed in this article which include functions like ‘sys.write()’, ‘print()’, and even more, you can also create your own function to print to stderr in Python.

Conclusion

To summarize this article, we can say that there are many methods you can use to print to stderr in Python, which includes many functions like ‘write()’, ‘print()’. Furthermore, we have also discussed how we can create our own function to print to stderr in Python. You can use the following functions as per your needs of the code and how your code allows you to use them.

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How to Print to stderr in Python?

The stderr in Python is short for Standard error is used to display information that is not intended to be part of the program’s regular output, for example, errors or exceptions. To print stderr in Python, we can use sys.stderr.write() function, besides this, there are other ways as well which we will discuss in this article.

1. Quick Examples of Printing stderr in Python

These examples will give you ideas to print to stderr, we will discuss each method in more detail in upcoming sections.

 # Method 1: using sys.stderr.write() import sys sys.stderr.write("This is an error message \n") # Method 2: using print() - Output Standard Error print("This is an error message", file=sys.stderr) # Method 3: Print stderr with logging module import logging logging.basicConfig(level=logging.ERROR) logging.error("This is an error message") 

2. What is stderr in Python?

The term stderr stands for “standard error” and is used to display error messages, warnings, and diagnostic information that may occur during Python program execution.

stderr is one of the three standard streams in a Unix-like operating system (including Linux and macOS) that provides a way for programs to communicate with the user.

Unlike the regular output stream stdout , which displays regular output, stderr is used to print information that is not intended to be part of the program’s normal output and is often displayed separately from regular output.

3. Print to stderr in Python

Whenever you review experts’ code, you will probably find the sys.stderr.write() function in their codes. They use it to write a string to the standard error stream ( stderr ). This is a simple way to print error messages, warnings, and diagnostic information to the console or log file.

The write() function takes a single argument, a string containing the message to be printed, and returns the number of characters written.

 # sys module needs to be imported import sys # This will print to stderr sys.stderr.write("message to be printed to stderr\n") 

Using the above basic syntax we can use the sys.stderr.write() function in any program to print to stderr. See the following example.

 # Program that open "textfile.txt" import sys import os filename = "testfile.txt" # Handle the exceptions try: with open(filename, "r") as f: # Your code here pass except FileNotFoundError: # Print error message to stderr sys.stderr.write(f"Error: not found.\n") # Exit with non-zero status code sys.exit(1) 

4. print() – Output Standard Error

You can use the print() function in Python to output messages to the console or to a file. By default, print() writes its output to the standard output stream ( stdout ). However, you can redirect its output to the standard error stream ( stderr ) by specifying the file argument as sys.stderr .

See the basic syntax of the print() function to print to stderr in Python:

 # Using print() function to print to stderr import sys print("message to be printed to stderr", file=sys.stderr) 

Using the same syntax we have developed our program that will print error using the print() function.

 # Program for the illustration import sys # Calculate the total cost def calculate_total_cost(price, quantity): if price ", file=sys.stderr) return total_cost calculate_total_cost(-10, 5) 

5. Print stderr with logging module

The logging module in Python provides a way to output log messages to various destinations, including the console, files, and network sockets. By default, the logging module writes Error log messages to the standard error stream ( stderr ).

 import logging logging.basicConfig(level=logging.ERROR) logger = logging.getLogger() logger.error("This is an error message") 

The logging module is thread-safe, which means that you can use it in multi-threaded programs without causing race conditions or other synchronization issues.

It supports multiple log levels, including DEBUG , INFO , WARNING , ERROR , and CRITICAL . By default, It only log messages with a severity level of WARNING or higher.

6. Summary and Conclusion

In this article, you have learned how to print standard errors using stderr in python. We have discussed 3 different methods for printing stderr in python. I hope this article was helpful. If you have any questions, please leave them in the comment section.

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