Understanding Python’s stdin, stdout, and stderr with Examples

Python provides several ways to interact with input and output streams. These streams are essential for communicating with a program during runtime, whether it’s taking input from the user, displaying output, or reporting errors. In Python, these streams are referred to as stdin, stdout, and stderr.

1. stdin, stdout, and stderr Explained.

1.1 stdin (Standard Input).

This is the input stream that a program uses to receive input data. It’s commonly used to read user input or data from files. By default, stdin is connected to the keyboard, allowing the program to read input from the user.

1.2 stdout (Standard Output).

This is the output stream that a program uses to write its normal output. It’s typically used to display information to the user. By default, stdout is connected to the console, so any print statements or output data will be displayed there.

1.3 stderr (Standard Error).

This is the output stream that a program uses to write error messages and diagnostics. It’s separate from stdout so that errors and normal output can be distinguished. Like stdout, stderr is also connected to the console by default.

2. Working with stdin, stdout, and stderr.

2.1 Redirecting standard output to a file.

Python provides a way to redirect the output of a program to a file instead of displaying it on the console. This can be achieved using the `>` operator in the command line.

Create a Python file with the name python_stdin_stdout_stderr.py, and write the below source code in it.

def print_text_to_stdout(text):


if __name__ == "__main__":

    print_text_to_stdout('Hello Python!')

When you run the above Python file in the command line python python_stdin_stdout_stderr.py, it will print the output in the command line console like below.

$ python python_stdin_stdout_stderr.py
Hello Python!

If you want to redirect the output from stdout to a file, you can run the command $ python python_stdin_stdout_stderr.py > out.txt. Then it will create a new file out.txt in the executing folder and the standard output (stdout) of `python_stdin_stdout_stderr.py` will be redirected to the file `out.txt`.

2.2 Using stdin.

To take input from the user, you can use the `input()` function, which reads a line of text from the user and returns it as a string. Below is an example code.

name = input("Enter your name: ")
print("Hello, " + name)

Below is the output of the above Python code.

Enter your name: jerry
Hello, jerry

2.3 Using stdout.

Printing to the standard output (stdout) is typically done using the `print()` function. This function takes one or more arguments and displays them as text.

def stdout_example():

    print("This is a message.")
    x = 10
    y = 20
    print("The sum of", x, "and", y, "is", x + y)

if __name__ == "__main__":


When you run the above source code, it will output the text as below.

This is a message.
The sum of 10 and 20 is 30

Besides the above basic usage of the print function, there are so many ways to use it, you can pass multiple arguments to the print function to implement different features. In the below, I will introduce the print function in detail.

2.4 The print function detail explanation.

The `print()` function in Python is used to display output to the console. It takes one or more arguments and displays their values as text in the console. Here’s the basic syntax:

print(object(s), sep=separator, end=end, file=file, flush=flush)

`object(s)`: This is the content you want to display. It can be one or more objects, separated by commas, that you want to print.

`sep`: This is the separator that is used to separate multiple objects when they are printed. The default separator is one whitespace (‘ ‘).

`end`: This is the string that is printed at the end of the output. The default is a newline character (‘\n‘).

`file`: This parameter allows you to specify the file object where the output will be written. The default is the standard output, which is the console.

`flush`: This parameter determines whether the output is flushed (written) immediately. The default is `False`, meaning that the output may be buffered and not written immediately.

Here are some examples to illustrate how the `print()` function works:

# Printing a single string
print("Hello, world!")

# Printing multiple objects separated by a custom separator
print("Hello", "world", sep=", ")

# Printing with a custom end character
print("Hello", end=" ")

# Printing to a file (in this case, writing to a text file)
with open("output.txt", "w") as f:
    print("Writing to a file", file=f)

# Flushing the output immediately
print("This will be flushed immediately.", flush=True)

Below is the above code output when you run it.

Hello, world!
Hello, world
Hello world!
(Contents written to "output.txt")
This will be flushed immediately.

Remember that the behavior of the `print()` function might vary depending on the environment or context in which you’re using it, such as the Python interpreter, a script, or an integrated development environment (IDE).

2.5 Using stderr.

Error messages and diagnostics should be written to the standard error (stderr) stream. In Python, you can achieve this by using the `sys.stderr` object from the `sys` module:

import sys

def stderr_example():
        # Some code that might raise an error
        x = 1 / 0
    except ZeroDivisionError:
        # Print an error message to sys.stderr
        sys.stderr.write("Error: Cannot divide by zero\n")
        # You can also use print function directly, which implicitly writes to sys.stderr
        #print("Error: Cannot divide by zero", file=sys.stderr)

if __name__ == "__main__":


Below is the above source code execution output.

Error: Cannot divide by zero

3. Conclusion.

Understanding and effectively using stdin, stdout, and stderr is crucial for building interactive and informative programs in Python. These streams provide a way to interact with users, display information, and report errors in a structured manner. Whether you’re taking user input, displaying output, or handling errors, Python’s stdin, stdout, and stderr streams are powerful tools to have at your disposal.

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