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Python sys Module

Last Updated : 18 Nov, 2023
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The sys module in Python provides various functions and variables that are used to manipulate different parts of the Python runtime environment. It allows operating on the interpreter as it provides access to the variables and functions that interact strongly with the interpreter. Let’s consider the below example.

Sys Module in Python

Python sys.version

In this example, sys.version is used which returns a string containing the version of Python Interpreter with some additional information. This shows how the sys module interacts with the interpreter. Let us dive into the article to get more information about the sys module.


import sys


3.6.9 (default, Oct  8 2020, 12:12:24) 
[GCC 8.4.0]

Input and Output using Python Sys

The sys modules provide variables for better control over input or output. We can even redirect the input and output to other devices. This can be done using three variables – 

  • stdin
  • stdout
  • stderr

Read from stdin in Python

stdin: It can be used to get input from the command line directly. It is used for standard input. It internally calls the input() method. It, also, automatically adds ‘\n’ after each sentence.


This code reads lines from the standard input until the user enters ‘q’. For each line, it prints “Input : ” followed by the line. Finally, it prints “Exit”.


import sys
for line in sys.stdin:
    if 'q' == line.rstrip():
    print(f'Input : {line}')


Python sys.stdout Method

stdout: A built-in file object that is analogous to the interpreter’s standard output stream in Python. stdout is used to display output directly to the screen console. Output can be of any form, it can be output from a print statement, an expression statement, and even a prompt direct for input. By default, streams are in text mode. In fact, wherever a print function is called within the code, it is first written to sys.stdout and then finally on to the screen. 


This code will print the string “Geeks” to the standard output. The sys.stdout object represents the standard output stream, and the write() method writes the specified string to the stream.


import sys



stderr function in Python

stderr: Whenever an exception occurs in Python it is written to sys.stderr. 


This code will print the string “Hello World” to the standard error stream. The sys.stderr object represents the standard error stream, and the print() function writes the specified strings to the stream.


import sys
def print_to_stderr(*a):
    print(*a, file = sys.stderr)
print_to_stderr("Hello World")



Command Line Arguments

Command-line arguments are those which are passed during the calling of the program along with the calling statement. To achieve this using the sys module, the sys module provides a variable called sys.argv. It’s main purpose are:

  • It is a list of command-line arguments.
  • len(sys.argv) provides the number of command-line arguments.
  • sys.argv[0] is the name of the current Python script.

Example: Consider a program for adding numbers and the numbers are passed along with the calling statement.

This code calculates the sum of the command-line arguments passed to the Python script. It imports the sys module to access the command-line arguments and then iterates over the arguments, converting each one to an integer and adding it to a running total. Finally, it prints the total sum of the arguments.


import sys
n = len(sys.argv)
print("Total arguments passed:", n)
print("\nName of Python script:", sys.argv[0])
print("\nArguments passed:", end = " ")
for i in range(1, n):
    print(sys.argv[i], end = " ")
Sum = 0
for i in range(1, n):
    Sum += int(sys.argv[i])
print("\n\nResult:", Sum)



Exiting the Program

sys.exit([arg]) can be used to exit the program. The optional argument arg can be an integer giving the exit or another type of object. If it is an integer, zero is considered “successful termination”.

Note: A string can also be passed to the sys.exit() method.


This code checks if the age is less than 18. If it is, it exits the program with a message “Age less than 18”. Otherwise, it prints the message “Age is not less than 18”. The sys.exit() function takes an optional message as an argument, which is displayed when the program exits.


import sys
age = 17
if age < 18:
    sys.exit("Age less than 18")    
    print("Age is not less than 18")


An exception has occurred, use %tb to see the full traceback.

SystemExit: Age less than 18

Working with Modules

sys.path is a built-in variable within the sys module that returns the list of directories that the interpreter will search for the required module. 

When a module is imported within a Python file, the interpreter first searches for the specified module among its built-in modules. If not found it looks through the list of directories defined by sys.path.

Note: sys.path is an ordinary list and can be manipulated.

Example 1: Listing out all the paths

This code will print the system paths that Python uses to search for modules. The sys.path list contains the directories that Python will search for modules when it imports them.


import sys


Example 2: Truncating the value of sys.path

This code will print an error message because the pandas module is not in the sys.path list. The sys.path list is a list of directories that Python will search for modules when it imports them. By setting the sys.path list to an empty list, the code effectively disables Python’s ability to find any modules.


import sys
sys.path = []
import pandas


ModuleNotFoundError: No module named 'pandas'

sys.modules return the name of the Python modules that the current shell has imported.


This code will print a dictionary of all the modules that have been imported by the current Python interpreter. The dictionary keys are the module names, and the dictionary values are the module objects.


import sys


Reference Count

sys.getrefcount() method is used to get the reference count for any given object. This value is used by Python as when this value becomes 0, the memory for that particular value is deleted.


This code prints the reference count of the object a. The reference count of an object is the number of times it is referenced by other objects. An object is garbage collected when its reference count reaches 0, meaning that it is no longer referenced by any other objects


import sys
a = 'Geeks'



More Functions in Python sys

Function Description
sys.setrecursionlimit() sys.setrecursionlimit() method is used to set the maximum depth of the Python interpreter stack to the required limit.
sys.getrecursionlimit() method sys.getrecursionlimit() method is used to find the current recursion limit of the interpreter or to find the maximum depth of the Python interpreter stack.
sys.settrace() It is used for implementing debuggers, profilers and coverage tools. This is thread-specific and must register the trace using threading.settrace(). On a higher level, sys.settrace() registers the traceback to the Python interpreter
sys.setswitchinterval() method sys.setswitchinterval() method is used to set the interpreter’s thread switch interval (in seconds).
sys.maxsize() It fetches the largest value a variable of data type Py_ssize_t can store.
sys.maxint maxint/INT_MAX denotes the highest value that can be represented by an integer.
sys.getdefaultencoding() method sys.getdefaultencoding() method is used to get the current default string encoding used by the Unicode implementation.


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