sys - System-specific Parameters

The sys module provides access to variables and functions that interact closely with the Python interpreter.

Module Import

import sys

Common Attributes

sys.argv

Command-line arguments passed to a Python script.

import sys

# script.py arg1 arg2 arg3
print(sys.argv)
# ['script.py', 'arg1', 'arg2', 'arg3']

# First argument is always the script name
script_name = sys.argv[0]

# Process arguments
if len(sys.argv) > 1:
    input_file = sys.argv[1]

sys.path

List of directories Python searches for modules.

import sys

# View current module search path
print(sys.path)
# ['', '/usr/lib/python3.11', ...]

# Add custom path
sys.path.append('/path/to/custom/modules')
sys.path.insert(0, '/priority/path')

sys.platform

Platform identifier.

print(sys.platform)
# 'linux', 'darwin', 'win32', etc.

if sys.platform == 'linux':
    print("Running on Linux")
elif sys.platform == 'darwin':
    print("Running on macOS")
elif sys.platform == 'win32':
    print("Running on Windows")

sys.version

Python version information.

print(sys.version)
# '3.11.2 (main, Mar 13 2023, 12:18:29) [GCC 12.2.0]'

print(sys.version_info)
# sys.version_info(major=3, minor=11, micro=2, releaselevel='final', serial=0)

if sys.version_info >= (3, 10):
    print("Python 3.10 or higher")

Standard Streams

sys.stdin, sys.stdout, sys.stderr

Standard input, output, and error streams.

import sys

# Read from stdin
for line in sys.stdin:
    print(line.strip())

# Write to stdout
sys.stdout.write("Hello, World!\n")

# Write to stderr
sys.stderr.write("Error message\n")

# Redirect output
original_stdout = sys.stdout
with open('output.txt', 'w') as f:
    sys.stdout = f
    print("This goes to the file")
    sys.stdout = original_stdout

Exit Functions

sys.exit()

Exit the program.

status

int | str

default:"0"

Exit status code (0 for success, non-zero for error) or exit message

import sys

if not valid_input:
    sys.exit(1)  # Exit with error code

if error_occurred:
    sys.exit("Fatal error occurred")  # Exit with message

sys.exit(0)  # Successful exit

sys.exc_info()

Get information about current exception.

import sys

try:
    1 / 0
except:
    exc_type, exc_value, exc_traceback = sys.exc_info()
    print(f"Exception type: {exc_type}")
    print(f"Exception value: {exc_value}")

Module and Import Functions

sys.modules

Dictionary of loaded modules.

import sys
import os

# Check if module is loaded
if 'os' in sys.modules:
    print("os module is loaded")

# Get loaded module
os_module = sys.modules['os']

# Reload module (use importlib.reload instead)
from importlib import reload
reload(os)

sys.meta_path

List of meta path finders.

import sys

print(sys.meta_path)
# Shows import hooks

Size and Memory

sys.getsizeof()

Get size of an object in bytes.

object

any

required

Object to get size of

return

int

Size in bytes

import sys

print(sys.getsizeof(42))  # 28 bytes
print(sys.getsizeof("hello"))  # String size
print(sys.getsizeof([1, 2, 3]))  # List size
print(sys.getsizeof({'a': 1}))  # Dict size

sys.getrecursionlimit() / sys.setrecursionlimit()

Get or set maximum recursion depth.

import sys

print(sys.getrecursionlimit())  # Default: 1000

# Increase for deep recursion
sys.setrecursionlimit(2000)

# Example: deep recursion
def factorial(n):
    if n <= 1:
        return 1
    return n * factorial(n - 1)

System Information

sys.maxsize

Maximum value a variable of type Py_ssize_t can take.

import sys

print(sys.maxsize)
# 9223372036854775807 (on 64-bit system)

# Check if 64-bit
if sys.maxsize > 2**32:
    print("64-bit Python")
else:
    print("32-bit Python")

sys.byteorder

Byte order of the system.

import sys

print(sys.byteorder)
# 'little' or 'big'

sys.executable

Path to Python interpreter.

import sys

print(sys.executable)
# '/usr/bin/python3' or similar

Debugging and Profiling

sys.settrace()

Set trace function for debugging.

import sys

def trace_calls(frame, event, arg):
    if event == 'call':
        filename = frame.f_code.co_filename
        lineno = frame.f_lineno
        funcname = frame.f_code.co_name
        print(f"Calling {funcname} at {filename}:{lineno}")
    return trace_calls

sys.settrace(trace_calls)

def example():
    print("Hello")

example()
sys.settrace(None)  # Disable tracing

sys.setprofile()

Set profiling function.

import sys
import time

def profile_func(frame, event, arg):
    if event == 'call':
        frame.f_locals['__start_time__'] = time.time()
    elif event == 'return':
        elapsed = time.time() - frame.f_locals.get('__start_time__', 0)
        funcname = frame.f_code.co_name
        print(f"{funcname} took {elapsed:.4f} seconds")
    return profile_func

sys.setprofile(profile_func)

Flags and Options

sys.flags

Command-line flags.

import sys

print(sys.flags)
# sys.flags(debug=0, inspect=0, interactive=0, optimize=0, ...)

if sys.flags.debug:
    print("Running in debug mode (-d flag)")

if sys.flags.optimize:
    print("Running in optimized mode (-O flag)")

Practical Examples

Command-Line Script

import sys

def main():
    if len(sys.argv) < 2:
        print("Usage: python script.py <filename>", file=sys.stderr)
        sys.exit(1)
    
    filename = sys.argv[1]
    try:
        with open(filename, 'r') as f:
            print(f.read())
    except FileNotFoundError:
        print(f"Error: {filename} not found", file=sys.stderr)
        sys.exit(1)
    except Exception as e:
        print(f"Error: {e}", file=sys.stderr)
        sys.exit(1)

if __name__ == '__main__':
    main()

Platform-Specific Code

import sys

if sys.platform.startswith('linux'):
    import fcntl
    # Linux-specific code
elif sys.platform == 'darwin':
    # macOS-specific code
    pass
elif sys.platform == 'win32':
    import msvcrt
    # Windows-specific code

Progress Bar with stderr

import sys
import time

def progress_bar(iterable, total=None):
    total = total or len(iterable)
    for i, item in enumerate(iterable, 1):
        percent = (i / total) * 100
        sys.stderr.write(f"\rProgress: {percent:.1f}%")
        sys.stderr.flush()
        yield item
    sys.stderr.write("\n")

# Usage
for item in progress_bar(range(100)):
    time.sleep(0.01)
    # Process item

Memory Usage Tracker

import sys

def get_size(obj, seen=None):
    """Recursively calculate size of objects"""
    size = sys.getsizeof(obj)
    if seen is None:
        seen = set()
    
    obj_id = id(obj)
    if obj_id in seen:
        return 0
    
    seen.add(obj_id)
    
    if isinstance(obj, dict):
        size += sum(get_size(k, seen) + get_size(v, seen) 
                   for k, v in obj.items())
    elif hasattr(obj, '__dict__'):
        size += get_size(obj.__dict__, seen)
    elif hasattr(obj, '__iter__') and not isinstance(obj, (str, bytes, bytearray)):
        size += sum(get_size(item, seen) for item in obj)
    
    return size

# Usage
data = {'a': [1, 2, 3], 'b': {'nested': [4, 5, 6]}}
print(f"Total size: {get_size(data)} bytes")

Complete Reference

Constants

Platform Constants

sys.platform - Platform identifier
sys.byteorder - Native byte order (‘little’ or ‘big’)
sys.maxsize - Maximum Py_ssize_t value
sys.maxunicode - Maximum Unicode code point

Version Information

sys.version - Version string
sys.version_info - Version tuple
sys.hexversion - Version as single number
sys.api_version - C API version

Executable Information

sys.executable - Python interpreter path
sys.prefix - Installation prefix
sys.base_prefix - Base installation prefix
sys.exec_prefix - Platform-specific installation prefix

Functions

Exit Functions

sys.exit(status) - Exit the interpreter
sys.excepthook(type, value, traceback) - Handle uncaught exceptions

Exception Functions

sys.exc_info() - Current exception information
sys.last_type, sys.last_value, sys.last_traceback - Last exception

Debugging Functions

sys.settrace(func) - Set trace function
sys.gettrace() - Get trace function
sys.setprofile(func) - Set profile function
sys.getprofile() - Get profile function

Module Functions

sys.getrefcount(object) - Reference count
sys.getsizeof(object) - Object size in bytes
sys.intern(string) - Intern string

Best Practices

Use sys.exit() for clean exits:

# Good
if error:
    sys.exit(1)

# Avoid
if error:
    exit()  # May not be available in all contexts

Be careful with sys.setrecursionlimit(): Setting it too high can cause stack overflow. Python’s default limit (usually 1000) is reasonable for most cases. If you need deeper recursion, consider iterative approaches instead.

sys.argv is always a list: Always check length before accessing elements to avoid IndexError.

if len(sys.argv) > 1:
    arg = sys.argv[1]

os

Operating system interfaces

platform

Platform identification

argparse

Command-line argument parsing

logging

Logging facility

Overview

Built-in Modules

Concurrency

Text Processing

File & Data

Networking

​Module Import

​Common Attributes

​sys.argv

​sys.path

​sys.platform

​sys.version

​Standard Streams

​sys.stdin, sys.stdout, sys.stderr

​Exit Functions

​sys.exit()

​sys.exc_info()

​Module and Import Functions

​sys.modules

​sys.meta_path

​Size and Memory

​sys.getsizeof()

​sys.getrecursionlimit() / sys.setrecursionlimit()

​System Information

​sys.maxsize

​sys.byteorder

​sys.executable

​Debugging and Profiling

​sys.settrace()

​sys.setprofile()

​Flags and Options

​sys.flags

​Practical Examples

​Command-Line Script

​Platform-Specific Code

​Progress Bar with stderr

​Memory Usage Tracker

​Complete Reference

​Constants

​Functions

​Best Practices

​Related Modules

os

platform

argparse

logging

Build docs developers (and LLMs) love

Module Import

Common Attributes

sys.argv

sys.path

sys.platform

sys.version

Standard Streams

sys.stdin, sys.stdout, sys.stderr

Exit Functions

sys.exit()

sys.exc_info()

Module and Import Functions

sys.modules

sys.meta_path

Size and Memory

sys.getsizeof()

sys.getrecursionlimit() / sys.setrecursionlimit()

System Information

sys.maxsize

sys.byteorder

sys.executable

Debugging and Profiling

sys.settrace()

sys.setprofile()

Flags and Options

sys.flags

Practical Examples

Command-Line Script

Platform-Specific Code

Progress Bar with stderr

Memory Usage Tracker

Complete Reference

Constants

Functions

Best Practices

Related Modules