Library and Extension FAQ¶

Answers to common questions regarding Python's standard library capabilities, system integration, and interfacing with low-level compiled code.

How do I run external shell commands from Python?¶

Always use the subprocess module. Avoid legacy functions like os.system() or os.popen(), which are vulnerable to shell injection attacks.

import subprocess

# Run command safely without shell interpretation
result = subprocess.run(
    ["git", "status", "--porcelain"],
    capture_output=True,
    text=True,
    check=True  # Automatically raises CalledProcessError on non-zero exit!
)

print(result.stdout)

Never pass untrusted user strings to shell=True

Setting shell=True invokes the system shell (/bin/sh or cmd.exe). If user-controlled strings are formatted into the command, attackers can inject arbitrary shell commands! Always pass arguments as a list of strings (["cmd", "arg1", "arg2"]).

How do I interface with external C/C++ code?¶

Python provides several powerful mechanisms depending on your performance and ergonomics requirements:

Technology	Best For	Requires C Compiler?
`ctypes`	Calling existing shared C libraries (`.dll`, `.so`) directly	No
`cffi`	Modern, safer C Foreign Function Interface	Optional
Cython	Compiling Python-like code to ultra-fast C extensions	Yes
CPython C-API	Deep low-level interpreter integration and extension modules	Yes
PyO3	Writing high-performance Python extensions in Rust	Yes (Rust compiler)

Quick Example with `ctypes`¶

import ctypes

# Call C standard library directly from pure Python!
# On Windows: ctypes.cdll.msvcrt
# On Linux:   ctypes.CDLL("libc.so.6")
import platform

if platform.system() == "Windows":
    libc = ctypes.cdll.msvcrt
else:
    libc = ctypes.CDLL("libc.so.6")

libc.puts(b"Hello from C standard library puts() called by Python!")

How do I parse XML and HTML in Python?¶

XML Parsing with `xml.etree.ElementTree`¶

The built-in ElementTree module provides a fast, lightweight XML parser:

import xml.etree.ElementTree as ET

xml_data = """<library>
    <book id="1"><title>Fluent Python</title></book>
    <book id="2"><title>Effective Python</title></book>
</library>"""

root = ET.fromstring(xml_data)
for book in root.findall("book"):
    print(book.find("title").text)

HTML Parsing¶

Python includes html.parser for basic tokenization. For modern web scraping that tolerates broken HTML, install BeautifulSoup4 (pip install beautifulsoup4).

How do I generate random numbers cryptographically?¶

Use the standard library secrets module, not random:

import secrets

# Cryptographically secure random token:
secure_token = secrets.token_hex(32)