subprocess

Q: Merge stderr?

stderr=subprocess.STDOUT into stdout pipe.

Q: Windows?

creationflags for new console; list args still preferred.

Q: Environment?

env={os.environ, "K": "V"} partial override.

Q: uv run?

["uv", "run", "python", "-m", "tool"] in list form for reproducible env.

subprocess spawns external programs, captures stdout/stderr, and manages exit codes. Prefer argument lists without shell unless shell features are explicitly required.

Recipe

import subprocess
 
result = subprocess.run(
    ["python", "--version"],
    capture_output=True,
    text=True,
    check=True,
)
print(result.stdout.strip())

When to reach for this:

Git, ffmpeg, openssl CLI wrappers
Legacy tools without Python API
Packaging scripts invoking compilers
Health checks calling curl binary
Sandboxed isolation via separate process

Working Example

import subprocess
from pathlib import Path
 
def run_git(args: list[str], cwd: Path) -> str:
    completed = subprocess.run(
        ["git", *args],
        cwd=cwd,
        capture_output=True,
        text=True,
        check=False,
    )
    if completed.returncode != 0:
        raise RuntimeError(completed.stderr.strip() or "git failed")
    return completed.stdout.strip()
 
def safe_ls(path: Path) -> str:
    # Never shell=True with user input
    proc = subprocess.run(
        ["ls", "-la", str(path)],
        capture_output=True,
        text=True,
        timeout=5,
    )
    proc.check_returncode()
    return proc.stdout
 
print(run_git(["--version"], Path.cwd()))

What this demonstrates:

List form args avoid shell injection
check_returncode or check=True for failures
timeout prevents hung children
text=True decodes bytes to str with encoding default

Deep Dive

run vs Popen

API	Use
`run`	One-shot wait for completion
`Popen`	Streaming IO, background

IO

capture_output=True pipes stdout/stderr
stdin=subprocess.DEVNULL when no input

Gotchas

shell=True with user input - command injection. Fix: list args, shlex.quote if shell required.
Unbounded output - memory exhaustion. Fix: stream with Popen iter or redirect to file.
Missing timeout - zombie hangs. Fix: always timeout in services.
CWD trust - relative binaries from wrong dir. Fix: explicit cwd and env.
Blocking event loop - subprocess in async route. Fix: asyncio.create_subprocess_exec.

Alternatives

Alternative	Use When	Don't Use When
shutil.which	Resolve binary path	Need not run
asyncio subprocess	Async streaming	Sync script
Pure Python lib	Stable API exists	One-off CLI ok

FAQs

shell=True ever?

Only trusted fixed commands in controlled scripts - never with external input.

Return code 0?

Success convention - check explicitly even if not using check=True.

Merge stderr?

stderr=subprocess.STDOUT into stdout pipe.

Binary output?

Omit text=True; handle bytes and decode explicitly.

Windows?

creationflags for new console; list args still preferred.

Environment?

env={**os.environ, "K": "V"} partial override.

Long running process?

Popen + communicate with timeout or poll loop.

Security audit?

Flag any shell=True and string concatenation in security review.

uv run?

["uv", "run", "python", "-m", "tool"] in list form for reproducible env.

pytest?

Use fixtures with tmp_path; mock subprocess.run for unit tests.

pathlib & os - cwd paths
argparse & configparser - CLI tools that wrap subprocess
Standard Library Best Practices - safety
Concurrency Basics - blocking vs async

Python Standard Library Reference - Complete documentation for built-in modules and functions.
Python HOWTOs (Official) - Practical topic-focused guides.
Stack Overflow – Python Tag - Q&A for Python programming problems.
Automate the Boring Stuff with Python (3rd Edition) - Practical automation (Al Sweigart).
Fluent Python (2nd Edition, O'Reilly) - Same book on O'Reilly.

Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.

Recipe

import subprocess
 
result = subprocess.run(
    ["python", "--version"],
    capture_output=True,
    text=True,
    check=True,
)
print(result.stdout.strip())

When to reach for this:

Git, ffmpeg, openssl CLI wrappers
Legacy tools without Python API
Packaging scripts invoking compilers
Health checks calling curl binary
Sandboxed isolation via separate process

Working Example

import subprocess
from pathlib import Path
 
def run_git(args: list[str], cwd: Path) -> str:
    completed = subprocess.run(
        ["git", *args],
        cwd=cwd,
        capture_output=True,
        text=True,
        check=False,
    )
    if completed.returncode != 0:
        raise RuntimeError(completed.stderr.strip() or "git failed")
    return completed.stdout.strip()
 
def safe_ls(path: Path) -> str:
    # Never shell=True with user input
    proc = subprocess.run(
        ["ls", "-la", str(path)],
        capture_output=True,
        text=True,
        timeout=5,
    )
    proc.check_returncode()
    return proc.stdout
 
print(run_git(["--version"], Path.cwd()))

What this demonstrates:

List form args avoid shell injection
check_returncode or check=True for failures
timeout prevents hung children
text=True decodes bytes to str with encoding default

Deep Dive

run vs Popen

API	Use
`run`	One-shot wait for completion
`Popen`	Streaming IO, background

IO

capture_output=True pipes stdout/stderr
stdin=subprocess.DEVNULL when no input

Gotchas

shell=True with user input - command injection. Fix: list args, shlex.quote if shell required.
Unbounded output - memory exhaustion. Fix: stream with Popen iter or redirect to file.
Missing timeout - zombie hangs. Fix: always timeout in services.
CWD trust - relative binaries from wrong dir. Fix: explicit cwd and env.
Blocking event loop - subprocess in async route. Fix: asyncio.create_subprocess_exec.

Alternatives

Alternative	Use When	Don't Use When
shutil.which	Resolve binary path	Need not run
asyncio subprocess	Async streaming	Sync script
Pure Python lib	Stable API exists	One-off CLI ok

FAQs

shell=True ever?

Only trusted fixed commands in controlled scripts - never with external input.

Return code 0?

Success convention - check explicitly even if not using check=True.

Merge stderr?

stderr=subprocess.STDOUT into stdout pipe.

Binary output?

Omit text=True; handle bytes and decode explicitly.

Windows?

creationflags for new console; list args still preferred.

Environment?

env={**os.environ, "K": "V"} partial override.

Long running process?

Popen + communicate with timeout or poll loop.

Security audit?

Flag any shell=True and string concatenation in security review.

uv run?

["uv", "run", "python", "-m", "tool"] in list form for reproducible env.

pytest?

Use fixtures with tmp_path; mock subprocess.run for unit tests.

pathlib & os - cwd paths
argparse & configparser - CLI tools that wrap subprocess
Standard Library Best Practices - safety
Concurrency Basics - blocking vs async

Python Standard Library Reference - Complete documentation for built-in modules and functions.
Python HOWTOs (Official) - Practical topic-focused guides.
Stack Overflow – Python Tag - Q&A for Python programming problems.
Automate the Boring Stuff with Python (3rd Edition) - Practical automation (Al Sweigart).
Fluent Python (2nd Edition, O'Reilly) - Same book on O'Reilly.

Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.

subprocess

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subprocess