subprocess Module Complexity¶

The subprocess module enables spawning new processes, connecting to I/O pipes, and obtaining return codes.

Complexity Reference¶

Operation	Time	Space	Notes
`run()` / `Popen()`	O(1) + exec time	O(1)	fork/exec overhead; process execution time dominates
`communicate()`	O(n)	O(n)	n = stdout + stderr size
`wait()`	O(1) + process	O(1)	Blocks until process exits

Basic Usage¶

import subprocess

# Simple execution - O(p)
result = subprocess.run(['ls', '-l'])  # O(p) - process time

# Capture output - O(n)
result = subprocess.run(['echo', 'hello'], capture_output=True, text=True)
print(result.stdout)  # 'hello\n'

# Check return code - O(1)
if result.returncode == 0:
    print("Success")

Process Communication¶

import subprocess

# Execute with input - O(n)
result = subprocess.run(
    ['wc', '-w'],
    input='hello world\n',
    text=True,
    capture_output=True
)
print(result.stdout)  # '2\n'

# Check for errors - O(1)
result = subprocess.run(['false'], capture_output=True)
if result.returncode != 0:
    print("Command failed")

Popen for Advanced Control¶

import subprocess

# Create process - O(p)
process = subprocess.Popen(
    ['cat'],
    stdin=subprocess.PIPE,
    stdout=subprocess.PIPE,
    text=True
)

# Communicate - O(n)
output, error = process.communicate(input='hello\n')
print(output)  # 'hello\n'

Pipe Chaining¶

import subprocess

# Pipeline - O(n)
proc1 = subprocess.Popen(['cat', 'file.txt'], stdout=subprocess.PIPE)
proc2 = subprocess.Popen(
    ['grep', 'pattern'],
    stdin=proc1.stdout,
    stdout=subprocess.PIPE,
    text=True
)
proc1.stdout.close()  # Allow proc1 to close

output, _ = proc2.communicate()  # O(n) - n = file size

Version Notes¶

Python 3.5+: run() recommended
Earlier: Use Popen() and communicate()

Best Practices¶

✅ Do:

Use run() for simple commands
Use capture_output=True
Use text=True for string output
Check return codes

❌ Avoid:

Shell injection (use list, not string)
Ignoring return codes
No timeout protection