runpy Module Complexity

The runpy module finds a module by name or a script by path and executes its code in a fresh namespace, the way python -m and python script.py do. Unlike import, it does not leave the module it runs in sys.modules: every call finds, loads and executes the code again, and returns a dictionary of the globals the code left behind.

s is the size of the code being run: the characters of its source when it is compiled, or of its bytecode when a cached .pyc is loaded instead. i is the entries in init_globals, g is the names in the globals the code leaves behind, and f is the import system's search for a module name, priced on the importlib page. Every bound is runpy's own cost; executing the module's top-level statements costs whatever they do, on top.

Complexity Reference

Functions

Operation	Time	Space	Notes
runpy.run_module(mod_name, init_globals=None, run_name=None, alter_sys=False)	O(f + s + i)	O(s + i)	Imports the parent packages of a dotted name, which stay in sys.modules; the module itself is executed afresh on every call and is not added to it. A package runs its __main__ submodule. Loads cached bytecode when it is current, as import does, and returns the namespace the code ran in
runpy.run_module(mod_name, alter_sys=True)	O(f + s + i + g)	O(s + i + g)	As above, but sys.argv[0] and sys.modules[run_name] point at the running module until it returns, and both are then restored; returns a copy of its globals
runpy.run_path(path_name, init_globals=None, run_name=None) on a file	O(s + i + g)	O(s + i + g)	A .py script is compiled from source on every call and no bytecode is written. Swaps and restores sys.argv[0] and sys.modules[run_name] and returns a copy of the globals, like alter_sys=True
runpy.run_path(path_name, init_globals=None, run_name=None) on a directory or zip file	O(f + s + i + g)	O(s + i + g)	Puts the path first on sys.path for the call and runs its __main__.py through the import system, so a directory reuses its __pycache__; the same swap and copy

Running Modules by Name

Every Call Executes the Code Again

run_module() is not an import. The module runs in a new namespace each time and is not left in sys.modules, so a second call pays for the code's top-level work again. Only the parent packages of a dotted name are imported, once, the ordinary way.

import pathlib
import runpy
import sys
import tempfile

with tempfile.TemporaryDirectory() as tmp:
    root = pathlib.Path(tmp)
    (root / 'runpy_demo_pkg').mkdir()
    (root / 'runpy_demo_pkg' / '__init__.py').write_text('')
    (root / 'runpy_demo_pkg' / 'task.py').write_text('calls.append(__name__)\nresult = 42\n')
    sys.path.insert(0, tmp)
    try:
        calls = []
        first = runpy.run_module('runpy_demo_pkg.task', {'calls': calls})   # O(f + s + i)
        second = runpy.run_module('runpy_demo_pkg.task', {'calls': calls})  # executes again
        assert calls == ['runpy_demo_pkg.task', 'runpy_demo_pkg.task']
        assert first['result'] == second['result'] == 42
        assert first is not second  # a fresh namespace each time

        assert 'runpy_demo_pkg' in sys.modules           # the parent was imported
        assert 'runpy_demo_pkg.task' not in sys.modules  # the module itself was not
    finally:
        sys.path.remove(tmp)
        sys.modules.pop('runpy_demo_pkg', None)

Running as __main__

Passing run_name='__main__' makes the module's if __name__ == '__main__': block run, which is what python -m does. The module reads the command line from sys.argv, so set it first and put it back afterwards.

import json
import os
import runpy
import sys
import tempfile

with tempfile.TemporaryDirectory() as tmp:
    source = os.path.join(tmp, 'in.json')
    target = os.path.join(tmp, 'out.json')
    with open(source, 'w') as f:
        f.write('{"b": 1, "a": [1, 2]}')

    saved = sys.argv
    sys.argv = ['json.tool', '--sort-keys', '--compact', source, target]
    try:
        # Equivalent to: python -m json.tool --sort-keys --compact in.json out.json
        runpy.run_module('json.tool', run_name='__main__')  # O(f + s) plus the tool's work
    finally:
        sys.argv = saved

    with open(target) as f:
        assert f.read() == '{"a":[1,2],"b":1}\n'

Altering sys

With alter_sys=True, the running code sees its file as sys.argv[0] and its module under run_name in sys.modules. Both are restored when the call returns, and the result is a copy of the globals, O(g).

import pathlib
import runpy
import sys
import tempfile

with tempfile.TemporaryDirectory() as tmp:
    script = pathlib.Path(tmp) / 'runpy_demo_sys.py'
    script.write_text(
        'import sys\n'
        'argv0 = sys.argv[0]\n'
        'registered = getattr(sys.modules.get(__name__), "__dict__", None) is globals()\n'
        'namespace = globals()\n'
    )
    sys.path.insert(0, tmp)
    saved_argv0 = sys.argv[0]
    try:
        result = runpy.run_module('runpy_demo_sys', alter_sys=True)  # O(f + s + i + g)
        assert result['argv0'] == str(script)
        assert result['registered'] is True
        assert result['namespace'] is not result  # a copy of the globals

        plain = runpy.run_module('runpy_demo_sys')  # O(f + s + i)
        assert plain['registered'] is False
        assert plain['namespace'] is plain  # the namespace the code ran in

        assert sys.argv[0] == saved_argv0
        assert 'runpy_demo_sys' not in sys.modules
    finally:
        sys.path.remove(tmp)

Running Files and Directories

Scripts Compile on Every Call

run_path() on a .py file reads and compiles the whole source each time and never writes bytecode, so running the same script repeatedly pays O(s) to compile it every time. A directory holding __main__.py is run through the import system instead, which writes and then reuses __pycache__, as run_module() does for a module.

import os
import pathlib
import runpy
import sys
import tempfile

sys.dont_write_bytecode = False

with tempfile.TemporaryDirectory() as tmp:
    root = pathlib.Path(tmp)
    script = root / 'script.py'
    script.write_text('answer = 6 * 7\n')

    result = runpy.run_path(str(script))  # O(s + i + g) - compiled from source
    assert result['answer'] == 42
    assert result['__name__'] == '<run_path>'
    assert not (root / '__pycache__').exists()  # nothing cached for next time

    app = root / 'app'
    app.mkdir()
    (app / '__main__.py').write_text('answer = 6 * 7\n')

    result = runpy.run_path(str(app), run_name='__main__')  # O(f + s + i + g)
    assert result['answer'] == 42
    assert os.listdir(app / '__pycache__')  # the next run loads bytecode

Common Patterns

Running a Script With Arguments

import pathlib
import runpy
import sys
import tempfile

with tempfile.TemporaryDirectory() as tmp:
    script = pathlib.Path(tmp) / 'greet.py'
    script.write_text(
        'import sys\n'
        'if __name__ == "__main__":\n'
        '    greeting = "Hello, " + sys.argv[1]\n'
    )

    saved = sys.argv
    sys.argv = [str(script), 'world']
    try:
        result = runpy.run_path(str(script), run_name='__main__')  # O(s + i + g)
    finally:
        sys.argv = saved

    assert result['greeting'] == 'Hello, world'

Performance Best Practices

✅ Do:

• Import a module and call its functions when the same code runs repeatedly: import executes it once, runpy executes it on every call
• Use run_module() or a directory over run_path() on a .py file for code you run more than once, so the compile is paid once and cached bytecode is loaded after
• Leave alter_sys at its default unless the code needs sys.argv[0] or its own sys.modules entry; it adds a copy of the globals

❌ Avoid:

• run_path() on the same large script in a loop - every call compiles it from source
• Expecting state to persist between runs - each call starts from an empty namespace plus init_globals

Version Notes

• All Python 3: run_path() and run_module(..., alter_sys=True) change sys.argv[0] and sys.modules for the duration of the call, so they are not thread-safe

Related Modules

• importlib - The search and loading run_module() uses; import once instead of running repeatedly
• zipapp - Builds the archives run_path() and python archive.pyz execute
• sys - sys.argv, sys.modules and sys.path, which these functions read and swap