Python · Modules and imports

A module is a .py file. A package is a folder of modules, traditionally with an __init__.py.

• import math gives you the module object, and you write math.sqrt.
• from math import sqrt imports one name.
• import numpy as np imports under an alias.
• Avoid from x import *, because it hides where names come from.

The first import of a module runs the module once and caches it in sys.modules. Later imports reuse that cached object. Python looks for modules in the directories listed in sys.path.

When a file is run directly, __name__ is "__main__". When it is imported, __name__ is the module’s name. The if __name__ == "__main__": guard uses this to keep demo or test code from running on import.

import sys, os, tempfile, importlib

# create a tiny module on the fly and import it
d = tempfile.mkdtemp()
with open(os.path.join(d, "mathx.py"), "w") as f:
    f.write(
        'print("mathx is being imported")\n'
        "def area(r):\n"
        "    return 3.14159 * r * r\n"
        'if __name__ == "__main__":\n'
        '    print("run directly")\n'
    )
sys.path.insert(0, d)

import mathx
import mathx            # second import: cached, no print
from mathx import area
print(area(2))
print(mathx.__name__, "mathx" in sys.modules)
print(importlib.import_module("math").sqrt(81))

Output:

mathx is being imported
12.56636
mathx True
9.0