What is __init__.py for?

It marks a directory as a package and runs when the package is imported. It can be empty, and since Python 3.3 a package without one mostly works — but including it avoids surprises.

Why does from. import x fail when I run the file?

Relative imports need a parent package, which a file run by path does not have. Run it with python -m package.module.

Does import run the whole module?

Yes, top to bottom, once per program.

How do I reload a module I changed?

importlib.reload(module) in an interactive session. In a script, restart it — reloading has enough sharp edges that it is not worth relying on.

Where is a module actually loaded from?

module.__file__ after importing it, which is the fastest way to confirm you have the one you meant.

Is import x inside a function slow?

Only the first time. After that it is a dictionary lookup in sys.modules.

What is the difference between a module and a package?

A module is a file; a package is a directory of modules. Both are imported the same way.

Can a module import itself?

It can, and it gets the partially initialised version from sys.modules. There is no good reason to.

What does if TYPE_CHECKING: do?

Guards imports that exist only for type hints, so they cost nothing at runtime and cannot cause a circular import.

Should I import inside a function to speed up startup?

Only for genuinely heavy optional dependencies. For anything else the cost is already paid once and the hidden dependency is not worth it.

Modules and import

PythonBy Ashish JangraUpdated 31 August 2026

The forms of import, what each one puts in your namespace, and why import * is discouraged.

Overview

The three forms

import math            # math.sqrt(9)
from math import sqrt  # sqrt(9)
import numpy as np     # np.array(...)

The first keeps the module as a prefix. That is a feature: reading math.sqrt a hundred lines later tells you immediately where it came from, and it cannot collide with anything of yours.

The second is shorter and right when you use one or two names heavily and there is no ambiguity.

as renames on the way in, for length (numpy as np) or to avoid a clash with a name you already have.

imports.py

imports.py Python 3

# The plain form: the module name becomes the namespace.
import math
print("math.sqrt(9)   :", math.sqrt(9))
print("math.pi        :", round(math.pi, 4))

# from ... import: the name lands directly in your namespace.
from math import sqrt, floor
print("sqrt(9)        :", sqrt(9))
print("floor(3.7)     :", floor(3.7))

# as: rename on the way in, usually to shorten or to avoid a clash.
import json as j
print("json as j      :", j.dumps({"a": 1}))

from datetime import datetime as dt
print("datetime as dt :", type(dt.now()).__name__)

# The module object itself.
print()
print("math is a", type(math).__name__)
print("a few names   :", [n for n in dir(math) if n.startswith("f")][:5])

Output

import_care.py

import_care.py Python 3

# import * dumps every public name in, and you cannot see what arrived.
from math import *
print("sqrt came in from somewhere:", sqrt(16))

# The problem: names collide silently and the last import wins.
def gamma(x):
    return "my own gamma"

print("my gamma :", gamma(1))
from math import gamma          # silently replaces it
print("after import *-style shadowing:", gamma(1))

# The standard library has the answer to a lot of things.
print()
import random, statistics, collections
random.seed(0)
nums = [random.randint(1, 10) for _ in range(8)]
print("nums    :", nums)
print("mean    :", round(statistics.mean(nums), 2))
print("counts  :", dict(collections.Counter(nums)))

# Imports are cached: importing twice does not run the module twice.
print()
import sys
print("math already loaded:", "math" in sys.modules)

Output

Worth knowing

import math keeps the module name as a prefix, which shows where a function came from.

from math import sqrt puts sqrt straight into your namespace.

import * hides where names came from and silently overwrites your own.

Imports are cached: a module runs once per program, no matter how often it is imported.