Imports and Modules

Imports and Modules

What it means

import brings a module’s names into your namespace. A module is any .py file; a package is a directory of modules with an __init__.py. __all__ is a module-level list of strings that controls exactly what from module import * pulls in — it doesn’t affect explicit imports like from module import specific_name.

Examples

import math                    # module import — access via math.sqrt(...)
from math import sqrt          # name import — access via sqrt(...) directly
from math import sqrt as s     # aliased import
import numpy as np             # conventional aliasing for a common library

mypackage/__init__.py

from .utils import helper_function
from .models import User
# now `from mypackage import helper_function, User` works directly

analytics.py

__all__ = ["compute_average", "compute_median"]

def compute_average(values):
    ...

def compute_median(values):
    ...

def _internal_helper():        # not in __all__, and leading underscore signals "private"
    ...

from analytics import *
# imports compute_average, compute_median only
# _internal_helper is NOT imported, even without __all__, because of the leading underscore

Common mistake

Assuming __all__ (or a leading underscore) makes a name truly private. It’s a convention, not enforcement — from analytics import _internal_helper and import analytics; analytics._internal_helper() both still work fine. __all__ only changes what import * grabs.

analytics.py

__all__ = ["compute_average"]

def _internal_helper():
    return "not actually private"

# elsewhere:
from analytics import _internal_helper  # works! __all__ didn't block this
print(_internal_helper())               # "not actually private"

Quick practice

1. What does __all__ actually control?

   Answer

   Only what `from module import *` imports — it has no effect on explicit `from module import name` or `import module; module.name`.

2. Does a name starting with _ get blocked from being imported?

   Answer

   No — it's only excluded from `import *`. You can still explicitly `from module import _private_name`; Python doesn't enforce true privacy.

3. Why would you re-export names in a package’s __init__.py?

   Answer

   To give users a flatter public API — `from mypackage import User` instead of requiring `from mypackage.models import User`, hiding the internal module layout.