You think import just brings code into your file. It does much more than that.

Today, you understand how Python loads, initializes, and caches modules.

Today’s Goal

By the end of today, you will:

  • Understand how Python resolves imports
  • Learn module execution model
  • Understand import caching
  • Avoid common import pitfalls

The Illusion

import math

You think:

Python just loads math

Reality:

Python searches, loads, compiles (if needed), executes, and caches the module

Import Pipeline

import statement
  ↓
check sys.modules cache
  ↓
find module (sys.meta_path / sys.path)
  ↓
load module (loader)
  ↓
execute module top-level code
  ↓
store in sys.modules

Step 1 — Cache Check

import sys
print('math' in sys.modules)

If present → reuse
If not → load

Step 2 — Finding the Module

Search order:

  1. built-in modules
  2. current directory
  3. sys.path
import sys
print(sys.path)

Step 3 — Loading & Execution

  • file is read
  • compiled to bytecode (if needed)
  • executed top-to-bottom

Important Insight

Importing a module runs its code.

Example

module.py

print("module executed")
x = 10

main.py

import module

Output:

module executed

Import Happens Once

import module
import module

Executed once due to sys.modules cache.

Import Styles

import math
math.sqrt(4)

from math import sqrt
sqrt(4)

import math as m
m.sqrt(4)

Avoid Wildcard Imports

from module import *

Problems:

  • namespace pollution
  • conflicts
  • harder debugging

Module as Singleton

Modules are singletons.

Shared state across imports.

Reloading

import importlib
import module
importlib.reload(module)

Circular Imports

Cause partially initialized modules and runtime errors.

__name__ and Entry Points

print(__name__)
  • Direct run → “main”
  • Imported → module name

Example

def run():
    print("running")

if __name__ == "__main__":
    run()

__init__.py

Marks directory as package and executes on import.

# __init__.py
print("package initialized")

Modern Note

Optional in Python 3.3+, but still useful.

Your Task

  • inspect sys.modules
  • print sys.path
  • create module and observe execution

Common Mistakes

  • assuming import is free
  • circular imports
  • wildcard imports

Think Deeper

  1. Why cache modules?
  2. What if import fails?
  3. How does order affect behavior?

Final Insight

Import system defines how your app is wired.

Tomorrow

Errors & Exceptions

Rule

  • Imports execute code
  • Modules are singletons
  • Structure matters

See you in Day 7.