Python Modules and Imports: What Exams Actually Test

Import syntax

There are several ways to import. Each has different implications for how you access the imported names.

import math              # access via math.sqrt()
import math as m         # alias: m.sqrt()
from math import sqrt    # direct: sqrt()
from math import sqrt, pi
from math import *       # imports everything (avoid)

print(math.sqrt(16))     # 4.0
print(m.sqrt(16))        # 4.0
print(sqrt(16))          # 4.0

__name__ == "__main__"

When you run a file directly, __name__ is "__main__". When imported as a module, __name__ is the module filename.

# mymodule.py
def helper():
    return "I help"

if __name__ == "__main__":
    # Only runs when executed directly
    # NOT when imported
    print(helper())

The standard library

Know these modules for exams.

import os, sys, math, random, datetime, json, re
from collections import defaultdict, Counter, deque

os.path.join("dir", "file.txt")
os.getcwd()
sys.argv
random.choice([1, 2, 3])
json.dumps({"key": "value"})
json.loads('{"key": "value"}')

Creating your own module

Any .py file is a module. Any directory with an __init__.py is a package.

# utils.py
def add(a, b):
    return a + b

# main.py
import utils
print(utils.add(2, 3))  # 5

# Package structure:
# mypackage/
#   __init__.py
#   math_utils.py

collections module

The collections module provides specialised data structures tested in Python exams.

from collections import Counter, defaultdict

words = ["apple", "banana", "apple", "cherry", "apple"]
count = Counter(words)
print(count.most_common(2))
# [('apple', 3), ('banana', 1)]

dd = defaultdict(list)
dd["fruits"].append("apple")  # no KeyError!

Exam tip

The `if __name__ == "__main__":` pattern is tested in virtually every Python exam. Know that this block runs when the file is executed directly but NOT when imported.