Importing Modules

Why this matters

Python ships with a huge standard library of ready-made tools: maths, dates, files, JSON, random numbers and more. You use them with import. The same idea lets you split your own code across files.

The idea

import math                       # use as math.sqrt(16)
from math import sqrt, pi         # use as sqrt(16) and pi
import statistics as stats        # a shorter name: stats.mean([1, 2, 3])

import math is the safest: you always see where a name comes from. Avoid from math import *, which dumps every name into your file.

To learn what a module offers, use help(math.ceil) or dir(math).

Every file has a built-in name, __name__. When you run the file it is "__main__"; when another file imports it, it is the module’s name. That gives the standard pattern:

def main():
    print("running")

if __name__ == "__main__":     # only when run directly, not when imported
    main()

Try it

In the editor, import math and print math.pi, math.sqrt(2) and math.floor(-2.5). Then add print(__name__) and see what it shows.

Your turn

Write:

• circle_area(radius): the area, rounded to 2 decimals (circle_area(2) is 12.57). Use math.pi.
• pages_needed(items, per_page): how many pages are needed to show items items with per_page on each page. 23 items at 10 per page need 3 pages. Use math.ceil.
• summarize(numbers): a dictionary {"mean": ..., "median": ...} using the statistics module, with the mean rounded to 2 decimals.
• main() that prints Area: 12.57 (using your circle_area(2)), called only under the if __name__ == "__main__": guard.

Recap

• import module then module.name; from module import name for a few names.
• The standard library is big: look before you write your own.
• if __name__ == "__main__": runs code only when the file is run directly.

Common mistakes

• Naming your own file like a module (random.py, math.py). Python imports yours instead and things break: AttributeError: module 'random' has no attribute 'randint'.
• Forgetting the module name: sqrt(4) after import math is a NameError.
• Putting code at the top level of a file you want to import, which then runs on import.