A library (also called a module or package) is a collection of pre-written code that you can use in your own scripts. Python comes with a large standard library of built-in modules covering common tasks, and you can install many more third-party libraries - later pages in this course use two popular ones, numpy and matplotlib.

Importing a Library

To use a library, first import it with the import statement. Everything the library provides is then accessed with library_name.thing_name:

import math

radius = 4.0
area = math.pi * radius**2

print(f"Area: {area:.2f}")
print(f"Square root of 81: {math.sqrt(81)}")


Area: 50.27
Square root of 81: 9.0

The math module is part of the standard library, so it’s always available - there’s nothing to install. It provides constants like math.pi and functions like math.sqrt() that aren’t built directly into the language.

Selective Imports

If you only need a few specific names from a library, you can import them directly with from ... import .... This lets you drop the library_name. prefix:

from math import sqrt, pi

radius = 4.0
area = pi * radius**2

print(f"Area: {area:.2f}")
print(f"Square root of 81: {sqrt(81)}")


Area: 50.27
Square root of 81: 9.0

This form is convenient for a couple of frequently used names, but importing an entire library with import library_name (as in the first example) is usually clearer in longer scripts, since math.sqrt(x) makes it obvious where sqrt came from, while a bare sqrt(x) does not.

Import Aliases

Libraries with long names are often imported under a shorter alias using import library_name as alias. This is especially common for the numpy and matplotlib libraries used later in this course:

import numpy as np
import matplotlib.pyplot as plt

These aliases (np and plt) are just convention - the library still works the same either way - but nearly all Python code you’ll encounter uses them, so it’s worth adopting the same convention early. See NumPy Arrays and Plotting with Matplotlib for more.

Installing Third-Party Libraries

Standard library modules like math ship with Python and never need to be installed. Third-party libraries, like numpy and matplotlib, do not - they must be installed once per computer using pip, Python’s package installer, from a terminal:

pip install numpy matplotlib

If a library isn’t installed and you try to import it, Python raises a ModuleNotFoundError. If you see that error, double check the spelling of the library name, then make sure it has been installed with pip for the same Python interpreter VS Code is using (see Selecting a Python Interpreter in VS Code).

Reading Questions

  1. What is the difference between the standard library and a third-party library?
  2. What command would you use to import the math module?
  3. After running import math, how would you access the value of $\pi$?
  4. What is the difference between import math and from math import sqrt?
  5. What do np and plt refer to, by convention?
  6. What error does Python raise if you try to import a library that isn’t installed?
  7. How do you install a third-party library like numpy?

Practice Problem: Descent Angle

Your Task

Write a script named descent_angle.py that starts from the given values:

horizontal_distance_m = 8000
altitude_loss_m = 500

Import the math module, then compute descent_angle_deg, the descent angle in degrees, using math.atan and math.degrees. Print the result with an f-string, rounded to 2 decimal places.

Your variable name for the answer must match exactly (descent_angle_deg) so that the checker below can find it.

Checking Your Work

Download check_descent_angle.py and save it in the same folder as your descent_angle.py script. Open a terminal in that folder and run:

python check_descent_angle.py

The checker runs your script and reports whether descent_angle_deg is correct. This is practice, not a graded assignment. If something doesn’t pass, use the feedback to find and fix the issue, then run the checker again.