Python data and control flow

Use variables, collections, conditions, and loops to express a small program.

6 min read Free course lessonWeeks 2–3

A variable gives a name to a value. Common values include integers, decimal numbers, strings, and Booleans. Lists keep an ordered sequence; dictionaries associate keys with values. A document-processing program might keep tokens in a list and count them in a dictionary.

Make the program follow the data

Conditions choose between paths, and loops repeat work. For example, a loop can visit each word, normalize it, then increment its count. Functions give a name to a reusable operation. Keep functions small enough that their input, output, and side effects are easy to describe.

Python

tokens = ["model", "learns", "patterns", "model"]
counts = {}
for token in tokens:
    counts[token] = counts.get(token, 0) + 1
print(counts)

Use a list when order matters and duplicates are meaningful.
Use a dictionary when you need to look up a value by a key.
Use explicit names such as `token_count` rather than unexplained abbreviations.
Test a function with an empty input and an ordinary input; edge cases reveal assumptions.

Return to the Python and software foundations stage · Suggested timing is flexible; move at your own pace.