Recipe

Turn repeated code into a function

Use when: the same few lines appear in two places, or one block of code does a job you can name

Goal: one small, named, documented, checked function used everywhere the code was

Taught in Designing functions

1. 1

   Pick the lines that do one job you can name in a sentence without "and".

   Example

   round(price * 1.2, 2), typed in three places: "add 20% tax".

   Why: a function with one clear job is easy to name, check and reuse.

2. 2

   The values that change between copies become parameters; the value the rest of the code needs becomes the return value.

   Example

   price varies, so it's the parameter; the taxed price is returned.

   Why: parameters and return are the function's whole connection to the outside, with no hidden globals.

3. 3

   A verb for an action, the value's name for a calculation, a question for yes or no. Add defaults for settings that rarely change.

   Example

   def with_tax(price, rate=0.2):

   Why: a good name makes every call site explain itself.

4. 4

   One line, as a command, saying what comes back.

   Example

   """Return price with tax added, rounded to cents."""

   Why: help() and your editor show it to everyone who calls the function.

5. 5

   Write two or three asserts with answers worked out by hand, then swap every copy of the old lines for a call.

   Example

   assert with_tax(10) == 12.0, then total = with_tax(price) in all three places.

   Why: the checks prove the function matches the code it replaces, before anything depends on it.