📎 This article includes 2 downloadable practice files ↓

📘 Python Beginner Course · Lesson 6 of 12

When you copy the same calculation into three places, a change means fixing it three times. A function names a piece of logic once; you call it wherever you need it. Excel 365’s LAMBDA is the same idea.

def and return

def gst(amount, rate=0.18):
    """Return GST on an amount (default 18%), rounded to paise."""
    return round(amount * rate, 2)

gst(1000)          # 180.0
gst(1000, 0.12)    # 120.0
gst(amount=2500, rate=0.05)

• amount and rate are parameters; rate=0.18 is a default used when you don’t pass one.
• return sends the answer back. Without it, the function returns None.
• The triple-quoted line is a docstring: hover over the function in VS Code to see it.

More useful functions

def fy_label(year, month):
    start = year - (month < 4)          # True counts as 1
    return f"FY {start}-{str(start + 1)[2:]}"

def grade(marks):
    if marks >= 90: return "A+"
    if marks >= 75: return "A"
    if marks >= 60: return "B"
    if marks >= 40: return "C"
    return "F"

[grade(m) for m in (95, 80, 61, 40, 12)]    # ['A+', 'A', 'B', 'C', 'F']

Scope

Variables created inside a function exist only inside it. Pass values in as parameters and get results out with return, rather than relying on outside variables.

Modules: functions in another file

Save your helpers in helpers.py, then in another script: from helpers import gst, fy_label. That’s how you build a personal toolkit.

Where beginners go wrong

Practice

Download the script (and data file, if listed) below into one folder. Open a terminal in that folder and run python lesson-NN.py. Compare with the expected output, then change something and run it again: that’s how Python is learned.