LECTURE 4.1

Functions

Write once, use everywhere, and how Python passes arguments

Introduction to Data Science and Machine Learning · Md Mahbub E Noor · University of Barishal

Today's key line of code

def add(a, b):
    return a + b

print(add(5, 6))

Where we are: Lecture 4, part 1

L2Python basics

L3Decisions and loops

Part 1L4Counting words

L5Real data

L6Build the model

Today you will

1Name every part of a function definition and call
2Send values in with parameters, get values out with return
3Use keyword and default arguments
4Know which variables a function can see
5Predict what a function can and cannot change in the caller

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal2 / 39

The problem: the same lines, again and again

Without a function

Code

account = 20
balance = 100
balance = balance + 100
print("Account:", account)
print("Balance:", balance)
balance = balance - 50
print("Account:", account)
print("Balance:", balance)
balance = balance - 10
print("Account:", account)
print("Balance:", balance)

With a function

Code

def show(account, balance):
    print("Account:", account)
    print("Balance:", balance)

account = 20
balance = 100
balance = balance + 100
show(account, balance)
balance = balance - 50
show(account, balance)
balance = balance - 10
show(account, balance)

Change the report format once, inside show, and every call uses the new format.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal3 / 39

You already use functions

Built-in

print(), len(), type(), input(), max(): written by Python's developers, ready to use.

From modules

math.sqrt(), pd.read_csv(): written by others, loaded with import.

Your own

Written with def. Today's topic.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal4 / 39

Anatomy of a function

Code

def hello():
    print("Hello World!")

hello()

Output

Hello World!

Keyword def defines a function

Function name: you choose it

Brackets: inputs go here (none yet)

Body: indented, runs only when the function is called

Call: runs the body. Brackets are required

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal5 / 39

Parameters and arguments

Code

def hello(name):
    print(f"Hello {name}!")

hello("Joy")
hello("Rini")

Output

Hello Joy!
Hello Rini!

Parameter: a name for the input, used inside the function

The parameter is used like a normal variable

Argument: the actual value sent in

Another call, another argument, same code

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal6 / 39

What if we forget the argument?

Predict first

Code

def printme(text):
    print(text)

printme()

What will this print? Write your prediction as a comment before you run the cell.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal7 / 39

Answer: What if we forget the argument?

Code

def printme(text):
    print(text)

printme()

Output

TypeError: printme() missing 1 required positional argument: 'text'

printme has one required parameter
The call sends nothing, so Python stops with a TypeError
The message names the missing parameter: text
Fix: printme("Hello")

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal8 / 39

return: hand a value back

Code

def add(a, b):
    s = a + b
    return s

x = add(5, 6)
print(x)

Output

Two parameters, separated by a comma

return sends the value back and ends the function

The returned value is stored in x

5 goes to a, 6 goes to b, by position

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal9 / 39

Watch it run: a function call

WATCH FOR

Defining a function does not run it
At the call, a new orange frame appears
Parameters a and b get the argument values
return sends 11 back, and the frame disappears

Silent video: press play, pause any time.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal10 / 39

print or return?

Predict first

Code

def double_print(x):
    print(x * 2)

def double_return(x):
    return x * 2

a = double_print(4)
b = double_return(4)
print(a, b)

What will this print? Write your prediction as a comment before you run the cell.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal11 / 39

Answer: print or return?

Code

def double_print(x):
    print(x * 2)

def double_return(x):
    return x * 2

a = double_print(4)
b = double_return(4)
print(a, b)

Output

8
None 8

double_print shows 8, but gives nothing back
A function without return gives back None
double_return gives back 8, stored in b
To use a result later, return it

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal12 / 39

Watch it run: print versus return

WATCH FOR

Both functions produce 8
Only double_return hands it back
Watch a become None, b become 8
Shown on screen is not the same as returned

Silent video: press play, pause any time.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal13 / 39

Returning several values

return a, b, c packs the values into a tuple
Unpack them into separate names (Lecture 3.2)
The number of names must match
Avoid names like str and sum: they hide built-ins

Code

def check():
    return "ok", 10, 3.14

result = check()
print(result)
print(type(result))
status, num1, num2 = check()
print(status, num1, num2)

Output

('ok', 10, 3.14)
<class 'tuple'>
ok 10 3.14

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal14 / 39

Keyword arguments: pass by name

Name the parameter in the call: age=50
Then order does not matter
Makes calls with many inputs easier to read
Lecture 6 uses this a lot: test_size=0.2

Code

def printinfo(name, age):
    print("Name:", name)
    print("Age:", age)

printinfo(age=50, name="Miki")

Output

Name: Miki
Age: 50

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal15 / 39

Default arguments: a value if none is given

age=35 in the definition is a default
Leave it out and 35 is used
Pass a value and it replaces the default
Parameters with defaults must come after those without

Code

def printinfo(name, age=35):
    print("Name:", name, "| Age:", age)

printinfo("Miki")
printinfo("Miki", 50)
printinfo(name="Joy", age=20)

Output

Name: Miki | Age: 35
Name: Miki | Age: 50
Name: Joy | Age: 20

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal16 / 39

The rules for arguments

Read the last line of an error first. It names the problem.

What you typed	What Python says	What it means
def f(age=35, name): pass	SyntaxError: parameter without a default follows parameter with a default	Parameters with defaults must come after required ones.
printinfo(name="Miki", 50)	SyntaxError: positional argument follows keyword argument	Positional arguments must come before keyword arguments.
printinfo("Miki", name="Joy")	TypeError: printinfo() got multiple values for argument 'name'	name got two values: one by position, one by keyword.

What you typed

What Python says

What it means

def f(age=35, name):
    pass

SyntaxError: parameter without a default follows parameter with a default

Parameters with defaults must come after required ones.

printinfo(name="Miki", 50)

SyntaxError: positional argument follows keyword argument

Positional arguments must come before keyword arguments.

printinfo("Miki", name="Joy")

TypeError: printinfo() got multiple values for argument 'name'

name got two values: one by position, one by keyword.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal17 / 39

Variables inside a function stay inside

greeting is a local variable: it exists only while the function runs
Outside, Python has never heard of it
Inputs come in through parameters
Results go out through return

Code

def make_greeting():
    greeting = "Hi"
    return greeting

print(make_greeting())
print(greeting)

Output

Hi
NameError: name 'greeting' is not defined

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal18 / 39

How arguments are passed: names point to objects

A variable is a name that points to an object
Passing an argument does not copy the object
The parameter becomes a second name for the same object
Python's rule is called pass by object reference (or pass by assignment)
It is neither classic call by value nor call by reference

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal19 / 39

Evidence: the same object inside and outside

id() gives an object's identity number
Same number means same object
So the function received the object itself, not a copy
Your numbers will differ; they only need to match

Code

def show_id(arg):
    print("inside: ", id(arg))

var = "Hello"
print("outside:", id(var))
show_id(var)

Output

outside: 139860846426944
inside:  139860846426944

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal20 / 39

Changing a list inside a function changes it outside

lst.append(100) changes the list object itself
nums names that same object
So the caller sees the change
Lists and dictionaries are mutable: they can be changed

Code

def add_item(lst):
    lst.append(100)

nums = [10, 20, 30]
add_item(nums)
print(nums)

Output

[10, 20, 30, 100]

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal21 / 39

But reassigning inside a function does not

lst = [99] points the local name at a new list
The caller's name still points to the old list
Under true call by reference, nums would now be [99]
So Python is not call by reference

Code

def replace(lst):
    lst = [99]
    print("inside: ", lst)

nums = [10, 20, 30]
replace(nums)
print("outside:", nums)

Output

inside:  [99]
outside: [10, 20, 30]

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal22 / 39

Numbers and strings can only be replaced

Integers and strings are immutable: no operation changes them
n = n + 1 makes a new object and moves the local name
The caller's x is untouched
To get a new value out, return it: x = add_one(x)

Code

def add_one(n):
    n = n + 1
    return n

x = 10
add_one(x)
print(x)
x = add_one(x)
print(x)

Output

10
11

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal23 / 39

Watch it run: change the object, or move the name

WATCH FOR

Green note: lst and nums name the same list
append changes that list, so nums changes too
lst = [99] moves only the local name
After replace, nums is unchanged

Silent video: press play, pause any time.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal24 / 39

The one rule to remember

`lst.append(x)`	Changes the object: caller sees it
`lst[0] = x`	Changes the object: caller sees it
`d[key] = x`	Changes the object: caller sees it

`lst = [...]`	Moves the local name: caller does not
`n = n + 1`	New object: caller does not see it
`return value`	The reliable way to send results out

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal25 / 39

Built-in math functions

You type	Python gives	Meaning
`abs(-5)`	`5`	distance from zero
`max(5, 9)`	`9`	largest of the arguments
`max([4, 9, 2])`	`9`	or largest item of a list
`min(5, 9)`	`5`	smallest
`pow(2, 3)`	`8`	2 ** 3
`pow(2, 3, 5)`	`3`	(2 ** 3) % 5
`round(5.8)`	`6`	nearest whole number
`round(3.14159, 2)`	`3.14`	round to 2 decimal places
`round(2.5)`	`2`	halves go to the even number
`sum([5, 6])`	`11`	sum needs a list, not sum(5, 6)

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal26 / 39

Mistakes to avoid

Read the last line of an error first. It names the problem.

What you typed	What Python says	What it means
sum(5, 6)	TypeError: 'int' object is not iterable	sum needs one list: sum([5, 6]).
sum = 10 print(sum([1, 2]))	TypeError: 'int' object is not callable	sum = 10 hid the built-in sum. Never use built-in names for variables.
nums = [1, 2] nums = nums.append(3) print(nums)	None	No error, but None: append returns nothing, and nums lost its list.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal27 / 39

Modules: borrowing more functions

A module is a file of ready-made functions
import math loads it; use math.name
from math import lcm imports one name directly
Use math.gcd to check your exercise answers

Code

import math
print(math.sqrt(16))
print(math.gcd(8, 12))
print(math.pi)

from math import lcm
print(lcm(8, 12))

Output

4.0
4
3.141592653589793
24

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal28 / 39

In-class exercises: write functions

Each task is a function that RETURNS its answer. Test it with the examples.

evens(n)

Return a list of even numbers from 1 to n. evens(10) gives [2, 4, 6, 8, 10].

gcd(a, b)

Greatest common divisor, with a loop. gcd(8, 12) gives 4.

lcm(a, b)

Use your gcd: a * b // gcd(a, b). lcm(8, 12) gives 24.

mex(lst)

Smallest non-negative integer not in the list. mex([2, 1, 4, 0, 5]) gives 3.

find_operator(number)

Return the operator name, or "INV" if not 11 characters or unknown prefix.

second_largest(lst)

Largest value smaller than the maximum. second_largest([4, 9, 2, 9, 7]) gives 7.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal29 / 39

Solutions

Show these after students have tried the exercises.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal30 / 39

Solution: evens

Build the answer in a list, return it at the end. The caller decides whether to print it.

Code

def evens(n):
    result = []
    for i in range(1, n + 1):
        if i % 2 == 0:
            result.append(i)
    return result

print(evens(10))

Output

[2, 4, 6, 8, 10]

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal31 / 39

Solutions: gcd and lcm

lcm reuses gcd: functions calling functions is how bigger programs are built.

Code

def gcd(a, b):
    result = 1
    for i in range(1, min(a, b) + 1):
        if a % i == 0 and b % i == 0:
            result = i
    return result

def lcm(a, b):
    return a * b // gcd(a, b)

print(gcd(8, 12), gcd(86, 44), lcm(8, 12))

Output

4 2 24

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal32 / 39

Watch it run: a function that calls a function

WATCH FOR

Frames stack up: main, sum_of_squares, square
Each caller waits until its call returns
square runs twice, each time with a fresh frame
Like lcm calling gcd in the solution

Silent video: press play, pause any time.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal33 / 39

Solutions: mex and second_largest

while m in lst keeps counting up until it finds a number that is missing.

Code

def mex(lst):
    m = 0
    while m in lst:
        m = m + 1
    return m

def second_largest(lst):
    largest = max(lst)
    others = []
    for x in lst:
        if x != largest:
            others.append(x)
    return max(others)

print(mex([2, 1, 4, 0, 5]), mex([2, 1, 4, 0, 5, 3]))
print(second_largest([4, 9, 2, 9, 7]))

Output

3 6
7

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal34 / 39

Solution: find_operator

return ends the function at once, so an invalid number can never also print an operator name.

Code

def find_operator(number):
    if len(number) != 11:
        return "INV"
    match number[:3]:
        case "017" | "013": return "Grameenphone"
        case "014" | "019": return "Banglalink"
        case "015": return "Teletalk"
        case "016": return "Airtel"
        case "018": return "Robi"
        case _: return "INV"

print(find_operator("01411111111"))
print(find_operator("0131234567890"))

Output

Banglalink
INV

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal35 / 39

Interactive lab: Functions Lab

In the lab you can

Call visualiserStep through calls: frames open, values go in, return comes back.
Argument matcherSee which argument fills which parameter, or why it fails.
Change or move?Predict whether a function changes the caller's list.
Write and test9 exercises with automatic tests checked against Python.

Open the lab ►︎

Opens in a new tab and works offline. All labs: labs/START_HERE.html

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal36 / 39

Today's toolkit

`def name(p1, p2):`	Define a function
`return value`	Send a result back
`name(a1, a2)`	Call it with arguments
`name(p2=x, p1=y)`	Keyword arguments

`def f(p, q=35):`	Default value for q
`a, b = f()`	Unpack several return values
`import math`	Load a module
`math.sqrt(x)`	Use a module's function

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal37 / 39

Homework 4.1

Due before Lecture 4.2 · [date]

Predict, then run

Predict first: def f(x): x.append(1); then a = [0]; f(a); print(a). And def g(x): x = x + [1]; with the same test.

Temperature

Write c_to_f(c) that returns c * 9 / 5 + 32. Print a table for 0, 25, 37, 100 using a loop.

Grade function

Turn the Lecture 3.1 grade code into grade(score) that returns A, B, C or F. Test all four.

Spam check

Write is_spam(message, words) that returns True or False. Default words: ["free", "win", "prize"].

Explain

In 3 sentences: why does f change a but g does not? Use the words object and name.

Submit on [LMS / Google Classroom]: your notebook file HW4_1_<your ID>.ipynb, or a Colab link set to “Anyone with the link can view”.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal38 / 39

Recap: Lecture 4.1 in 90 seconds

Watch it again at home to revise.

Md Mahbub E NoorLecture 4.1: FunctionsUniversity of Barishal39 / 39