Session 2

Topics Covered

• Functions, methods & modules
• Reading Files
• The For loop
• The if statement

Welcome Back

!figlet Welcome to Session 2

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\ \      / /__| | ___ ___  _ __ ___   ___  | |_ ___  
 \ \ /\ / / _ \ |/ __/ _ \| '_ ` _ \ / _ \ | __/ _ \ 
  \ V  V /  __/ | (_| (_) | | | | | |  __/ | || (_) |
   \_/\_/ \___|_|\___\___/|_| |_| |_|\___|  \__\___/ 
                                                     
 ____                _               ____  
/ ___|  ___  ___ ___(_) ___  _ __   |___ \ 
\___ \ / _ \/ __/ __| |/ _ \| '_ \    __) |
 ___) |  __/\__ \__ \ | (_) | | | |  / __/ 
|____/ \___||___/___/_|\___/|_| |_| |_____|
                                           

Review of Last Session

Any questions?

A small problem.

%load_problem countdown

Problem: Countdown

Write a program countdown.py to print numbers from n to 1.

You can verify your solution using:

%verify_problem countdown

The program should take the number n as command-line argument.

$ python countdown.py 5
5
4
3
2
1

%%file countdown.py

print(1)

Writing countdown.py

!python countdown.py 5

%verify_problem countdown

ERROR: Unable to find program countdown.py.

Functions, Methods and Modules

Python has many built-in functions.

sum([1, 2, 3, 4, 5])

15

len("hello")

5

print("hello")

hello

Python doesn’t support operations on incompatible datatypes.

1 + "2"

TypeError: unsupported operand type(s) for +: 'int' and 'str'

1 + int("2")

3

str(1) + "2"

'12'

int("2")

2

str(1)

'1'

Example: counting the number of digits in a number

How to count the number of digits in a number?

12345

12345

2 ** 1000

10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376

number = 1234

number

1234

str(number)

'1234'

len(str(number))

4

len(str(2**1000))

302

len(str(2**10000))

3011

Writing our own functions

def square(x):
    return x*x

square(4)

16

def square(x):
    y = x*x
    return y

square(4)

16

%load_problem cube

Problem: Cube

Write a function cube to compute cube of a number.

>>> cube(2)
8

You can verify your solution using:

%verify_problem cube

# your code here

%verify_problem cube

ERROR: Unable to find function with name cube.

Print vs Return

def square1(x):
    return x*x

def square2(x):
    print(x*x)

square1(4)

16

square2(4)

16

1 + square1(4)

17

1 + square2(4)

16

TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'

square1(square1(4))

256

Functions are values too!

def square(x):
    return x*x

square(4)

16

square() # error

TypeError: square() missing 1 required positional argument: 'x'

square

<function __main__.square(x)>

x = 5

x

5

y = x

type(x)

int

square

<function __main__.square(x)>

type(square)

function

f = square

f

<function __main__.square(x)>

f(4)

16

def square(x):
    return x*x

def sum_of_squares(x, y):
    return square(x) + square(y)

sum_of_squares(3, 4)

25

def cube(x):
    return x*x*x

def sum_of_cubes(x, y):
    return cube(x) + cube(y)

sum_of_cubes(3, 4)

91

def sq1(x):
    return x*x

def sq2(y):
    return y*y

If you notice, the sum_of_squares and sum_of_cubes are almost the same.

Can we generalize these two functions into a single one?

def sumof(f, x, y):
    return f(x) + f(y)

sumof(square, 3, 4)

25

sumof(cube, 3, 4)

91

sumof(len, "hello", "python")

11

1 + 2

3

functions = [square, cube]

functions

[<function __main__.square(x)>, <function __main__.cube(x)>]

functions[0]

<function __main__.square(x)>

functions[0](4)

16

Passing functions as arguments is so useful that there are many built-in functions that accept functions as arguments.

max(3, 4)

4

max([3, 4, 5])

5

max(["one", "two", "three", "four", "five"])

'two'

What if I want to find the longest one?

"one" > "two"

False

max(["one", "two", "three", "four", "five"], key=len)

'three'

def mylen(word):
    print("mylen", word)
    return len(word)

max(["one", "two", "three", "four", "five"], key=mylen)

mylen one
mylen two
mylen three
mylen four
mylen five

'three'

"two" < "three"

False

# with key=len
len("two") < len("three")

True

Methods

Methods are special kind of functions that work on a value/object.

x = "Hello"

x.upper()

'HELLO'

upper is a method available on objects of type str.

type(x)

str

n = 1

n.upper() # doesn't work

AttributeError: 'int' object has no attribute 'upper'

"mathematics".count("mat")

2

"mathematics".replace("mat", "rat")

'ratheratics'

Spliting and joining strings

sentence = "Antyhing that can go wrong, will go wrong."

sentence.split()

['Antyhing', 'that', 'can', 'go', 'wrong,', 'will', 'go', 'wrong.']

len(sentence.split())

8

def wordcount(sentence):
    return len(sentence.split())

wordcount(sentence)

8

sentences = [
    "one two three",
    "one two three four",
    "one two three four five",
    "a b c d e f g"]    

max(sentences, key=wordcount)

'a b c d e f g'

sentence.split() # split on any white space

['Antyhing', 'that', 'can', 'go', 'wrong,', 'will', 'go', 'wrong.']

sentence.split(",") # split on the given delimiter

['Antyhing that can go wrong', ' will go wrong.']

Let’s see how to join the strings back.

"-".join(["a", "b", "c"])

'a-b-c'

":".join(["a", "b", "c"])

'a:b:c'

"".join(["a", "b", "c"])

'abc'

sep = "-x-"

sep.join(["a", "b", "c"])

'a-x-b-x-c'

Modules

import time

time.asctime()

'Mon Sep 25 05:25:51 2023'

time.asctime()

'Mon Sep 25 05:25:56 2023'

time 

<module 'time' (built-in)>

print(time)

<module 'time' (built-in)>

t = time

type(time)

module

t

<module 'time' (built-in)>

t.asctime()

'Mon Sep 25 05:26:43 2023'

!date

Mon Sep 25 05:27:12 AM UTC 2023

Let’s see how to implement it in Python.

%%file date.py
import time

print(time.asctime())

Writing date.py

!python date.py

Mon Sep 25 05:27:45 2023

!date

Mon Sep 25 05:27:48 AM UTC 2023

The os module

import os

os.listdir(".")

['index.qmd',
 'assignments.qmd',
 'assignment-01.qmd',
 '_quarto.yml',
 '_book',
 'course.qmd',
 'references.qmd',
 '.ipynb_checkpoints',
 'date.py',
 'args.py',
 'styles.css',
 'echo.py',
 'syllabus.qmd',
 'countdown.py',
 'session2.ipynb',
 'square.py',
 '.quarto',
 'lab.qmd',
 'script.js',
 'session1.ipynb.invalid',
 'schedule.qmd',
 'session1.ipynb',
 'hello.py',
 'live-notes.qmd',
 'schedule.ipynb',
 'references.bib']

len(os.listdir("."))

26

# os.path.getsize("session-02.ipynb")
os.path.getsize("session2.ipynb")

82059