More Python

Functions, Methods & Modules

Functions

Problem: Write a function sum_of_squares that takes two numbers as arguments and returns sum of their squares.

>>> sum_of_squares(3, 4)
25
>>> sum_of_squares(2, 5)
29

Methods

Methods of special kind of functions that work on a value or an object.

x = "Hello"

x.upper()

'HELLO'

"Python".upper()

'PYTHON'

x = 1

x.upper() # doesn't work

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[5], line 1
----> 1 x.upper() # doesn't work

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

Let’s see some more useful methods on strings.

"mathematics".count("mat")

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

'ratheratics'

Spliting and Joining strings

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

sentence.split()

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

We can also specify a delimeter to split it differently.

sentence.split(",")

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

Can we write a function to compute the number of words in a sentence?

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

wordcount("one two three")

Let’s also see how to join strings.

"-".join(["one", "two", "three"])

'one-two-three'

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

'abc'

Reading Files

%%file three.txt
one
two
three

Writing three.txt

ls

index.ipynb  three.txt

open("three.txt").read()

'one\ntwo\nthree\n'

open("three.txt").readlines()

['one\n', 'two\n', 'three\n']

How to print the contents of three.txt in upper case?

contents = open("three.txt").read()
print(contents.upper())

ONE
TWO
THREE

Modules

import time

time.asctime()

'Mon Oct 28 11:52:28 2024'

print(time.asctime())

Mon Oct 28 11:52:40 2024

%%file date.py
import time
print(time.asctime())

Writing date.py

!python date.py

Mon Oct 28 11:53:00 2024

There are mutliple ways to import things in Python.

import time
time.asctime()

'Mon Oct 28 11:57:40 2024'

from time import asctime
asctime()

'Mon Oct 28 11:58:04 2024'

from time import asctime as mytime 
mytime()

'Mon Oct 28 11:59:05 2024'

Let’s look at some more useful modules.

import os

List all files in the current directory.

os.listdir()

['index.ipynb', 'three.txt', 'date.py']

# .. parent directory
os.listdir("..")

['2024-08-19',
 '2024-10-14',
 '2024-10-21',
 '02',
 '2024-08-12',
 '2024-10-28',
 '2024-09-30',
 '01',
 'index.qmd']

You can also try giving whole path.

# works only on unix
os.listdir("/home")

['anand']

You could try the following if you are on windows.

os.listdir("c:\\")

Reading Command-line arguments

Let’s see how to write programs that take command-line arguments.

In Python, the sys module keeps tracks of the command-line arguments passed to the program in variable sys.argv

%%file args.py
import sys
print(sys.argv)

Writing args.py

!python args.py

['args.py']

!python args.py hello

['args.py', 'hello']

!python args.py hello world

['args.py', 'hello', 'world']

Example: Square

Let’s write a script to compute square of a number.

%%file square.py
import sys
n = int(sys.argv[1])
print(n*n)

Writing square.py

!python square.py 5

Lists

names = ["Alice", "Bob", "Charlie"]

len(names)

names[0]

'Alice'

for name in names:
    print("Hello", name)

Hello Alice
Hello Bob
Hello Charlie

numbers = [1, 2, 3, 4, 5]

sum(numbers)

Let’s see how to compute product of numbers.

def product(numbers):
    result = 1
    for n in numbers:
        result *= n
    return result

product([1, 2, 3, 4])

We can add a new element to a list using append method.

numbers = [1, 2, 3, 4, 5]
numbers.append(6)

numbers

[1, 2, 3, 4, 5, 6]

How to computes squares of a list of numbers?

numbers

[1, 2, 3, 4, 5, 6]

numbers + numbers

[1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 5, 6]

def squares(numbers):
    result = []
    for n in numbers:
        result.append(n*n)
    return result

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

[1, 4, 9, 16, 25]

[n*n for n in numbers]

[1, 4, 9, 16, 25, 36]

[n*n for n in range(10) if n%2 == 0]

[0, 4, 16, 36, 64]

# sum of squares of all even numbers below one million
sum([n*n for n in range(1000000) if n%2 == 0])

166666166667000000

How to find all the python files in the current directory?

import os
[f for f in os.listdir() if f.endswith(".py")]

['args.py', 'date.py', 'square.py']

Example: sumfile

Write a program sumfile.py that takes a filename as command-line argument and computes the sum of all numbers in a file.

%%file 5.txt
1
2
3
4
5

Writing 5.txt

%%file sumfile.py
import sys
filename = sys.argv[1]
lines = open(filename).readlines()
print(lines)
numbers = [int(line) for line in lines]
print(numbers)
print(sum(numbers))

Overwriting sumfile.py

!python sumfile.py 5.txt

['1\n', '2\n', '3\n', '4\n', '5\n']
[1, 2, 3, 4, 5]
15

List Indexing & Slicing

x = ["a", "b", "c", "d", "e", "f", "g", "h"]

x[0:2] # from index 0 to index 2 (end is not included)

['a', 'b']

x[:2] # up to index 2

['a', 'b']

x[2:] # from index 2 onwards

['c', 'd', 'e', 'f', 'g', 'h']

x[1:7:3] # from index 1 to 7 in steps of 3

['b', 'e']

x[::-1] # reverse

['h', 'g', 'f', 'e', 'd', 'c', 'b', 'a']

How to get the last element of a list?

['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']

x[len(x)-1]

'h'

x[-1]

'h'

sentence = "one two three"

# last word
sentence.split()[-1]

'three'

Numerical Computing

If you don’t have numpy and matplotlib installed, you can install them using:

!python -m pip install numpy matplotlib

import numpy as np

x = np.array([1, 2, 3, 4, 5])

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

x.dtype

dtype('int64')

x.shape

(5,)

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

x + 10

array([11, 12, 13, 14, 15])

x * x

array([ 1,  4,  9, 16, 25])

1/x

array([1.        , 0.5       , 0.33333333, 0.25      , 0.2       ])

Example: Euclidian distance between two points

Euclidian distance between two vectors is defined as:

\(E(p, q) = \sqrt{\Sigma_{i=1}^{n}(p_{i}-q_{i})^2}\)

P = np.array([1, 2, 3, 4, 5])
Q = np.array([10, 20, 30, 40, 50])

d = P-Q

np.sqrt(np.sum(d*d))

np.float64(66.74578638386096)

def euclidean_distance(P, Q):
    d = P-Q
    return np.sqrt(np.sum(d*d))

euclidean_distance(P, Q)

np.float64(66.74578638386096)

Plotting Graphs

Install matplotlib using:

!python -m pip install matplotlib

import numpy as np
import matplotlib.pyplot as plt

Let’s plot \(y = x^2\).

# divide [-10, 10] into 100 numbers
x = np.linspace(-10, 10, 100)
y = x*x

plt.plot(x, y)
plt.grid()
plt.title("$y = x^2$")

Text(0.5, 1.0, '$y = x^2$')

plt.plot(x, x*x, label="$x^2$")
plt.plot(x, 2*x, label="$2 x$")
plt.grid()
plt.legend()