Scope & Modules (math, random)

What is scope?

The scope of a variable is the part of the program where that variable can be used. A variable created inside a function lives only in that function — like a classroom announcement that is heard only inside that classroom, while a school-wide announcement (global) is heard everywhere.

Local and global variables

school = 'KV No. 1'           # global variable

def show():
    percent = 88              # local variable
    print('Inside:', school, percent)

show()
print('Outside:', school)
print(percent)                # percent does not exist here

percent was created inside show(), so it is destroyed when the function ends. Parameters of a function are also local variables.

Same name inside and outside

If a function assigns to a name, Python creates a new local variable, even if a global one has the same name. The global stays unchanged.

x = 5

def change():
    x = 50                    # creates a NEW local x
    print('Local x =', x)

change()
print('Global x =', x)

The global keyword

To change a global variable inside a function, declare it with global.

count = 0

def visit():
    global count              # use the global variable
    count += 1

visit(); visit(); visit()
print('Visitors =', count)

The Python standard library

Python comes with hundreds of ready-made modules called the standard library. A module is simply a .py file with useful functions. Related modules grouped in a folder form a package.

Ways to import a module

from math import sqrt, pi        # import only what you need
print(sqrt(81), round(pi, 2))
import math as m                 # short alias
print(m.floor(9.99))

math module

import math
print(math.sqrt(144), math.pow(2, 5))
print(math.ceil(4.2), math.floor(4.8), math.fabs(-7.5))
print(math.factorial(5), math.gcd(36, 60))
print(round(math.pi, 4), round(math.e, 4))
print(round(math.sin(math.radians(30)), 2), math.log10(1000))

random module

Random values change every run, so your output will differ from the sample below.

import random
print(random.random())          # float 0.0 <= x < 1.0
print(random.randint(1, 6))     # dice: 1 to 6 (both included)
print(random.randrange(10, 50, 5))  # 10, 15, ... 45
print(random.choice(['Red', 'Green', 'Blue']))

import random
# 4-digit OTP
otp = random.randint(1000, 9999)
print('Your OTP is', otp)

statistics module

import statistics
marks = [85, 92, 78, 92, 66, 70]
print('Mean  :', statistics.mean(marks))
print('Median:', statistics.median(marks))
print('Mode  :', statistics.mode(marks))

Mean = sum ÷ count = 483 ÷ 6 = 80.5. Median = middle value of the sorted list [66, 70, 78, 85, 92, 92] = (78 + 85) ÷ 2 = 81.5. Mode = most frequent value = 92.

Creating your own module

Save the functions below as shapes.py. In another file in the same folder, write import shapes and call shapes.circle_area(7).

# file: shapes.py  (your own module)
def circle_area(r):
    return 3.14159 * r * r

def square_area(s):
    return s * s

import shapes
print(round(shapes.circle_area(7), 2))
print(shapes.square_area(9))

Summary & practice

• Local variables live inside a function; global variables live outside.
• Use global name to change a global inside a function.
• Import styles: import m, import m as x, from m import f.
• math, random (random, randint, randrange, choice), statistics (mean, median, mode).