Phone makers buy their parts

A phone maker does not make the screen, the camera and the battery from nothing. They buy parts that other people made and tested.

Programmers do the same. Other people wrote and tested code for common jobs. You can use it. This lesson shows where it comes from, and its good and bad points.

Key words

Word	What it means	Example
Pre-written code	Code that somebody else wrote, tested and shared.
User-written code	Code that you write yourself for your own program.
Built-in function	A function that is part of Python. It is always there.	len(), max(), round()
Library	A collection of pre-written code for one kind of job.	math, random
import	The Python word that makes a library ready to use.	import math
Third-party library	A library made by another company or group. You install it first.	pandas
Function	A named piece of code that gives back a result.
Licence	The rules for how you may use somebody else's code.	free to use, but you must name the author

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Four places code comes from

Where from	Who wrote it	Example	Do you import it?
User-written	You	def ticket_cost()	No
Built into Python	The Python team	len(), print(), round()	No
Python library	The Python team	`math.sqrt()`, `random.randint()`	Yes: import math
Third-party library	Another group	pandas, matplotlib	Install it, then import it

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Try it: built-in functions

Code: python

prices = [3.5, 1.2, 2.0]
print(len(prices))
print(max(prices))
print(min(prices))
print(round(7.6))
print(prices.count(2.0))
print(ord("A"))
print(chr(66))

# Add another price to the list and run it again.

Try it: the math library

Code: python

import math

print(math.floor(7.6))
print(math.ceil(7.6))
print(math.trunc(-7.6))
print(math.sqrt(81))
print(math.pow(2, 5))
print(math.pi)

# Try math.floor(-7.6). What do you expect?

What the math functions give

You write	It gives	In words
math.floor(7.6)	7	Down to the whole number below
math.ceil(7.6)	8	Up to the whole number above
math.trunc(-7.6)	-7	Cut off the decimal part
round(7.6)	8	To the nearest whole number
`math.sqrt(81)`	9.0	The square root: 9 times 9 is 81
math.pow(2, 5)	32.0	Five 2s multiplied: 2 x 2 x 2 x 2 x 2

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Try it: the random library

Code: python

import random

print(random.randint(1, 6))
print(random.random())
print(random.uniform(1, 10))
print(random.sample(["Sam", "Aisha", "Tom", "Mei"], 2))

# Run it three times. The answers change.