Python Basics Cheat Sheet

A Programming › Python cheat sheet in English by davidhemer

Contains the basics of Python programming

Python Variables Data Strings Loops Types

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Data types

Strings	Represent text	`"Hello"`
Integers	Represent whole numbers	`3`
Floating point numbers	Represent decimal values	`3.258`
Scientific notation	Used to represent very large or very small numbers	`3.798e17`

Mathematical operators

+	addition	`3+2`
-	subtraction	`72.2-83.7`
*	multiplication	`42.8*21.3`
/	division	`22.2/3.78`
//	integer division	`25//12`
%	modulus (remainder)	`35%2`

Comparing numbers

`==`	True if two numbers are equal
`!=`	True if two numbers are not equal
`<=`	True if the first number is less than or equal to the second
`<`	True if the first number is strictly less than the second
`>=`	True if the first number is greater than or equal to the second
`>`	True if the first number is strictly greater than the second

Importing modules

import math
from sigfig import *
from sigfig import round

The first import statement requires all calls to functions from this module to be prefixed with the module name, e.g. math.gcd(4,8)
The second import statement allows functions from the sigfig module to be called without prefixes. Name clashes can provide unexpected results.
The third import statement allows us to select the module functions that we want to use.

Math module functions

square root	math.sqrt	`math.sqrt{25.2}`
greatest common divisor	math.gcd	`math.gcd(25,8)`
power	math.pow	`math.pow(3,5)`
pi	math.pi	`math.pi`

sigfig module

from sigfig import *

# round to a given number of decimal places
round(number, decimals=3)

# round to the given number of significant figures
round(number, sigfigs=4)

Variables

import math

# assign values to variables
r=8.2
c=2\*math.pi\*r
print(c)

# update the value of a variable
r=r+5
c=2\*math.pi\*r
print(c)

Strings

`"Fred"`	`'Fred'`	`"The cat sat on the mat"`
`'The answer is 3.24'`	`"$56.85"`	`"Beware of the llama!"`
`'###IMPORTANT###'`	`"46**2"`	`"()$@!\#"`

Strings are enclosed in double quotes or single quotes.

Special characters in strings

`\n`	New line
`\t`	Horizontal tab
`\\`	Backslash
`\'`	Single quote
`\"`	Double quote

Print statements

print("Hello World")
print("\"Hello World\"") # Enclosed in quotes
print("Hello\nWorld") # New line between words
print("Hello\tWorld") # Tab between words

Formatting strings

name="Clarence"
print("Hello my name is {}".format(name))

pet="cat"
place="mat"
print("The {0} sat on the {1}. The {0} is very lazy".format(pet, place))

Replacement fields are replaced by the corresponding string from the list of arguments in the format function.
The indexes start at 0, so {0} is replaced by the value of the first argument.

Format specifiers

d	`{0:d}`	Represents a number as a whole number
f	`{0:.2f}`	Represents a number with a fixed number of decimal places
e	`{0:.3e}`	Represents a number using scientific notation.
%	`{0:.1%}`	Represent a decimal as a percentage with the specified number of decimal places
b	`{0:b}`	Represent a number in binary form

Format specifiers are used in print statements to describe how a number is represented.

Formatting numbers in strings

aNum=3234.374

print("{0} with 0 dps is {0:.0f}".format(aNum))
print("{0} with 1 dp is {0:.1f}".format(aNum))
print("{0} with 2 dps is {0:.2f}".format(aNum))
print("{0} in scientific notation is {0:e}".format(aNum))
print("{0} in scientific notation with one decimal place is {0:.1e}".format(aNum))
print("{0} in scientific notation with two decimal places is {0:.2e}".format(aNum))

Formatting percentages

student1="Beryl"
score1=0.828
student2="Marvin"
score2=0.738

print("{0} got {1:.0%} in her Maths test".format(student1, score1))
print("{0} got {1:.1%} in his Maths test".format(student2, score2))

The format specifier {0:.2%} formats the first argument in the format list as a percentage with two decimal places.
Note that the number being formatted should be represented as a decimal.

Input statements

name=input("Enter your name: ")
favFood=input("Enter your favourite food: ")

print("Hi {0}, seems like your favourite food is {1}".format(name, favFood))

Input statements store the resulting value as a string. These strings can then be used in a print statement.

Converting strings

int	Converts a string to an integer	`int("32")`
float	Converts a string to a floating point number	`float("56.78")`

String conversions can be used with input statements, where values are always entered as strings.

Conditional statements

age=int(input("How old are you? "))

if (age >= 18):
    print("Don't forgot to vote.")

Check whether a person is 18 years. Remember to convert the age into an integer before doing the check.

If then else statement

import random

target=random.randint(1,10)

guess=int(input("Enter your guess between 1 and 10: " ))

if (guess==target):
    print("Correct! Well done.")
else:
    print("Wrong. The correct answer was {:d}.".format(target))

print("Thanks for playing.")

A lucky number guessing game. Uses the random module to generate a random number between 1 and 10.

Else if statements

import random

age=random.randint(1,99)

print("Age is {:d}.".format(int(age)))
if (age <= 12):
    print("You are a child, tickets cost $10.50")
elif (age <60):
    print("You are an adult, tickets cost $14.00")
else:
    print("You are a senior, tickets cost $12.00")

Random number generator used to generate a random age between 1 and 99.
Elif branch used to include an additional case.
Multiple

While loops

import random

target=random.randint(1,10)

guess=input("Enter your guess between 1 and 10: " )
num_guesses=1

while (int(guess) != target):
    print("That was wrong. Try again.")
    guess=input("Enter your guess between 1 and 10: " )
    num_guesses=num_guesses+1

print("Correct! Well done. You took {0:d} guesses.".format(num_guesses))
print("Thanks for playing.")

The while loop repeats until the condition is no longer true.
In this case the while loop repeats while the guess is not correct.

For loops

print("The first ten perfect square numbers are:")

for i in range(1,11):
    print("{:d}".format(i**2))

This loop is repeated 10 times. The values of the loop variable i range from 1 up to, but not including, 11.

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