Python cheat sheet

# A comment starts with a number sign.
print("Good morning!")
print('Two kinds of quotes work')

• print shows a message on the screen.
• A string is text in quotes.
• A line has no semicolon at the end. The end of the line ends the instruction.

city = "Lisbon"
score = 10
score = 12        # a new value
final_score = 97  # lower case and underscores

Type	Example	What it is
str	"Mars"	text
int	42	a whole number
float	3.5	a number with decimals
bool	True	True or False

type(value) tells you the type of a value.

Operator	What it does	Example	Result
+ - *	add, subtract, multiply	7 * 2	14
/	divide. The result is a float.	7 / 2	3.5
//	divide and drop the decimals	7 // 2	3
%	remainder	7 % 2	1
**	power	7 ** 2	49

name = "Sam"
print(f"Hello, {name}!")       # Hello, Sam!
print(f"The sum is {2 + 2}.")  # The sum is 4.

An f-string has the letter f before the first quote. Braces put a value into the text.

A method gives a new string. It does not change the old one.

Code	Result
" hi ".strip()	"hi"
"Hello".lower()	"hello"
"hello".upper()	"HELLO"
"a quick fox".title()	"A Quick Fox"
"a-b-c".split("-")	["a", "b", "c"]
"a fox".replace("a", "the")	"the fox"
"hello".startswith("he")	True
len("python")	6
"python"[0]	"p"
"python"[-1]	"n"
"python"[1:4]	"yth"
"th" in "python"	True

Positions start at 0. A slice stops before its end position.

if score >= 90:
    print("Great")
elif score >= 50:
    print("Pass")
else:
    print("Try again")

• A line with if, elif or else ends with a colon.
• The block under it has an indent of four spaces.
• Python runs the first block whose condition is true.

Operator	Meaning
==	is equal to
!=	is not equal to
> <	greater than, less than
>= <=	greater or equal, less or equal
a and b	true when the two sides are true
a or b	true when one side or the two sides are true
not a	True becomes False, False becomes True

One = stores a value. Two == compare two values.

for i in range(3):
    print(i)    # 0, 1, 2

n = 3
while n > 0:
    print(n)    # 3, 2, 1
    n -= 1      # without it, the loop never ends

total = 0
for i in range(1, 4):
    total += i  # the same as total = total + i
print(total)    # 6

Code	Numbers
range(4)	0, 1, 2, 3
range(1, 4)	1, 2, 3
range(0, 10, 3)	0, 3, 6, 9

The end number is not included. To join a number to text, use str(3) or an f-string.

colors = ["red", "green", "blue"]
print(colors[0])    # red
print(colors[-1])   # blue, the last item
print(len(colors))  # 3
print(colors[0:2])  # ['red', 'green']

for color in colors:
    print(color)

Code	What it does
colors.append("pink")	adds an item to the end
colors.pop()	removes the last item and gives it back
colors.remove("red")	removes the first item that matches
colors.sort()	puts the items in order
"red" in colors	True when the item is in the list
", ".join(colors)	one string from all text items
sum([10, 7, 9])	adds the numbers of a list

A list can change. A method such as append changes the list that you have.

pet = {"name": "Biscuit", "age": 4}

print(pet["name"])     # Biscuit
pet["age"] = 5         # change a value
pet["color"] = "brown" # add a pair

print(pet.get("weight"))     # None
print(pet.get("weight", 0))  # 0
print("name" in pet)         # True
print(len(pet))              # 3

for key, value in pet.items():
    print(key, value)

• pet["weight"] stops with a KeyError when the key does not exist. get does not.
• pet.keys() gives the keys, and pet.values() gives the values.

A list of dictionaries

pets = [
    {"name": "Biscuit", "age": 4},
    {"name": "Pepper", "age": 2},
]

for pet in pets:
    print(pet["name"], "is", pet["age"])

def add(a, b):        # a and b are parameters
    return a + b


print(add(2, 3))      # 5. 2 and 3 are arguments.


def welcome(name, greeting="Hello"):
    return f"{greeting}, {name}!"


print(welcome("Sam"))        # Hello, Sam!
print(welcome("Sam", "Hi"))  # Hi, Sam!

• return gives a value back to the code that called the function, and it stops the function.
• print shows a value to a person. A function that only prints gives back None.
• A parameter with a default value goes after the parameters without one.
• f"{value:.1f}" puts a number into text with one digit after the decimal point.