# a number
x = 5
y = -10.6
# strings
hello = "Hi, Python!"
# booleans
pythonIsHard = False
pythonIsFun = True
print() is a function that prints things to the console. Those things can either be strings, or variables (which Python will convert to strings)
print("hello") # prints "hello"
print(hello) # prints "Hi, Python!"
You can 'add' strings together ("concatenation") with +
city0 = "Evanston"
city1 = "Chicago"
cities = city0 + " and " + city1
print(cities)
This can be used to print multiple things to the console on the same line. But all those + can be hard to read, so you can also make "F-strings" which can contain Python
print("the best number is " + str(x) + ", but " + str(y) + " is good too")
print(f"the worst number is {x}, but {y} is bad too.")
print(f"....but {y + x + 1000} would be the best")
Asserts are powerful testing tools that let you stop the program if something is False, but will not do anything if it is True
assert city0 == city1, f"cities should be the same! {city0} is not the same as {city1}"
You can compare things with ==, !=, <, >, <=, >= . Be careful to not use = by itself, that assigns a new value!
notTheSame = x != y
stillNotTheSame = not x == y
if x > y:
print(f"{x} is bigger than {y}")
elif x == y:
print(f"{x} is the same as {y}")
else:
print(f"{x} is less than {y}")
You can also compare strings. The same string will be equal but < or > will compare them alphabetically
if city0 > city1:
print(f"{city0} is after {city1}")
elif city0 == city1:
print(f"{city0} is the same as {city1}")
else:
print(f"{city0} is before {city1}")
and, or, and not are also operators. If your statements get complex, add parentheses.
city0 != city1 or not x == y
(city0 != city1) or not (x == y)
Or even better, save each clause as a variable, that way you can test them independently:
citiesNotTheSame = city0 != city1
numbersNotTheSame = not (x == y)
allDifferent = citiesNotTheSame and citiesNotTheSame
print(f"citiesNotTheSame:{citiesNotTheSame}, numbersNotTheSame:{numbersNotTheSame}")
print(f"allDifferent:{allDifferent}")
You can create an list explicitly, or use a range function to create an list of numbers that count up. These both create the same list.
arr0 = [0,1,2,3,4]
arr1 = range(0,5)
You can also create an list by multiplying a string and a number
allExes = "x"*15
print(f"allExes is: {allExes}")
will print:
allExes is: xxxxxxxxxxxxxxx
String comprehensions are special syntax in [] that create lists
squareList = [number*number for number in arr0]
evenSquareList = [x for x in squareList if (x%2==0)]
print(f"squareList is: {squareList}")
print(f"evenSquareList is: {evenSquareList}")
this will print
squareList is: [0, 1, 4, 9, 16]
evenSquareList is: [0, 4, 16]
You can "subscript" (that is, "look up data at a key") with [index]. You can also set items with subscription. Negative numbers count from back from the end of this list, so are good for asking for the last element. Asking for an index that is outside the list (ie. the 5th element in a 5 element list). The code below will print water, love, skittles, then give an IndexError error.
elements = ["water", "air", "fire", "earth", "love"]
print(f"The first element is {elements[0]}")
print(f"The final element is {elements[-1]}")
elements[0] = "skittles"
print(f"The first element is {elements[0]}")
You can also use subscript notation to make new lists from an existing list
print(f"The middle elements are: {elements[1:3]}")
print(f"All but the first elements: {elements[0:-1]}")
If you try to use subscription notation on a non-list Python will let you know with a subscription error.
You can add an element to the end of alist with myList.append(someElement), add at an index with myList.insert(index,someElement), remove the last element with mylist.pop(), or clear it with myList.clear(). Length is a bit different len(myList)
laughter = []
while len(laughter) < 4:
laughter.append("ha")
print(laughter)
You can also make Tuples, which are a lot like lists, but you can't modify them after you make them. This code will cause a 'tuple' object does not support item assignment error.
planets = ("jupiter", "saturn", "uranus", "neptune", "pluto")
planets[5] = "xena"
In Python, for loops iterate over every element of an list. So you need an list to pass them. You can create a new one with range()
for i in range(5):
print (f"The square of {i} is {i*i}")
Or you can use an existing list variable
for number in evenSquareList:
print (f"{number} is an even square")
while loops execute while the condition evaluates to True
count = 5
while count > 0:
count = count - 1
print(f"countdown: {count}")
print("*boom!*")
# run forever....
count = 5
while count > 0:
count = count + 1
print(f"never gonna: {count}")
print("give you up!")
If your condition is never False, they will run forever, and you have to hit CMD-C or CTRL-C to escape the process.
Importing libraries is easy, just import libraryName or from libraryName import someSmallPartOfLibrary for just a part of a library. The most common libraries to import is math with gives you common math operations and sys which lets you do file system operations.
Functions are defined with def and have parameters in (). Make sure you have def () and : and indent the function body!
import math
def getDistance(x, y, z):
sum = x*x + y*y + z*z
return math.sqrt(sum)
vx = 5
vy = 10
vz = 15
dist = getDistance(vx, vy, vz)
print(f"The length of a vector {vx} {vy} {vz} is {dist}, about {round(dist)}")
Dictionaries are lookup tables from string keys to some values, which may be numbers, strings, lists or even other dictionaries. Note that keys are always in "", and every line but the last needs a ,.
translations = {
"cat": {
"spanish": "gato",
"german": "katze",
"finnish": "kissa",
"yoruba": "ologbo"},
"dog": {
"afrikaans": "hond",
"gujarati": "kutto",
"hawaiian": "ilio"
}
}
You can "subscript" (that is, "look up data at a key")
catDictionary = translations["cat"]
You can also ask for an list of all the keys a dictionary has, and use that in a for loop.
catLanguages = catDictionary.keys()
for lang in catLanguages:
print(f"'cat' in {lang} is '{catDictionary[lang]}' ")
key in list will return True or False depending if the key is in the dictionary or not. Asking for a key that is not in the dictionary will give you an error, so this is helpful!
canTranslateFish = "fish" in translations
print(f"Can I translate 'fish' with this dictionary? {canTranslateFish}")
if "french" in catLanguages:
print(f"'A French cat is '{catDictionary['french']}' ")
else:
print("No cat translation for French")
Classes create objects that have their own individual data, but also access to class methods, special functions that can run with self.myMethodName()
class Cat:
catWord = "meow"
def __init__(self, name):
self.fish = 0
self.mood = "happy"
self.name = name
def speak(self):
return f"{self.name} says '{self.catWord}'"
cat0 = Cat("Bustopher")
cat1 = Cat("Grizabella")
cat1.mood = "sad"
print(cat0) will print:
<__main__.Cat object at 0x7fd7d4b3af40>
Not so helpful! So we can add a __str__ method that turns it into a more useful string.
def __str__(self):
return f"{self.name}, a {self.mood} cat"
Bustopher, a happy cat
Grizabella, a sad cat
You can also inherit from existing classes
class MagicalCat(Cat):
catWord = "abracadabra"
def __init__(self, name):
super().__init__(name)
def speak(self):
return f"{self.name} says ✨'{self.catWord}'✨"
cat2 = MagicalCat("Mistoffelees")
print(cat2.speak())
Grizabella says 'meow'
Mistoffelees says ✨'abracadabra'✨