Python basics

Everyday Python: types, collections, comprehensions, functions and common idioms.

25 cardsBeginnerCode

Open the downloaded file on your iPhone or iPad and choose StackCards. It is also in the app: Library tab, then Add to my library.

What is inside

Print a valueWrites text to the console.

print("Hello")

Make a listOrdered, changeable, and allows duplicates.

numbers = [1, 2, 3]

Make a dictionaryMaps keys to values.

ages = {"Ana": 30, "Luis": 25}

Make a tupleOrdered and unchangeable.

point = (4, 7)

Make a setUnordered collection of unique items.

tags = {"a", "b", "a"}  # {'a', 'b'}

Loop with an indexenumerate gives the position and the item together.

for i, name in enumerate(names):
    print(i, name)

Loop over two lists togetherzip pairs items up and stops at the shortest.

for a, b in zip(xs, ys):
    print(a, b)

List comprehensionBuilds a list from an expression in one line.

squares = [n * n for n in range(5)]

Comprehension with a filterAdds a condition at the end.

evens = [n for n in nums if n % 2 == 0]

Dictionary comprehensionBuilds a dict the same way.

lengths = {w: len(w) for w in words}

Define a functiondef, a name, parameters and a return value.

def add(a, b=0):
    return a + b

Accept any number of arguments*args collects positionals, **kwargs collects keywords.

def f(*args, **kwargs):
    ...

Anonymous functionA small function without a name.

double = lambda x: x * 2

Format a stringAn f-string puts values straight into the text.

f"{name} is {age}"

Read a file safelywith closes the file even if something fails.

with open("data.txt") as f:
    text = f.read()

Handle an errorCatch a specific exception and keep going.

try:
    int(text)
except ValueError:
    print("not a number")

Define a classMethods take self as the first parameter.

class Dog:
    def __init__(self, name):
        self.name = name

Slice a liststart:stop:step, where stop is not included.

nums[1:4]   # items 1, 2, 3
nums[::-1]  # reversed

Check if a key exists safelyget returns a default instead of raising KeyError.

ages.get("Eva", 0)

Sort a list by a rulekey says what to compare.

people.sort(key=lambda p: p.age)

Swap two variablesTuple unpacking needs no temporary.

a, b = b, a

Truthy and falsyEmpty collections, 0, None and "" count as False; most else is True.

== vs is== compares values. is checks whether both names point to the very same object.

Mutable vs immutableLists, dicts and sets can change in place. Strings, tuples and numbers can't.

What is a generator?A function that yields values one at a time instead of building the whole list in memory.