Python cheat sheet

Python cheat sheet · BrainBit Infotech Teaching · brainbit-teaching.com/courses/python/notes/cheatsheet/

Basics

Output, input, comments

print("Hi", name, sep=" ", end="\n")
name = input("Name: ")      # always a string
age = int(input("Age: "))   # convert to a number
# a comment

Data types

type(x) tells you the type of x.

Operators

7 // 2 is 3, 7 % 2 is 1, 2 ** 3 is 8. Order: (), **, * / // %, + -.

Strings

s = "Python"
s[0]      # 'P'      s[-1]   # 'n'
s[0:3]    # 'Pyt'    s[::-1] # 'nohtyP'
len(s)    # 6
s.upper()  s.lower()  s.strip()  s.replace("P", "J")
"a,b,c".split(",")   # ['a', 'b', 'c']
"-".join(["a", "b"]) # 'a-b'
f"{name} scored {marks:.1f}%"

Strings cannot be changed in place: methods return a new string.

Decisions

if marks >= 90:
    grade = "A"
elif marks >= 60:
    grade = "B"
else:
    grade = "C"

result = "Pass" if marks >= 33 else "Fail"

match day:
    case "Sat" | "Sun":
        print("Holiday")
    case _:
        print("School")

Loops

for i in range(1, 6):      # 1 2 3 4 5
    print(i)
for ch in "abc": ...
while n > 0:
    n -= 1

range(stop), range(start, stop), range(start, stop, step): the stop is never included. break leaves the loop; continue skips to the next turn.

Collections

List

nums = [5, 3, 8]
nums.append(1); nums.insert(0, 9)
nums.remove(3); nums.pop()
nums.sort(); sorted(nums, reverse=True)
sum(nums); max(nums); len(nums)
copy = nums.copy()   # b = a is NOT a copy

Dictionary

d = {"name": "Asha", "marks": 91}
d["city"] = "Ranchi"         # add or update
d.get("age", 0)              # no KeyError
for key, value in d.items(): ...
del d["city"]

Set

a | b   # union      a & b   # common
a - b   # difference

Functions

def area(length, width=1):
    """Return the area of a rectangle."""
    return length * width

area(5, 2); area(length=5)
def total(*nums): return sum(nums)
square = lambda x: x * x

A variable made inside a function is local to it.

Files and errors

with open("notes.txt", "w") as f:   # "r" read, "a" append
    f.write("Hello\n")
with open("notes.txt") as f:
    for line in f:
        print(line.strip())

try:
    n = int(input())
except ValueError as e:
    print("Not a number:", e)
else:
    print("OK")
finally:
    print("Always runs")

Common errors: ValueError, TypeError, KeyError, IndexError, ZeroDivisionError, FileNotFoundError.

Classes

class Student:
    school = "DPS"                 # class attribute
    def __init__(self, name, marks):
        self.name = name           # instance attribute
        self.marks = marks
    def passed(self):
        return self.marks >= 33
    def __str__(self):
        return f"{self.name}: {self.marks}"

class Monitor(Student):            # inheritance
    def __init__(self, name, marks, section):
        super().__init__(name, marks)
        self.section = section

Pythonic shortcuts

squares = [n * n for n in range(5)]
evens = [n for n in nums if n % 2 == 0]
for i, name in enumerate(names, start=1): ...
for n, m in zip(names, marks): ...
sorted(students, key=lambda s: s["marks"])

Modules

import math, random
from datetime import date
math.sqrt(16); random.randint(1, 6); date.today()

Install a package from a terminal: pip install requests.