QUICK REFERENCE - PYTHON 3 - Python Essentials, at a glance - Syntax & Collections - Control Flow & Practical Tools

Oct 13, 2026

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Quick Reference for Python 3 provides a compact, at-a-glance cheat sheet covering core language syntax and everyday programming patterns. It explains essentials such as variables and types, operators and truthiness, strings, lists, indexing and slicing, dictionaries, tuples and sets, and comprehensions. It also covers control flow with conditionals and loops, function definitions and arguments, input and exception handling, file I/O with JSON, useful built-ins, class basics, virtual environments, and key reminders for correctness.

LanguageEnglish

CategoryPresentations

FormatPDF

Pages2

Reading Time1 min

AccessFree to download

01 / Syntax & Collections

01 Start here

02 Variables & types

03 Operators & truthiness

04 Strings

05 Lists

06 Indexing & slicing

07 Dictionaries

08 Tuples & sets

09 Comprehensions

02 / Control Flow & Practical Tools

10 Conditionals

11 Loops

12 Functions

13 Input & exceptions

14 Files & JSON

15 Useful built-ins

16 Classes

17 Virtual environments

18 Remember these

QUICK REFERENCE / PYTHON 3 Python The essentials, at a glance. DevSheets 01 / SYNTAX & COLLECTIONS 01 Start here Install: [python.org/downloads](python.org/downloads) python3 --version python3 app .py Windows: use python or py. Run python3 alone to open the interactive shell. Use 4 spaces per indent. A \# starts a comment. 02 Variables & types name = "Maya"age = 28 price = 19.95 active = True result = None \# str \# int \# float \# bool \# no value Use snake_case names. True, False, and None are capitalized. No type declaration is required. 03 Operators & truthiness + - * /// % **== != < > <= >= and or not in / not in 7 / 2 7 // 2 2 ** 3 \# 3.5 \# 3 \# 8 Arithmetic Floor, remainder, power Compare values Boolean operators Membership False, None, zero, and empty strings or collections are falsy. Check None with is None. 04 Strings Text is immutable; methods return new values. name = "Maya" f"Hi, {name}!" \# 'Hi, Maya!' f"{3.14159 : .2f}" \# '3 .14' " hi " .strip() \# 'hi' "Hello" .lower() \# 'hello' "a,b" .split(",") \# ['a', 'b'] " - " .join([ "a" , "b"]) \# 'a-b' "cat" . replace( "c" , "b") \# 'bat' 05 Lists Ordered and mutable; duplicates are allowed. nums = [10 , 20 , 30] nums.append(40) \# add one nums.extend([50 , 60]) \# add many nums . remove(20) \# first match last = nums.pop() \# remove last len(nums) \# item count nums.sort() \# change nums ordered = sorted(nums) \# new list sort() changes the list and returns None. sorted() returns anew sorted list. 06 Indexing & slicing text = "Python" text[0] text[-1] text[1:4] text[::-1] \# 'P' \# 'n' \# 'yth' \# 'nohtyP' sequence[start:stop:step]. Start is included; stop is excluded. Works with lists and tuples too. 07 Dictionaries Mutable key-value mappings. Keys are unique and must be hashable, such as strings or numbers. user = { "name" : "Maya" , "age" : 28} user["name"] \# 'Maya' user["age"] = 29 user.get( "role" , "guest") \# fallback "name" in user \# True for key, value in user .items(): print(key, value) d[key] raises KeyError for a missing key. get() reads a fallback without adding the key. 08 Tuples & sets point = (10 , 20) x, y = point \# unpack single = (42,) \# one-item tuple tags = { "web" , "api" , "web"} tags.add("python") tags.discard("api") \# safe if absent {1 , 2} | {2 , 3} \# union: {1, 2, 3} {1 , 2} & {2 , 3} \# intersection: {2} Tuples are immutable. Sets hold unique, hashable values in no guaranteed order. Empty set: set(). 09 Comprehensions nums = [1 , 2 , 3 , 4] squares = [n * n for n in nums] evens = [n for n in nums if n % 2 == 0] unique = {n % 2 for n in nums} Use [] for a list, {key: value} for a dict, and {value} for a set. Add if to filter. QUICK REFERENCE / PYTHON 3 Python Everyday patterns, ready to use. DevSheets 02 / CONTROL FLOW & PRACTICAL TOOLS 10 Conditionals score = 82 if score >= grade = 90 : "A" elif score >= 80 : grade = else : grade = "B" "C" 11 Loops for n in range (3): print(n) \# 0, then 1, then 2 names = [ "Maya" , "Leo"] for i, name in enumerate(names, start=1): print(i, name) count = 3 while count > 0 : count - = 1 range(start, stop, step) excludes stop. break exits the loop; continue skips to the next iteration. 12 Functions def greet(name, greeting="Hello"): return f"{greeting}, {name}!" greet("Maya") greet( "Leo" , greeting="Hi") *args collects positional arguments; **kwargs collects keyword arguments. Without return, a function returns None. 13 Input & exceptions input() returns a string. Convert it when you need a number, and handle invalid input. try : age = int (input("Age: " )) except ValueError : print ("Enter a whole number." ) else : print(f"Next year: {age + 1}") Also: float("3.5") converts to a float; str(42) converts to text. Catch specific error

FAQs

How do you start Python 3 from the command line and check the version?

Run python3 --version to see the installed version. On Windows, use python or py to open the interactive shell. You can also run scripts like python3 app.py.

What are the differences between list sort() and sorted()?

sort() modifies the existing list and returns None. sorted() creates and returns a new sorted list without changing the original.

How do you handle invalid user input and convert types in Python?

Use input() to read text and convert it as needed (e.g., int(input(...))). Wrap conversions in try/except for specific exceptions like ValueError, and use else to run code when conversion succeeds.

Oct 13, 2026

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