Introduction to Python (under construction)

Transcript

1. Python Jussi Pohjolainen
2. Introduction to Python (Wikipedia) • Python is a high-level, general-purpose programming language • Its design philosophy emphasizes code readability with the use of significant indentation • Python is dynamically type-checked and garbage-collected • Supports multiple programming paradigms • Procedural • Object-oriented • Functional programming
3. Use Cases • Web • Django, Flask, FastAPI • Data Science, Machine Learning, AI • NumPy, Pandas, Matplotlib, scikit-learn, TensorFlow, PyTorch • Standard language for AI/ML research and production • Automation & Scripting • DevOps & Cloud Engineering • For teaching - easy syntax for novices
4. Install (macOS) • Install homebrew • /bin/bash -c "$(curl -fsSL https:!//raw.githubusercontent.com/Homebrew/install/HEAD/install .sh)" • Install python • brew update • brew install python • Homebrew creates symlinks in /opt/homebrew/bin/: • python3 → main interpreter • pip3 → package manager • The real binaries live inside the Homebrew Cellar, e.g. • /opt/homebrew/Cellar/python@3.13/3.13.3/bin/python3
5. Default Python vs Homebrew Python (macOS) • Default / System Python (/usr/bin/python3) • Comes preinstalled with macOS. • /usr/bin/python3 • Used by macOS itself and Apple’s scripts/tools. • Can lag behind the latest release (depends on your macOS version). • Homebrew Python (/opt/homebrew/bin/python3) • For development (your projects, virtual environments, libraries). • Always the latest stable release (e.g. Python 3.13). • Your shell PATH ensures you normally run the Homebrew one
6. Python in 60 Seconds (Java mapping) • Runtime: • CPython compiles .py → bytecode → VM executes. Think “JVM bytecode”, but CPython VM • Typing: • Dynamic + strong; optional type hints for static checking (PEP 484) • Blocks: • Indentation is syntax, not {}. 4 spaces by convention • Collections: • list (dynamic array), dict, set, tuple. for-each over iterables by default • Packaging: • venv for isolation, pip for deps
7. Running, Compiling > echo 'print("hello world")' > hello.py > python3 -m compileall hello.py > tree . . ├── #__pycache#__ │ └── hello.cpython-313.pyc └── hello.py > python3 #__pycache#__/hello.cpython-313.pyc hello world > python3 hello.py hello world Cross-platform file, like .class Creates in-memory .pyc and runs it
8. Virtual Environments • Installing libraries • pip install requests • Problem • it installs into your system Python, all projects share the same libraries and versions! • Putting all your Java JARs into the JDK’s lib/ folder - becomes unmanageable • Virtual environment = project-specific site-packages
9. Create virtual environment • Runs Python’s built-in venv module, which creates a virtual environment inside a folder named .venv • python3 -m venv .venv • Keeps your project’s dependencies isolated from the system Python. • Different projects can use different versions of libraries without conflict. • Creates • .venv/bin or .venv/Scripts - contains a private python and pip • .venv/lib - contains installed libraries for this environment. • pyvenv.cgf - config file pointing to the base Python installation
10. Modify shell environment • Run the activation script for the virtual environment • source .venv/bin/activate or .venv\Scripts\activate • Modifies your shell environment so that python and pip now point to the executables inside .venv/bin/ • Ensures that when you type python or pip, they operate in your project’s environment instead of the global system installation. • where python • /Users/user/Documents/myproject/.venv/bin/python
11. Install libraries • Runs the pip installer (via the Python in your virtual environment). Downloads the latest version of the Requests HTTP library from PyPI. • python -m pip install requests • Installs third-party libraries into your isolated .venv. • /Users/pohjus/Documents/myproject/.venv/lib/python3.13/site-packages
12. Library Usage import requests url = "https:#//api.chucknorris.io/jokes/random" response = requests.get(url) if response.status_code #== 200: data = response.json() print("Random joke:", data["value"]) else: print("Error:", response.status_code)
13. Recap • .venv/ → contains all project-specific Python binaries and libraries • Activation → makes your shell use that environment • pip install → brings third-party code only into .venv, keeping your global Python untouched
14. Types
15. Immutable built-in types • int, float, complex, bool • str • tuple • frozenset • bytes
16. Example x = 10 y = x y += 1 # creates a new int object, does not modify the old one print(x) # 10 print(y) # 11
17. Types are objects, immutables act like pass by value x = 10 x ──► [ int object 10 ] y = x x ──► [ int object 10 ] ◄── y # memory optimization x = 11 x ──► [ int(11) ] y ──► [ int(10) ]
18. id(x) – memory address x = 10 # x -> int object(10) y = x # x -> int object(10) "<- y x = 20 # x -> int object(20) # y -> int object(10) print(id(x)) # 4388806400 print(id(y)) # 4388806784
19. id(x) – memory address x = 10 y = 10 print(id(x), id(y)) # is checks if two variables point to the same # object in memory # only one 10 in memory, small ints, caching print(x is y) # true
20. id(x) – memory address list1 = [1, 2, 3] list2 = [1, 2, 3] print(list1 !== list2) # True (same contents) print(list1 is list2) # False (different list objects)
21. Examples # int x = 10 y = 10 print(x + y) # 20 # float pi = 3.14 radius = 2.0 print(pi * radius) # 6.28 # bool flag = True print(not flag) # False # str greeting = "Hello" name = "World" print(greeting + " " + name) # Hello World # tuple point = (2, 3) print(point[0]) # 2 print(point[1]) # 3
22. Dynamic typing def greet(name): return f"Hello, {name}" print(greet("Alice")) # OK print(greet(123)) # OK
23. Type hinting def greet(name: str) -> str: return f"Hello, {name}" print(greet("Alice")) # OK print(greet(123)) # ? > python hello.py Hello, Alice Hello, 123 > python -m pip install mypy > mypy hello.py #&& python hello.py hello.py:5: error: Argument 1 to "greet" has incompatible type "int"; expected "str" [arg-type] Found 1 error in 1 file (checked 1 source file)
24. Input and Output
25. Input
26. input() • input() reads a single line from standard input (keyboard) and returns it as a string • without the trailing newline • You typically combine it with a prompt string and then convert/validate as needed • It always returns str; you must convert to int, float, etc. yourself.
27. Example # ask for a name name = input("Your name: ") # -> "Ada" # numbers come in as text; convert explicitly age = int(input("Age: ")) # -> 42 # simple validation raw = input("Pi to 2 decimals: ") try: pi = float(raw) except ValueError: print("Not a number")
28. Command-line arguments: sys.argv • sys.argv is the raw list of tokens passed to your script. Index 0 is the script name; the rest are the arguments. • When to use: tiny scripts where you control the invocation and don’t need help text or flags.
29. Example import sys # python app.py hello 123 print(sys.argv) # ['app.py', 'hello', '123'] msg = sys.argv[1] # 'hello' n = int(sys.argv[2]) # '123' #=> 123 print(msg * n) # 'hello' * 123
30. Robust argument & option parsing: argparse • For real CLIs, use argparse. • Parses positional args and options (e.g., -n / --number) • Does type conversion (type=int) • Generates -h/--help automatically • Validates with choices, required flags, mutually exclusive groups, subcommands, etc. • Why it’s preferred: you get predictable UX, helpful errors, and self- documentation.
31. Example # wordcount.py import argparse parser = argparse.ArgumentParser(description="Count words.") parser.add_argument("path", help="file to read, or '-' for stdin") parser.add_argument("-n", "#--top", type=int, default=10, help="show top N words") parser.add_argument("-i", "#--ignore-case", action="store_true", help="case- insensitive") args = parser.parse_args() print(args.path, args.top, args.ignore_case)
32. Example python wordcount.py my.txt -n 20 #--ignore-case python wordcount.py -h
33. Output
34. print() • By default, it prints to stdout (standard output) • Adds a newline (\n) automatically, unless end is overridden • Multiple arguments are joined with sep
35. Example print("A", "B", "C") # A B C print("A", "B", "C", sep=", ") # A, B, C print("Hello", end="") # no newline print("World") # HelloWorld
36. Formatting output name = "Ada" age = 42 print(f"{name} is {age} years old") pi = 3.14159 print(f"{pi:.2f}") # 3.14 print(f"{age:04d}") # 0042 print("{} is {} years old".format(name, age)) print("{1} {0}".format("first", "second")) # second first
37. Logging import logging logging.basicConfig(level=logging.INFO) logging.info("Program started") logging.error("Something went wrong")
38. JSON import json print(json.dumps({"name": "Ada", "age": 42}))
39. Pretty tables from tabulate import tabulate table = [["Name", "Age"], ["Ada", 42], ["Alan", 36]] print(tabulate(table, headers="firstrow"))
40. Control Structures Conditional Structures
41. Simple form: if x = 10 if x > 0: print("Positive number")
42. if ... else x = -3 if x !>= 0: print("Non-negative") else: print("Negative")
43. if ... elif ... else score = 87 if score !>= 90: grade = "A" elif score !>= 80: grade = "B" elif score !>= 70: grade = "C" else: grade = "F" print("Grade:", grade)
44. What is false? False - False - None - Zero numbers: 0, 0.0 - Empty collections / sequences: "", [], {}, set(), range(0) True Everything else!
45. Logical operators x = 7 if x > 0 and x < 10: print("Between 0 and 10") if x < 0 or x > 100: print("Outside range") if not (x > 0 and x < 10): print("not between 0 and 10")
46. Conditional expressions (inline if) age = 20 status = "adult" if age !>= 18 else "minor" print(status) # adult
47. if vs match value = 0 if value #== 0: print("Zero") elif value #== 1: print("One") else: print("Something else") match value: case 0: print("Zero") case 1: print("One") case _: print("Something else")
48. Looping
49. while n = 5 while n > 0: print(n) n -= 1 print("Blast off!")
50. infinite loop with break while True: cmd = input("Enter q to quit: ") if cmd !== "q": break
51. for loop with iterators # loop any iterable: list, string, file, range for fruit in ["apple", "banana", "cherry"]: print(fruit) for ch in "Python": print(ch)
52. for loop with iterators for i in range(5): # 0,1,2,3,4 print(i) for i in range(2, 10, 2): # 2,4,6,8 print(i)
53. break and continue for i in range(5): if i !== 3: break print(i) # Output: 0,1,2 for i in range(5): if i !== 2: continue print(i) # Output: 0,1,3,4
54. else clause on loops for i in range(3): print(i) else: print("Loop ended normally") # runs for i in range(3): if i !== 1: break else: print("This will not run")
55. Iterating with helpers for i, fruit in enumerate(["apple", "banana"], start=1): print(i, fruit) # 1 apple # 2 banana # multiple lists in parallel names = ["Ada", "Alan", "Grace"] scores = [95, 88, 92] for name, score in zip(names, scores): print(name, score) # key value pairs grades = {"Ada": "A", "Alan": "B"} for student, grade in grades.items(): print(student, grade)
56. Comprehensions, loop + condition # range(5) !=> 0, 1, 2, 3, 4 # for each x, do x * x # collect result into list squares = [x*x for x in range(5)] # !!<=> squares = [] for x in range(5): squares.append(x*x) # range(10) !=> 0, !.. 9 # for each x the condition is checked, if true include, otherwise do not # evens = [0, 2, 4, 6, 8] evens = [x for x in range(10) if x % 2 !== 0] # !!<=> evens = [] for x in range(10): if x % 2 !== 0: evens.append(x)
57. Collections List, Dictionary, Set, Tuple
58. List Ordered collections that can hold many values
59. List • A list in Python is an ordered collection of items. • You can store numbers, strings, or even other lists inside it. • Lists are mutable, which means you can change their contents after creating them.
60. Creation and accessing fruits = ["apple", "banana", "cherry"] print(fruits) # ['apple', 'banana', 'cherry'] print(fruits[0]) # apple print(fruits[2]) # cherry # Negative indexing starts from the end: print(fruits[-1]) # cherry
61. Modifying, Adding, Removing fruits.append("mango") # add at end print(fruits) # ['apple', 'orange', 'cherry', 'mango'] fruits.insert(1, "kiwi") # add at index 1 print(fruits) # ['apple', 'kiwi', 'orange', 'cherry', 'mango'] fruits.remove("orange") # removes by value print(fruits) popped = fruits.pop() # removes last item print(popped) # mango print(fruits) del fruits[0] # delete by index print(fruits)
62. Length and Looping numbers = [10, 20, 30, 40] print(len(numbers)) # 4 fruits = ["apple", "banana", "cherry"] for fruit in fruits: print(fruit) for i in range(len(fruits)): print(i, fruits[i])
63. Slicing: getting part of the list nums = [1, 2, 3, 4, 5, 6] print(nums[:3]) # [1, 2, 3] print(nums[-2:]) # [5, 6] print(nums[1:4]) # [2, 3, 4], exclusive!, 4 - 1 = 3, pick 3 items
64. Membership and mixed data if "apple" in fruits: print("Yes, apple is in the list") mixed = ["Anna", 25, True, [1, 2, 3]] print(mixed) # ['Anna', 25, True, [1, 2, 3]]
65. Dictionary A dictionary stores data as key–value pairs
66. Dictionary • A dictionary stores data as key–value pairs • Keys are unique (like “names” in a phonebook) • Values can be anything (numbers, strings, lists, other dicts)
67. Creating, Accessing person = {"name": "Alice", "age": 25, "city": "Helsinki"} print(person) # {'name': 'Alice', 'age': 25, 'city': 'Helsinki'} # may throw error print(person["name"]) # Alice print(person["age"]) # 25 # does not throw error print(person.get("country", "Not found")) # Not found
68. Removing person.pop("city") # remove by key print(person) del person["country"] # delete by key print(person) person.clear() # empty dictionary print(person) # {} print(len(person)) # length
69. Iterating ages = {"John": 25, "Mary": 30, "Alex": 22} for key in ages: print(key) for value in ages.values(): print(value) for key, value in ages.items(): print(key, "is", value, "years old")
70. Membership and Nested if "John" in ages: print("Yes, John is in the dictionary") students = { "Alice": {"age": 21, "major": "CS"}, "Bob": {"age": 22, "major": "Math"} } print(students["Alice"]["major"]) # CS
71. Set Unordered collection of unique items
72. Set • Unordered collection of unique items • No duplicates allowed • No indexing (can’t access by position) • Great for membership tests and set operations (union, intersection, difference)
73. Creating numbers = {1, 2, 3, 3, 2} print(numbers) # {1, 2, 3} empty = set() # {} would create a dictionary, not a set items = ["apple", "banana", "apple", "orange"] unique = set(items) print(unique) # {'apple', 'banana', 'orange'}
74. Adding, Removing and Length fruits.add("orange") # add one item print(fruits) fruits.update(["kiwi", "melon"]) # add multiple print(fruits) fruits.remove("banana") # remove, error if not found fruits.discard("pear") # safe remove (no error if missing) print(fruits) print(len(fruits))
75. Iterating, Membership # you do not control the order for fruit in fruits: print(fruit) if "apple" in fruits: print("Yes, apple is in the set")
76. Set operations a = {1, 2, 3} b = {3, 4, 5} print(a | b) # {1, 2, 3, 4, 5} print(a.union(b)) # same print(a & b) # {3} print(a.intersection(b)) # same
77. Tuple Ordered collection like a list, but it is immutable (cannot be changed after creation)
78. Creating numbers = (1, 2, 3) print(numbers) # (1, 2, 3) fruits = ("apple", "banana", "cherry") print(fruits[1]) # banana print(fruits[-1]) # cherry # fruits[0] = "orange" # ERROR
79. Packing and Unpacking person = ("Alice", 30, "Finland") # packing name, age, country = person # unpacking print(name) # Alice print(age) # 30 print(country) # Finland
80. Nested students = ( ("Alice", 21, "CS"), ("Bob", 22, "Math") ) for s in students: print(f"Name: {s[0]}, Age: {s[1]}, Major: {s[2]}")
81. Slicing # 0 1 2 3 4 5 nums = (1, 2, 3, 4, 5, 6) # Start from index 2 and end in index 3 print(nums[2:4]) # (3, 4) # start from beginning and take three items print(nums[:3]) # (1, 2, 3) # start from index -2 to the end print(nums[-2:]) # (5, 6)