Python

> Everything is an object: a key difference from C++

In Python, values such as integers, strings, lists, and functions are all objects. They have types and methods, which is different from C++ primitive values.

| Function | Purpose |
| --- | --- |
| `type()` | returns the type of an object |
| `dir()` | lists the available attributes and methods |

```python

type(5) type(“hello”)

    ```python
dir(int)
    ```

    > Methods wrapped in `__...__` are called magic methods. They allow Python to implement operators like `+`, `==`, `[]`, and `len()`.

    ```python
a = 5
b = 3
print(a + b)
    ```

    ---

    ## Container types

    ### 1. List

    Lists are ordered, mutable sequences.

    ```python
a = [1, "hi", 3.14]
a.append(5)
print(a)
    ```

    ### 2. Tuple

    Tuples are ordered, immutable sequences.

    ```python
b = (5, "Xiao is handsome", True)
print(b)
    ```

    ### 3. Dictionary

    Dictionaries store key-value pairs.

    ```python
d = {"apple": 5, "banana": 8}
d["apple"] = 10
print(d)
    ```

    ### 4. Set

    Sets store unique elements.

    ```python
s = {1, 2, 3, 3, 1, 4}
s.add(8)
print(s)
    ```

    ## Example exercises

    ### Exercise 1: Count occurrences

    ```python
labels = ["cat", "dog", "cat", "rabbit", "dog", "cat"]
count = {}

for label in labels:
    if label in count:
        count[label] += 1
    else:
        count[label] = 1
print(count)
    ```

    ### Exercise 2: Reverse a score dictionary

    ```python
scores = {'Amy': 85, 'Bob': 90, 'Cathy': 85, 'David': 92}
new = {}

for student in scores.keys():
    if scores[student] in new:
        new[scores[student]].append(student)
    else:
        new[scores[student]] = [student]
print(new)
    ```

    ## Summary

    Python is object-oriented, flexible, and expressive. The correct understanding of objects and container types is fundamental to writing clean Python programs.