Python Basic Syntax

# Verify installation
python --version   # Python 3.13.x

# Interactive mode (great for debugging)
python

# Run a script
python hello.py

# hello.py
print("Hello, World!")

# Single-line comment: starts with #

"""
Multi-line comment:
wrapped in triple quotes; commonly used as docstrings for functions/classes
"""

def add(a, b):
    """Return the sum of two numbers."""
    return a + b

age = 18           # integer
name = "Alice"     # string
height = 1.75      # float
is_adult = True    # boolean

# Three key attributes of a variable
print(id(age))     # memory address
print(type(age))   # type
print(age)         # value

# Valid variable names
user_name = "Bob"
_private = 42
MAX_RETRY = 3

# Multiple assignment
x = y = z = 0          # Chained assignment; all three point to the same object
a, b = 10, 20          # Unpacking assignment
a, b = b, a            # Swap two variables

# Extended unpacking (Python 3)
first, *rest = [1, 2, 3, 4, 5]
# first=1, rest=[2, 3, 4, 5]

a = 100
b = 100
print(a is b)   # True, shared object

a = 300
b = 300
print(a is b)   # False (in CPython), outside the cached range

# int (arbitrary precision, no overflow)
age = 18
big = 10 ** 100          # Supports arbitrarily large integers

# float (64-bit double precision)
pi = 3.14159
sci = 1.5e-3             # Scientific notation: 0.0015

# complex
c = 3 + 4j
print(c.real, c.imag)    # 3.0  4.0

# Type conversion
int("42")        # 42
float("3.14")    # 3.14
int(3.9)         # 3 (truncates, does not round)

s1 = 'hello'
s2 = "world"
s3 = """multi-line
string"""

# Concatenation and repetition
greeting = "Hello" + ", " + "Alice"
line = "-" * 40

# Indexing and slicing
s = "Python"
print(s[0])      # P (forward index starts at 0)
print(s[-1])     # n (reverse index starts at -1)
print(s[1:4])    # yth
print(s[::-1])   # nohtyP (reversed)

# Ordered, mutable, elements can be any type
fruits = ["apple", "banana", "cherry"]
print(fruits[0])          # apple
print(fruits[-1])         # cherry

# Nested list
matrix = [[1, 2, 3], [4, 5, 6]]
print(matrix[1][2])       # 6

# Key-value pairs; keys must be immutable
person = {"name": "Alice", "age": 25, "city": "Beijing"}
print(person["name"])     # Alice

# Python 3.9+ dict merge
defaults = {"color": "red", "size": 10}
custom = {"color": "blue"}
merged = defaults | custom   # {'color': 'blue', 'size': 10}

# Ordered, immutable
coords = (10.0, 20.0)
x, y = coords             # Unpacking

# Single-element tuple requires a trailing comma
single = (42,)            # Without the comma it is just parentheses around 42

# Unordered, no duplicates
s = {1, 2, 3, 2, 1}
print(s)                  # {1, 2, 3}

# Set operations
a = {1, 2, 3}
b = {2, 3, 4}
print(a | b)              # Union: {1, 2, 3, 4}
print(a & b)              # Intersection: {2, 3}
print(a - b)              # Difference: {1}

print(True, False)
print(type(True))         # <class 'bool'>
print(int(True))          # 1
print(int(False))         # 0

# Falsy values
# 0, 0.0, "", [], {}, (), set(), None are all False
if []:
    print("this will not execute")

# Explicit type conversion
str(123)          # '123'
list("abc")       # ['a', 'b', 'c']
tuple([1, 2, 3])  # (1, 2, 3)
set([1, 1, 2])    # {1, 2}
dict([("a", 1)])  # {'a': 1}

name = "Alice"
age = 25
print("Name: %s, Age: %d" % (name, age))

print("Name: {}, Age: {}".format("Alice", 25))
print("{name} is {age} years old".format(name="Bob", age=30))
print("{0:>10}".format("right"))   # Right-align, width 10
print("{:.2f}".format(3.14159))    # Two decimal places

name = "Alice"
age = 25
print(f"Name: {name}, Age: {age}")

# Expressions
print(f"Next year: {age + 1}")
print(f"π ≈ {3.14159:.2f}")

# Python 3.12+: any quote style inside f-strings (no escaping needed)
items = ["apple", "banana"]
print(f"First item: {items[0]!r}")

# Debug output (Python 3.8+, = shows variable name and value)
x = 42
print(f"{x=}")   # x=42

import re

# Traditional approach
data = "Hello 42 World"
m = re.search(r"\d+", data)
if m:
    print(m.group())

# Using the walrus operator
if m := re.search(r"\d+", data):
    print(m.group())   # 42

# Simplify while loops
while chunk := input("Enter text (blank line to stop): "):
    print(f"You entered: {chunk}")

# Decimal to other bases
print(bin(10))    # 0b1010
print(oct(10))    # 0o12
print(hex(255))   # 0xff

# Other bases to decimal
print(int("1010", 2))    # 10 (binary → decimal)
print(int("ff", 16))     # 255 (hexadecimal → decimal)

# Thousands separator formatting
print(f"{1234567890:,}")   # 1,234,567,890

# Comparison operators return bool
print(3 > 2)     # True
print(3 == 3.0)  # True (equal values)
print(3 is 3.0)  # False (different types, different objects)

# Chained comparison (Python-specific)
x = 5
print(1 < x < 10)  # True

# Logical operators: not > and > or
print(True and False)   # False
print(True or False)    # True
print(not True)         # False

# Short-circuit evaluation: and/or return the operand that determines the result
print(0 or "default")   # "default"
print(1 and "ok")       # "ok"

x = 10
x += 1      # x = 11
x -= 2      # x = 9
x *= 3      # x = 27
x //= 4     # x = 6
x **= 2     # x = 36

a = 60   # 0011 1100
b = 13   # 0000 1101

print(a & b)    # 12   bitwise AND
print(a | b)    # 61   bitwise OR
print(a ^ b)    # 49   bitwise XOR
print(~a)       # -61  bitwise NOT
print(a << 2)   # 240  left shift
print(a >> 2)   # 15   right shift

# Immutable types: int, float, str, tuple, frozenset
# Changing the value creates a new object (id changes)
a = "hello"
print(id(a))
a = a + "!"
print(id(a))    # id has changed

# Mutable types: list, dict, set
# Modified in-place; id remains the same
lst = [1, 2, 3]
print(id(lst))
lst.append(4)
print(id(lst))  # id unchanged

import sys
print(sys.getdefaultencoding())   # utf-8

# In Python 3, str is Unicode; bytes is a byte sequence
s = "你好"
b = s.encode("utf-8")             # str → bytes
print(b)                          # b'\xe4\xbd\xa0\xe5\xa5\xbd'
print(b.decode("utf-8"))          # bytes → str

# Source files default to UTF-8; no need for # -*- coding: utf-8 -*- at the top

import gc

# Trigger garbage collection manually
gc.collect()

# Check reference count
import sys
x = [1, 2, 3]
print(sys.getrefcount(x))   # 2 (x itself + the getrefcount argument)