Functions & Arguments

1# ============================================================
2# Basic function definition
3# ============================================================
4def greet(name: str, greeting: str = "Hello") -> str:
5    """Return a greeting message.
6
7    Args:
8        name: The name of the person to greet.
9        greeting: The greeting word. Defaults to "Hello".
10
11    Returns:
12        A formatted greeting string.
13    """
14    return f"{greeting}, {name}!"
15
16print(greet("Alice"))              # "Hello, Alice!"
17print(greet("Bob", "Hey"))         # "Hey, Bob!"
18print(greet(greeting="Hi", name="Charlie"))  # "Hi, Charlie!"
19
20# ============================================================
21# *args and **kwargs
22# ============================================================
23def log_message(level: str, *messages, **metadata):
24    """Log messages with metadata."""
25    combined = " ".join(str(m) for m in messages)
26    meta_str = ", ".join(f"{k}={v}" for k, v in metadata.items())
27    print(f"[{level}] {combined}" + (f" ({meta_str})" if meta_str else ""))
28
29log_message("INFO", "Server started", "on port", 8080)
30# [INFO] Server started on port 8080
31
32log_message("ERROR", "Connection failed", host="db.server.com", retry=3)
33# [ERROR] Connection failed (host=db.server.com, retry=3)
34
35# ============================================================
36# Positional-only and keyword-only parameters (Python 3.8+)
37# ============================================================
38def divide(a, b, /, *, round_to=None):
39    """
40    a, b: positional-only (before /)
41    round_to: keyword-only (after *)
42    """
43    result = a / b
44    if round_to is not None:
45        result = round(result, round_to)
46    return result
47
48print(divide(10, 3))               # 3.3333...
49print(divide(10, 3, round_to=2))   # 3.33
50# divide(a=10, b=3)  # TypeError: positional-only
51
52# ============================================================
53# Functions as first-class objects
54# ============================================================
55def add(a, b):
56    return a + b
57
58def subtract(a, b):
59    return a - b
60
61# Assign to variable
62operation = add
63print(operation(5, 3))  # 8
64
65# Store in data structures
66operations = {
67    "+": add,
68    "-": subtract,
69    "*": lambda a, b: a * b,
70}
71print(operations["*"](4, 5))  # 20
72
73# Pass as argument
74def apply_operation(func, a, b):
75    return func(a, b)
76
77print(apply_operation(add, 10, 5))  # 15
78
79# Return from function
80def make_multiplier(factor):
81    def multiply(x):
82        return x * factor
83    return multiply
84
85double = make_multiplier(2)
86triple = make_multiplier(3)
87print(double(5))    # 10
88print(triple(5))    # 15
89
90# ============================================================
91# Type hints (PEP 484)
92# ============================================================
93from typing import Optional, Union
94
95def find_user(
96    user_id: int,
97    include_inactive: bool = False,
98) -> Optional[dict]:
99    """Find user by ID. Returns None if not found."""
100    users = {1: {"name": "Alice", "active": True}}
101    user = users.get(user_id)
102    if user and (include_inactive or user.get("active")):
103        return user
104    return None
105
106# Union types (Python 3.10+ can use | syntax)
107def process(value: int | str) -> str:
108    return str(value).upper()
109
110# Complex type hints
111from typing import Callable
112
113def retry(func: Callable[..., str], attempts: int = 3) -> str:
114    for i in range(attempts):
115        try:
116            return func()
117        except Exception:
118            if i == attempts - 1:
119                raise
120    return ""
121
122# ============================================================
123# Default argument pitfall (CRITICAL!)
124# ============================================================
125# BAD — mutable default argument is shared across calls!
126def append_to_bad(item, lst=[]):
127    lst.append(item)
128    return lst
129
130print(append_to_bad(1))  # [1]
131print(append_to_bad(2))  # [1, 2] ← Unexpected! Same list!
132print(append_to_bad(3))  # [1, 2, 3] ← Accumulating!
133
134# GOOD — use None as sentinel
135def append_to_good(item, lst=None):
136    if lst is None:
137        lst = []
138    lst.append(item)
139    return lst
140
141print(append_to_good(1))  # [1]
142print(append_to_good(2))  # [2] ← Correct! New list each time.
143
144# ============================================================
145# Unpacking arguments
146# ============================================================
147def create_user(name, age, city):
148    return {"name": name, "age": age, "city": city}
149
150# Unpack list/tuple as positional args
151args = ["Alice", 30, "NYC"]
152user = create_user(*args)
153
154# Unpack dict as keyword args
155kwargs = {"name": "Bob", "age": 25, "city": "LA"}
156user = create_user(**kwargs)
157
158# Forwarding args (common in decorators)
159def wrapper(*args, **kwargs):
160    print(f"Called with args={args}, kwargs={kwargs}")
161    return create_user(*args, **kwargs)