Python Syntax Basics: Types, Control Flow, Functions & Comprehensions
Numbers, strings, lists, tuples, dicts and sets; if/for/while; functions, *args and **kwargs; decorators; and comprehensions — everything needed to write real Python.
Built-in Data Types
# 数字
42 # int
3.14 # float
2 ** 64 # 大整数自动支持
# 字符串
s = "BackendWiki"
s.upper() # BACKENDWIKI
f"len={len(s)}" # f-string 插值
"a,b,c".split(",") # ['a', 'b', 'c']
# 列表（可变，有序）
nums = [1, 2, 3]
nums.append(4)
nums[0] = 10
nums[1:3] # 切片 [2, 3]
# 元组（不可变）
point = (3, 4)
x, y = point # 解包
# 字典（键值对，3.7+ 保持插入序）
cfg = {"port": 8080, "host": "0.0.0.0"}
cfg["port"]
cfg.get("timeout", 30) # 带默认值
# 集合（去重）
tags = {"java", "python", "java"}
print(tags) # {'java', 'python'}Control Flow
score = 85
# 条件（没有 switch 也不需要，用 elif 链）
if score >= 90:
grade = "A"
elif score >= 60:
grade = "B"
else:
grade = "F"
# for 遍历可迭代对象
for i in range(3):
print(i) # 0 1 2
for k, v in {"a": 1, "b": 2}.items():
print(k, v)
# while 与 break / continue
while True:
if score > 100:
break
score += 1
# match 语句（3.10+，类似 switch）
match score:
case 100:
print("full")
case s if s >= 60:
print("pass")
case _:
print("fail")Functions, *args and **kwargs
def add(a: int, b: int, *args: int, **kwargs) -> int:
"""可变位置参数 args 与关键字参数 kwargs"""
total = a + b + sum(args)
if kwargs.get("debug"):
print("total =", total)
return total
add(1, 2) # 3
add(1, 2, 3, 4) # 10
add(1, 2, debug=True) # 打印并返回 3
# 关键字参数与默认值
def connect(host, port=3306, timeout=5): ...
connect(host="localhost", timeout=10)
# lambda 表达式
sorted([3, 1, 2], key=lambda x: -x) # [3, 2, 1]Strings, tuples and numbers are passed by object reference but immutable — reassignment inside a function does not affect the caller. Mutable objects (lists, dicts) share state.
Comprehensions and Decorators
# 列表推导式
squares = [x * x for x in range(10)]
evens = [x for x in squares if x % 2 == 0]
# 字典 / 集合推导式
word_len = {w: len(w) for w in ["go", "java"]}
# 装饰器：在函数前后加逻辑
import time
def timed(fn):
def wrapper(*args, **kwargs):
start = time.time()
result = fn(*args, **kwargs)
print(f"{fn.__name__} took {time.time()-start:.3f}s")
return result
return wrapper
@timed
def slow():
time.sleep(1)
slow() # slow took 1.001s内置数据类型
# 数字
42 # int
3.14 # float
2 ** 64 # 大整数自动支持
# 字符串
s = "BackendWiki"
s.upper() # BACKENDWIKI
f"len={len(s)}" # f-string 插值
"a,b,c".split(",") # ['a', 'b', 'c']
# 列表（可变，有序）
nums = [1, 2, 3]
nums.append(4)
nums[0] = 10
nums[1:3] # 切片 [2, 3]
# 元组（不可变）
point = (3, 4)
x, y = point # 解包
# 字典（键值对，3.7+ 保持插入序）
cfg = {"port": 8080, "host": "0.0.0.0"}
cfg["port"]
cfg.get("timeout", 30) # 带默认值
# 集合（去重）
tags = {"java", "python", "java"}
print(tags) # {'java', 'python'}控制流
score = 85
# 条件（没有 switch 也不需要，用 elif 链）
if score >= 90:
grade = "A"
elif score >= 60:
grade = "B"
else:
grade = "F"
# for 遍历可迭代对象
for i in range(3):
print(i) # 0 1 2
for k, v in {"a": 1, "b": 2}.items():
print(k, v)
# while 与 break / continue
while True:
if score > 100:
break
score += 1
# match 语句（3.10+，类似 switch）
match score:
case 100:
print("full")
case s if s >= 60:
print("pass")
case _:
print("fail")函数、*args 与 **kwargs
def add(a: int, b: int, *args: int, **kwargs) -> int:
"""可变位置参数 args 与关键字参数 kwargs"""
total = a + b + sum(args)
if kwargs.get("debug"):
print("total =", total)
return total
add(1, 2) # 3
add(1, 2, 3, 4) # 10
add(1, 2, debug=True) # 打印并返回 3
# 关键字参数与默认值
def connect(host, port=3306, timeout=5): ...
connect(host="localhost", timeout=10)
# lambda 表达式
sorted([3, 1, 2], key=lambda x: -x) # [3, 2, 1]字符串、元组、数字虽是引用传递但不可变，函数内重新赋值不影响调用方；列表、字典等可变对象则共享状态。
推导式与装饰器
# 列表推导式
squares = [x * x for x in range(10)]
evens = [x for x in squares if x % 2 == 0]
# 字典 / 集合推导式
word_len = {w: len(w) for w in ["go", "java"]}
# 装饰器：在函数前后加逻辑
import time
def timed(fn):
def wrapper(*args, **kwargs):
start = time.time()
result = fn(*args, **kwargs)
print(f"{fn.__name__} took {time.time()-start:.3f}s")
return result
return wrapper
@timed
def slow():
time.sleep(1)
slow() # slow took 1.001s