Python中数据类型转换与格式化输出实战指南

# 字符串转整型
print(int("123"))  # 成功 → 123
print(int("123.5")) # 报错：字符串含小数点

# 浮点数转整型(直接截断小数)
print(int(3.99))  # 3 (不是四舍五入！)

# 布尔值转整型
print(int(True))  # 1
print(int(False)) # 0

# 进制转换
print(int("1010", 2))  # 二进制转十进制 → 10
print(int("A", 16))    # 十六进制转十进制 → 10

s = "123a"
if s.isdigit():
    num = int(s)
else:
    print("不是有效数字")

print(float("3.14"))  # 3.14
print(float("3"))     # 3.0
print(float(True))    # 1.0

user_input = "12.34元"
try:
    price = float(user_input[:-1])  # 去掉"元"字
except ValueError:
    price = 0.0

print(str(42))        # "42"
print(str(3.14))      # "3.14"
print(str([1,2,3]))   # "[1, 2, 3]"

class Person:
    def __init__(self, name):
        self.name = name
        
    def __str__(self):
        return f"Person: {self.name}"

print(str(Person("Alice")))  # "Person: Alice"

print(bool(0))     # False
print(bool(""))    # False
print(bool([]))    # False
print(bool("Hi"))  # True
print(bool(1))     # True

# 不好的写法
if len(items) > 0:
    do_something()

# Pythonic写法
if items:
    do_something()

print(3 + 4.5)   # int自动转float → 7.5
print(True + 1)  # bool转int → 2

print("价格：" + 100)  # TypeError!

name = "Alice"
age = 25
print("姓名：%s，年龄：%d" % (name, age))

print("PI: %.2f" % 3.14159)  # 保留2位小数 → "PI: 3.14"
print("%10s" % "hello")      # 右对齐10字符 → "     hello"

print("{}今年{}岁".format(name, age))

# 位置参数
print("{1}先于{0}".format("B", "A"))  # "A先于B"

# 命名参数
print("{name}的分数是{score}".format(name="Bob", score=90))

# 字典解包
data = {"x":1, "y":2}
print("坐标：{x},{y}".format(**data))

# 格式控制
print("{:.2%}".format(0.25))  # 百分比 → "25.00%"
print("{:>10}".format("hi"))  # 右对齐 → "        hi"

print(f"{name}今年{age}岁")

# 表达式计算
print(f"10年后{name}将{age + 10}岁")

# 函数调用
print(f"名字长度：{len(name)}")

# 格式控制
num = 3.14159
print(f"PI：{num:.3f}")  # "PI：3.142"
print(f"{num:10.3f}")    # 宽度10，3位小数 → "     3.142"

# 对齐
print(f"{'left':<10}")   # 左对齐
print(f"{'center':^10}") # 居中
print(f"{'right':>10}")  # 右对齐

# 安全转换函数示例
def safe_convert(value, to_type, default=None):
    try:
        return to_type(value)
    except (ValueError, TypeError):
        return default

age = safe_convert(input("年龄："), int, 0)

# 不好的写法
for i in range(1000000):
    print(f"当前计数：{i}")

# 更好的写法
template = "当前计数：{}"
for i in range(1000000):
    print(template.format(i))

print(0.1 + 0.2 == 0.3)  # False！

from decimal import Decimal
print(Decimal('0.1') + Decimal('0.2') == Decimal('0.3'))  # True

# str转bytes
s = "你好"
b = s.encode('utf-8')  # b'\xe4\xbd\xa0\xe5\xa5\xbd'

# bytes转str
s2 = b.decode('utf-8')  # "你好"

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    
    def __format__(self, format_spec):
        if format_spec == 'json':
            return f'{{"x":{self.x},"y":{self.y}}}'
        return f"({self.x}, {self.y})"

p = Point(3,4)
print(f"{p}")        # (3, 4)
print(f"{p:json}")   # {"x":3,"y":4}

def generate_report(data):
    # 安全转换数据
    safe_data = {
        'name': str(data.get('name', '')),
        'sales': safe_convert(data.get('sales'), float, 0.0),
        'growth': safe_convert(data.get('growth'), float, 0.0)
    }
    
    # 生成格式化报表
    report = f"""
    {safe_data['name']}销售报表
    {'='*30}
    销售额：¥{safe_data['sales']:,.2f}
    同比增长：{safe_data['growth']:.1%}
    """
    return report

data = {'name': '华东区', 'sales': '1234567.89', 'growth': '0.125'}
print(generate_report(data))

def progress_bar(current, total, length=50):
    percent = current / total
    filled = int(length * percent)
    bar = '█' * filled + '-' * (length - filled)
    return f"[{bar}] {percent:.1%}"

for i in range(101):
    print(f"\r{progress_bar(i, 100)}", end='')
    time.sleep(0.05)
print()