在Python中,变量是用来存储和操作数据的标识符。可以把变量看作是一个容器,用来存储值或者引用其他对象。
在Python中,变量的使用有以下几个特点:
# 变量赋值
x = 5
name = 'Alice'
pi = 3.14
# 使用变量
print(x) # 输出:5
print('Hello, ' + name) # 输出:Hello, Alice
radius = 2.5
area = pi * radius ** 2
print('圆的面积为:', area) # 输出:圆的面积为: 19.625
# 变量类型推断
x = 10 # x是整数类型
x = 'Hello' # x变为字符串类型
x = [1, 2, 3] # x变为列表类型
# 多个变量赋值
a = b = c = 0
print(a, b, c) # 输出:0 0 0
# 整数
x = 5
y = -10
z = 0
# 浮点数
pi = 3.14
radius = 2.5
# 复数
a = 2 + 3j
b = -1j
# 布尔值
is_valid = True
is_greater = x > y
# 数值计算和操作
print(x + y) # 输出:-5
print(pi * radius ** 2) # 输出:19.625
print(a.real) # 输出:2.0
print(b.imag) # 输出:-1.0
print(not is_valid) # 输出:False
# 数值转换
x_str = '10'
x_int = int(x_str)
y_float = float(y)
z_complex = complex(x, y)
''
)或双引号(""
)括起来。name = 'Alice'
message = "Hello, world!"
sentence = "I'm learning Python."
print(name) # 输出:Alice
print(message) # 输出:Hello, world!
print(sentence) # 输出:I'm learning Python.
greeting = "Hello"
name = "Alice"
message = greeting + ", " + name
print(message) # 输出:Hello, Alice
sentence = "I'm learning Python."
print(sentence[2:9]) # 输出:m learn
word = "Python"
print(word[0]) # 输出:P
print(word[-1]) # 输出:n
print(word[2:4]) # 输出:th
sentence = "I'm learning Python."
words = sentence.split() # 按空格分割字符串
print(words) # 输出:["I'm", 'learning', 'Python.']
print(sentence.upper()) # 输出:"I'M LEARNING PYTHON."
print(sentence.lower()) # 输出:"i'm learning python."
print(sentence.replace('Python', 'Java')) # 输出:"I'm learning Java."
[]
)来表示,其中的元素使用逗号(,
)分隔。numbers = [1, 2, 3, 4, 5]
names = ['Alice', 'Bob', 'Charlie']
mixed = [1, 'Hello', True, 3.14]
nested = [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
empty = []
[]
)来访问列表中的元素。索引从0开始,负数索引表示从列表末尾开始倒数。numbers = [1, 2, 3, 4, 5]
print(numbers[0]) # 输出:1
print(numbers[-1]) # 输出:5
print(numbers[2:4]) # 输出:[3, 4]
[]
)来修改列表中的元素。numbers = [1, 2, 3, 4, 5]
numbers[0] = 10
print(numbers) # 输出:[10, 2, 3, 4, 5]
append()
方法向列表末尾添加新元素,或使用拼接操作符(+
)将两个列表合并。numbers = [1, 2, 3]
numbers.append(4)
print(numbers) # 输出:[1, 2, 3, 4]
names1 = ['Alice', 'Bob']
names2 = ['Charlie', 'Dave']
combined = names1 + names2
print(combined) # 输出:['Alice', 'Bob', 'Charlie', 'Dave']
del
语句或remove()
方法删除列表中的元素。numbers = [1, 2, 3, 4, 5]
del numbers[2]
print(numbers) # 输出:[1, 2, 4]
names = ['Alice', 'Bob', 'Charlie']
names.remove('Bob')
print(names) # 输出:['Alice', 'Charlie']
len()
获取列表的长度(即元素个数)。numbers = [1, 2, 3, 4, 5]
print(len(numbers)) # 输出:5
()
)来表示,其中的元素使用逗号(,
)分隔。coordinates = (3, 4)
colors = ('red', 'green', 'blue')
mixed = (1, 'Hello', True, 3.14)
nested = ((1, 2), (3, 4, 5), (6, 7, 8, 9))
empty = ()
[]
)来访问元组中的元素,索引从0开始。coordinates = (3, 4)
print(coordinates[0]) # 输出:3
print(coordinates[1]) # 输出:4
coordinates = (3, 4)
coordinates[0] = 10 # TypeError: 'tuple' object does not support item assignment
coordinates = (3, 4)
x, y = coordinates
print(x) # 输出:3
print(y) # 输出:4
colors = ('red', 'green', 'blue')
print(len(colors)) # 输出:3
{}
)或者使用set()函数
来创建。fruits = {'apple', 'banana', 'orange'}
numbers = set([1, 2, 3, 4, 5])
empty = set()
fruits = {'apple', 'banana', 'orange', 'apple'}
print(fruits) # 输出:{'apple', 'banana', 'orange'}
add()
方法向集合中添加新元素。fruits = {'apple', 'banana', 'orange'}
fruits.add('grape')
print(fruits) # 输出:{'apple', 'banana', 'orange', 'grape'}
remove()
方法从集合中删除指定的元素,如果元素不存在会引发KeyError错误。fruits = {'apple', 'banana', 'orange'}
fruits.remove('banana')
print(fruits) # 输出:{'apple', 'orange'}
set1 = {1, 2, 3, 4, 5}
set2 = {4, 5, 6, 7, 8}
union = set1.union(set2) # 并集
intersection = set1.intersection(set2) # 交集
difference = set1.difference(set2) # 差集
print(union) # 输出:{1, 2, 3, 4, 5, 6, 7, 8}
print(intersection) # 输出:{4, 5}
print(difference) # 输出:{1, 2, 3}
len()
获取集合的大小(即元素个数)。:
)来分隔,而不同的键值对使用逗号(,
)分隔。字典使用花括号({}
)来表示。person = {'name': 'Alice', 'age': 25, 'city': 'New York'}
fruits = {'apple': 1, 'banana': 2, 'orange': 3}
empty = {}
person = {'name': 'Alice', 'age': 25, 'city': 'New York'}
print(person['name']) # 输出:Alice
print(person['age']) # 输出:25
person = {'name': 'Alice', 'age': 25, 'city': 'New York'}
person['age'] = 26
print(person) # 输出:{'name': 'Alice', 'age': 26, 'city': 'New York'}
person = {'name': 'Alice', 'age': 25}
person['city'] = 'New York'
print(person) # 输出:{'name': 'Alice', 'age': 25, 'city': 'New York'}
del
语句删除字典中的元素。person = {'name': 'Alice', 'age': 25, 'city': 'New York'}
del person['age']
print(person) # 输出:{'name': 'Alice', 'city': 'New York'}
len()
获取字典中键值对的数量。person = {'name': 'Alice', 'age': 25, 'city': 'New York'}
print(len(person)) # 输出:3