Python系统学习第六课

#变量作用域
·全局作用域
·局部作用域

#提升局部变量为全局变量
·使用global 在局部变量前加上global

def fun():
    global a
    a = 100

fun()   #这里必须要调用函数吗？如果没有的话，为什么不能直接输出a的值？
print(a)

100

def fun():
    global a = 100   #好像并不能这样赋值
fun()
print(a)

  File "", line 2
    global a = 100   #好像并不能这样赋值
             ^
SyntaxError: invalid syntax

#globals,locals函数
·可以通过这两个函数，显示出局部变量和全局变量

#eval()函数
·把一个字符串当成一个表达式来执行，返回执行后的结果
·语法 eval(string code, globals=None, locals=None)
#exec()函数
·跟和eval函数类似，执行表达式，但是不能返回结果
·语法 exec（string code, globals=None, locals=None)）

x = 100
y = 200
z = eval("x+y")
print(z)

300

x = 100
y = 200
z = exec("x+y")
print(z)

None

x = 100
y = 200
z = eval("print('x+y=',x+y)")
print(z)

x+y 300
None

#递归函数
·函数调用自身
·优点：简洁，高效
·python中对递归深度有限制，超过限制报错
·注意结束条件

x = 0
def fun():
    global x
    x += 1
    print(x)
    fun()
fun()

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---------------------------------------------------------------------------

RecursionError                            Traceback (most recent call last)

 in ()
      6     print(x)
      7     fun()
----> 8 fun()


 in fun()
      5     x += 1
      6     print(x)
----> 7     fun()
      8 fun()


... last 1 frames repeated, from the frame below ...


 in fun()
      5     x += 1
      6     print(x)
----> 7     fun()
      8 fun()


RecursionError: maximum recursion depth exceeded in comparison

#内置数据结构（变量类型）

• list
• set
• dict
• tuple

#列表

• 一组有顺序的数据组合

#列表
a = [100, 200, 300]
print(a)
type(a)

[100, 200, 300]





list

a = list():创建列表
type(a)

list

#查找列表索引，从0开始
a = [1,2,3,4,5,6]
print(a[2])

3

#分片操作
print(a[2:6])#不包含左边，只包含右边
print(a[2:])
print(a[:4])

[3, 4, 5, 6]
[3, 4, 5, 6]
[1, 2, 3, 4]

#负数为下标
print(a[-4:-1])  #为负数表示，从右往左
print(a[-1:-4])  #正常情况下，左边比右边的数小
print(a[-1:-4:-1])  #如果一定要左边比右边的大，就需要将步长设置为负数

[3, 4, 5]
[]
[6, 5, 4]

分片操作是生成一个新的list

• 内置函数id, 负责显示一个变量或者数据的唯一确定编号

a = 100
b = 200
c = a
print(id(a))
print(id(b))
print(id(c))

a = 300
print(id(a))
print(id(c))

1908570272
1908573472
1908570272
1300341147984
1908570272