第二章 语法基础
课时10 Python基础-06-变量作用域和列表
1.变量作用域
- 全局变量：作用域为全局（global），在函数外部定义
- 局部变量：作用域为局部（local），在函数内部定义
- LEGB原则：
L（local）：局部作用域
E（enclosing function locale）：外部嵌套函数作用域
G（global module）：函数定义所在模块作用域
B（buildin）：Python内置模块的作用域 - 举例
a) 不报错：函数内部调用函数外部的变量
#代码
a=1
def fun():
print(a)
fun()
print(a)
#效果
1
1
b) 报错：函数内部重新给函数外部定义的变量赋值
报错原因：变量a为已定义的全局变量，不能在函数fun中定义或赋值
#代码
a=1
def fun():
print(a)
a=2
fun()
print(a)
#效果
---------------------------------------------------------------------------
UnboundLocalError Traceback (most recent call last)
in ()
3 print(a)
4 a=2
----> 5 fun()
6 print(a)
in fun()
1 a=1
2 def fun():
----> 3 print(a)
4 a=2
5 fun()
UnboundLocalError: local variable 'a' referenced before assignment
c) 函数外部给函数内部定义的变量赋值（无法赋值，而是新建一个名称相同的全局变量）
#代码
def fun():
a=1
print(a)
a=2
a=3
fun()
print(a)
fun() #本次调用函数时，被赋值的变量a为函数内部的变量，而不是给外部的全局变量赋值
#效果
1
3
1
?? 2.提升局部变量为全局变量
在函数内部定义全局变量：
*juoyter notebook中，一旦某一段函数使用了global p，那么在本文档中新建的函数代码都会将p作为全局变量来处理；此问题可以通过按下重启按钮进行恢复。
#代码
def fun():
global a
a = 1
fun() #必须进行调用，否则报错：a未定义
print(a)
#效果
1
3.globals、locals函数
由此可见local具有相对性！
#代码
a = 1
def fun(c):
b = 2
print("globals={0}".format(globals()))
print("locals={0}".format(locals()))
fun(3)
print("locals={0}".format(locals()))
#效果
globals={'__name__': '__main__', '__doc__': 'Automatically created module for IPython interactive environment', '__package__': None, '__loader__': None, '__spec__': None, '__builtin__': , '__builtins__': , '_ih': ['', 'a=1\ndef fun():\n print(a)\n a=2\nprint(a)', 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', 'a=1\ndef fun():\n print(a)\nfun()\nprint(a)', 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)', 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\na=3\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', 'def fun()\n global a = 1\nprint(a)', 'def fun():\n global a = 1\nprint(a)', 'def fun():\n global a \n a = 1\nprint(a)', 'def fun():\n global n \n n = 1\nprint(n)', 'def fun():\n global n = 1\nprint(n)', 'a = 1\ndef fun(c):\n b = 2\n print(globals())\n print(locals())\nfun(3)', 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)', 'a = 1\ndef fun(c):\n b = 2\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))', 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))'], '_oh': {}, '_dh': ['/home/tlxy/桌面'], 'In': ['', 'a=1\ndef fun():\n print(a)\n a=2\nprint(a)', 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', 'a=1\ndef fun():\n print(a)\nfun()\nprint(a)', 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)', 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\na=3\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', 'def fun()\n global a = 1\nprint(a)', 'def fun():\n global a = 1\nprint(a)', 'def fun():\n global a \n a = 1\nprint(a)', 'def fun():\n global n \n n = 1\nprint(n)', 'def fun():\n global n = 1\nprint(n)', 'a = 1\ndef fun(c):\n b = 2\n print(globals())\n print(locals())\nfun(3)', 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)', 'a = 1\ndef fun(c):\n b = 2\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))', 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))'], 'Out': {}, 'get_ipython': >, 'exit': , 'quit': , '_': '', '__': '', '___': '', '_i': 'a = 1\ndef fun(c):\n b = 2\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))', '_ii': 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)', '_iii': 'a = 1\ndef fun(c):\n b = 2\n print(globals())\n print(locals())\nfun(3)', '_i1': 'a=1\ndef fun():\n print(a)\n a=2\nprint(a)', 'a': 1, 'fun': , '_i2': 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', '_i3': 'a=1\ndef fun():\n print(a)\nfun()\nprint(a)', '_i4': 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', '_i5': 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)', '_i6': 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)\nfun()', '_i7': 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', '_i8': 'def fun():\n a=1\n print(a)\na=3\nfun()\nprint(a)\nfun()', '_i9': 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', '_i10': 'def fun()\n global a = 1\nprint(a)', '_i11': 'def fun():\n global a = 1\nprint(a)', '_i12': 'def fun():\n global a \n a = 1\nprint(a)', '_i13': 'def fun():\n global n \n n = 1\nprint(n)', '_i14': 'def fun():\n global n = 1\nprint(n)', '_i15': 'a = 1\ndef fun(c):\n b = 2\n print(globals())\n print(locals())\nfun(3)', '_i16': 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)', '_i17': 'a = 1\ndef fun(c):\n b = 2\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))', '_i18': 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))'}
locals={'b': 2, 'c': 3}
locals={'__name__': '__main__', '__doc__': 'Automatically created module for IPython interactive environment', '__package__': None, '__loader__': None, '__spec__': None, '__builtin__': , '__builtins__': , '_ih': ['', 'a=1\ndef fun():\n print(a)\n a=2\nprint(a)', 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', 'a=1\ndef fun():\n print(a)\nfun()\nprint(a)', 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)', 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\na=3\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', 'def fun()\n global a = 1\nprint(a)', 'def fun():\n global a = 1\nprint(a)', 'def fun():\n global a \n a = 1\nprint(a)', 'def fun():\n global n \n n = 1\nprint(n)', 'def fun():\n global n = 1\nprint(n)', 'a = 1\ndef fun(c):\n b = 2\n print(globals())\n print(locals())\nfun(3)', 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)', 'a = 1\ndef fun(c):\n b = 2\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))', 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))'], '_oh': {}, '_dh': ['/home/tlxy/桌面'], 'In': ['', 'a=1\ndef fun():\n print(a)\n a=2\nprint(a)', 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', 'a=1\ndef fun():\n print(a)\nfun()\nprint(a)', 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)', 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\na=3\nfun()\nprint(a)\nfun()', 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', 'def fun()\n global a = 1\nprint(a)', 'def fun():\n global a = 1\nprint(a)', 'def fun():\n global a \n a = 1\nprint(a)', 'def fun():\n global n \n n = 1\nprint(n)', 'def fun():\n global n = 1\nprint(n)', 'a = 1\ndef fun(c):\n b = 2\n print(globals())\n print(locals())\nfun(3)', 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)', 'a = 1\ndef fun(c):\n b = 2\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))', 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))'], 'Out': {}, 'get_ipython': >, 'exit': , 'quit': , '_': '', '__': '', '___': '', '_i': 'a = 1\ndef fun(c):\n b = 2\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))', '_ii': 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)', '_iii': 'a = 1\ndef fun(c):\n b = 2\n print(globals())\n print(locals())\nfun(3)', '_i1': 'a=1\ndef fun():\n print(a)\n a=2\nprint(a)', 'a': 1, 'fun': , '_i2': 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', '_i3': 'a=1\ndef fun():\n print(a)\nfun()\nprint(a)', '_i4': 'a=1\ndef fun():\n print(a)\n a=2\nfun()\nprint(a)', '_i5': 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)', '_i6': 'def fun():\n a=1\n print(a)\na=2\nfun()\nprint(a)\nfun()', '_i7': 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', '_i8': 'def fun():\n a=1\n print(a)\na=3\nfun()\nprint(a)\nfun()', '_i9': 'def fun():\n a=1\n print(a)\n a=2\na=3\nfun()\nprint(a)\nfun()', '_i10': 'def fun()\n global a = 1\nprint(a)', '_i11': 'def fun():\n global a = 1\nprint(a)', '_i12': 'def fun():\n global a \n a = 1\nprint(a)', '_i13': 'def fun():\n global n \n n = 1\nprint(n)', '_i14': 'def fun():\n global n = 1\nprint(n)', '_i15': 'a = 1\ndef fun(c):\n b = 2\n print(globals())\n print(locals())\nfun(3)', '_i16': 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)', '_i17': 'a = 1\ndef fun(c):\n b = 2\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))', '_i18': 'a = 1\ndef fun(c):\n b = 2\n print("globals={0}".format(globals()))\n print("locals={0}".format(locals()))\nfun(3)\nprint("locals={0}".format(locals()))'}
4.eval、exec函数：把字符串当做代码来执行（神器！）
eval("string_code",global=None,local=None)
#代码
x = 1
y = 2
z1 = eval("x + y")
z2 = eval("x + y")
print(z1)
print(z2)
#效果
3
None
5.递归函数：定义一个函数，在内部调用之（调用自己）
注意：Python中队递归深度限制约1965层，超出报错
一定要写结束条件
#代码
a = 0
def fun():
global a
a += 1
print(a)
fun()
fun()
#效果
1
2
... #太多了，省略中间一千多行
1964
1965
---------------------------------------------------------------------------
RecursionError Traceback (most recent call last)
in ()
5 print(a)
6 fun()
----> 7 fun()
in fun()
4 a += 1
5 print(a)
----> 6 fun()
7 fun()
... last 1 frames repeated, from the frame below ...
in fun()
4 a += 1
5 print(a)
----> 6 fun()
7 fun()
RecursionError: maximum recursion depth exceeded in comparison
a） 斐波那契数列
f(1) = 1,f(2) = 2,f(n) = f(n-2) + f(n-1)
#代码
def fib(n):
if n <= 0 or n%1 != 0:
return "请输入大于等于1的整数"
elif n == 1 or n == 2: #不可以写成“if n == 1 or 2:”
return 1
else:
return fib(n-2) + fib(n-1)
print(fib(-1))
print(fib(1))
print(fib(3))
#效果
请输入大于等于1的整数
1
2
数字不能过大，否则超出计算机计算能力，建议设置n在20以内
6.内置数据结构（变量类型）
- list 列表（有先后顺序，元素可重复）
创建空列表：list1 = []
创建单元素列表：list2 = [100]
创建多元素列表：list3 = [1,2,3]
其他语法：list4 = list()#括号内至多1个参数，且不为数值类型
a) 访问列表中的单个元素：使用下标操作（索引），下标都是从0开始！！！
#代码
l = [2,141,2,32,34,1]
print(l[3])
#效果
32
b) 访问列表中的连续多个元素（分片）
注意：l[a:b]表示从a到b-1！！！
若a小于0，表示从右往左数，最右数的下标值为-1，如图所示：
#代码
l = [0,141,9,32,34,1]
print(l[2:4])
print(l[2:])
print(l[:4])
print(l[:])
print(l[1:4:1])
print(l[1:4:2])
print(l[-4:-2])
print(l[-2:-4]) #切片默认从左往右切
print(l[-2:-4:-1])
#效果
[9, 32]
[9, 32, 34, 1]
[0, 141, 9, 32]
[0, 141, 9, 32, 34, 1]
[141, 9, 32]
[141, 32]
[9, 32]
[]
[34, 32]
- set
- dict
- tuple
- etc)
id函数：显示某个变量或数据的唯一确定的编号
#代码
a = 1
print(id(a))
b = 2
print(id(b))
a = 3 #对a赋新值
print(id(a))
c = a #将a复制给c
print(id(c))
a = 4
print(id(a))
#效果
94112357272800
94112357272832
94112357272864 #对a赋新值
94112357272864 #将a复制给c
94112357272896 #再次对a赋新值