py-函数基础

定义: 函数是指将一组语句的集合通过一个名字(函数名)封装起来，要想执行这个函数，只需调用其函数名即可

特性:

1.减少重复代码
2.使程序变的可扩展
3.使程序变得易维护

函数参数

形参变量

只有在被调用时才分配内存单元，在调用结束时，即刻释放所分配的内存单元。因此，形参只在函数内部有效。函数调用结束返回主调用函数后则不能再使用该形参变量

实参
可以是常量、变量、表达式、函数等，无论实参是何种类型的量，在进行函数调用时，它们都必须有确定的值，以便把这些值传送给形参。因此应预先用赋值，输入等办法使参数获得确定值

def calc(x,y):    # x,y 是形参，用来接受参数
res = x*y
return res

t = calc(2,3)    # 2,3 是实参，实际传入的参数，
print(t)

默认参数

def staff(name,age,skill,year=2018):    #默认参数一般放到实参后面
　　print('staff info'.center(20,'-'))
　　print("name:",name)
　　print("age:",age)
　　print("skill:",skill)
　　print("year:",year)
staff("lemi",22,"python_devops")

关键参数
正常情况下，给函数传参数要按顺序，不想按顺序就可以用关键参数，只需指定参数名即可(指定了参数名的参数就叫关键参数)，但记住一个要求就是，关键参数必须放在位置参数(以位置顺序确定对应关系的参数)之后

def staff(name,age,skill,year=2018):
print('staff info'.center(20,'-'))
print("name:",name)
print("age:",age)
print("skill:",skill)
print("year:",year)

staff("lemi",skill="python_devops",22) # 不可以
staff("kevin",skill="linux",age=22) #可以这样
staff("walk",25,"linux",age=22) #不可以，相当于两次赋值
# 总结：不想按位置参数，一一对应的去传值，
# 就使用关键参数（参数名=值），但得只能在位置参数后面，
# 相当于，你不想排队一个个进，想按名进去，
# 那你就只能等到前面排队的进完再进，不然混乱（报错）

非固定参数

若你的函数在定义时不确定用户想传入多少个参数，就可以使用非固定参数
# *args 会把多传入的参数变成一个元组形式（args也可以其他形式命名，相当于形参）
# args,如果没传值,就为空 （）

def staff(name,age,*args):
print(name,age,args)
staff('lemi',22,'python','CQ')
# 输出
# lemi 22 ('python', 'CQ')

当*args在其他形参前面时会截胡，传递参数时，他后面的参数用关键参数可以解决
def staff(name,*args,age):
pass
staff(‘lemi’,’python’,’linux’,age=22)
staff(‘lemi’,*[‘python’,’linux’],age=22)
解包，传给形参args

# *kwargs 会把多传入的参数(key-value型)变成一个dict形式{}
# kwargs,如果没传值,就为空 {}

def staff(name,age,*args,**kwargs):
print(name,age,args,kwargs)
staff('lemi',22,'linux','web',skill='python',place='CQ')
# 输出
#lemi 22 ('linux', 'web') {'skill': 'python', 'place': 'CQ'}

staff('lemi',22,'linux','web',**{’skill’:'python',’place’:'CQ'})
效果跟上面一样

返回值

函数外部的代码要想获取函数的执行结果，就可以在函数里用return语句把结果返回

def staff(name,age,skill,year=2018):
    print('staff info'.center(20,'-'))
    print("name:",name)
    print("age:",age)
    print("skill:",skill)
    print("year:",year)
    if age>22:
        return False
    else:
        return True
interview = staff("lemi",22,"python_devops")
if interview:
    print(interview)
    print('面试成功')
else:
    print('have to improve')

output:
-----staff info-----
name: lemi
age: 22
skill: python_devops
year: 2018
面试成功
True

注意

函数在执行过程中只要遇到return语句，就会停止执行并返回结果，也可以理解为 return 语句代表着函数的结束
如果未在函数中指定return,那这个函数的返回值为None
函数,只能返回一个值

全局与局部变量

status = 'poor man'
def work(status):
    print('before learn python:',status)
    status = 'Charming male'
    print('after change:',status)

work(status)

print('看看外面status改变了吗？',status)

output:
before learn python: poor man
after change: Charming male
看看status改变了吗？ poor man

不用传name 值到函数里，也可以在函数里调用外面的变量
# 局部函数可以调用全局变量，但是不能修改
status = 'poor man'
def work():
    status = 'Charming male'
    print('after change:',status)
work()
print('看看外面status改变了吗？',status)
output:
after change: Charming male
看看外面status改变了吗？ poor man

1.在函数中定义的变量称为局部变量，在程序的一开始定义的变量称为全局变量。
2.全局变量作用域是整个程序，局部变量作用域是定义该变量的函数。
3.当全局变量与局部变量同名时，在定义局部变量的函数内，局部变量起作用；在其它地方全局变量起作用。

修改全局变量

status = 'poor man'
def work():
global status
status = 'Charming male'
print('after change:',status)
work()
print('看看外面status改变了吗？',status)
output
after change: Charming male
看看外面status改变了吗？ Charming male

global status的作用就是要在函数里声明status是全局变量,最上面的status='poor man'即使不写，程序最后面的print也可以打印status

变量是列表时，
当修改的是列表时，可以删除元素，修改元素

skill = ['linux','python','html']
def change_skill():
    skill = ['linux','python']
    print(skill)    #此处不能修改
change_skill()
print(skill)

['linux', 'python']
['linux', 'python', 'html']

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

skill = ['linux','python','html']
def change_skill():
    skill[2] = 'hadoop'
    skill.remove('linux')
    print('inside',skill)    #这里可以该，改的是元素，并不是列表对象
                            #因为在内存里，列表对象整体是一个地址，元素又是另一个地址
change_skill()
print('outside',skill)

inside ['python', 'hadoop']
outside ['python', 'hadoop']

作用域

作用域（scope），程序设计概念，通常来说，一段程序代码中所用到的名字并不总是有效/可用的，而限定这个名字的可用性的代码范围就是这个名字的作用域。
python中函数就是一个作用域，局部变量放置在其作用域中。
代码定义完，作用域已经生成，无论在任何地方调用，作用域链向上查找

age = 21
def func1():
    age = 22
    def func2():
        print(age)
    return func2
val = func1()
print(val)
val()

output
<function func1.<locals>.func2 at 0x000002AB58965BF8>
22

嵌套函数

name = 'bird kevin'
def work():
    name = 'common staff'
    def work_1():
        name = 'outstanding staff'
        print('level 3',name)
    work_1()
    print('level 2',name)
work()
print('level now',name)

OUTPUT
level 3 outstanding staff
level 2 common staff
level now bird kevin
#    最外层不能调用work_1（）里层函数

#小游戏
age = 19
def func1():
    global age
    def func2():
        print(age)
    age = 73
    func2()
func1()
print(age)      #output>>> 73 73

age = 19
def func1():
    def func2():
        print(age)
    func2()
    age = 73

func1()
print(age)      #会报错，不知道找哪一个

匿名函数

匿名函数就是不需要显式的指定函数名

#a = lambda x,y:x*y
print(a(1,2))
lambda 不能有复杂的逻辑。可以三元运算

res = map(lambda x:x**2,[1,5,7,4,8])
for i in res:
print(i)
输出

1
25
49
16
64
节省代码

高阶函数

变量可以指向函数，函数的参数能接收变量，那么一个函数就可以接收另一个函数作为参数，这种函数就称之为高阶函数。

def add(x,y,f):
    return f(x) + f(y)
res = add(3,-6,abs)
print(res)

只需满足以下任意一个条件，即是高阶函数

1.接受一个或多个函数作为输入

2.return 返回另外一个函数

递归

在函数内部，可以调用其他函数。如果一个函数在内部调用自身本身，这个函数就是递归函数

def fac(n):
    if n == 1:
        return 1
    else:
        return n *fac(n-1)
print(fac(3))

def calc(n):
    v = int(n/2)
    print(v)
    if v > 0:
        calc(v)
    print(n)

calc(10)

output

5
2
1
0
1
2
5
10

先一层层进去，再往外走

递归特性:

1.必须有一个明确的结束条件

2.每次进入更深一层递归时，问题规模相比上次递归都应有所减少

3.递归效率不高，递归层次过多会导致栈溢出（在计算机中，函数调用是通过栈（stack）这种数据结构实现的，每当进入一个函数调用，栈就会加一层栈帧，每当函数返回，栈就会减一层栈帧。由于栈的大小不是无限的，所以，递归调用的次数过多，会导致栈溢出）

堆栈扫盲

尾递归，调用下一层的时候就退出了
def cal(n):
    print(n)
    return cal(n+1)
cal(1)
在python并没有优化

内置函数

内置参数详解

>>> min([2,3,4])     #返回数字序列最小值

2
>>> max([2,3,4]) # 返回数字序列最大值

4

>>> help(all)
Help on built-in function all in module builtins:
all(iterable, /)
    Return True if bool(x) is True for all values x in the iterable.
    If the iterable is empty, return True.

>>> a = [1,2,3]
>>> all(a)
True
>>> a.append(0)
>>> all(a)
False
>>> a.append([])
>>> all(a)
False
>>> all([])
True

>>>help(any)
Help on built-in function any in module builtins:

any(iterable, /)
    Return True if bool(x) is True for any x in the iterable.

    If the iterable is empty, return False.

dir    打印当前下变量

>>> help(hex)

Help on built-in function hex in module builtins:

hex(number, /)
    Return the hexadecimal representation of an integer.

    >>> hex(12648430)
    '0xc0ffee'

>>> 10//3
3
>>> divmod(10,3)
(3, 1)
>>> sorted([22,31,13,3,14,1,34,3])
[1, 3, 3, 13, 14, 22, 31, 34]
>>> d = {}
>>> for i in range(20):
    d[i] = i-2

>>> d
{0: -2, 1: -1, 2: 0, 3: 1, 4: 2, 5: 3, 6: 4, 7: 5, 8: 6, 9: 7, 10: 8, 11: 9, 12: 10, 13: 11, 14: 12, 15: 13, 16: 14, 17: 15, 18: 16, 19: 17}
>>> sorted(d)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
>>> d.items
<built-in method items of dict object at 0x0000023C083AF5E8>
>>> d.items()
dict_items([(0, -2), (1, -1), (2, 0), (3, 1), (4, 2), (5, 3), (6, 4), (7, 5), (8, 6), (9, 7), (10, 8), (11, 9), (12, 10), (13, 11), (14, 12), (15, 13), (16, 14), (17, 15), (18, 16), (19, 17)])
>>> sorted(d.items(),key)
Traceback (most recent call last):
  File "<pyshell#11>", line 1, in <module>
    sorted(d.items(),key)
NameError: name 'key' is not defined
>>> sorted(d.items(),key=lambda x:x[1])
[(0, -2), (1, -1), (2, 0), (3, 1), (4, 2), (5, 3), (6, 4), (7, 5), (8, 6), (9, 7), (10, 8), (11, 9), (12, 10), (13, 11), (14, 12), (15, 13), (16, 14), (17, 15), (18, 16), (19, 17)]
>>> sorted(d.items(),key=lambda x:x[1],reverse = True)
[(19, 17), (18, 16), (17, 15), (16, 14), (15, 13), (14, 12), (13, 11), (12, 10), (11, 9), (10, 8), (9, 7), (8, 6), (7, 5), (6, 4), (5, 3), (4, 2), (3, 1), (2, 0), (1, -1), (0, -2)]
>>> ascii('n')
"'n'"
>>> ascii('你')
"'\\u4f60'"
>>> oct(10)
'0o12'
>>> bin(10)
'0b1010'
>>> help(eval)
Help on built-in function eval in module builtins:

eval(source, globals=None, locals=None, /)
    Evaluate the given source in the context of globals and locals.

    The source may be a string representing a Python expression
    or a code object as returned by compile().
    The globals must be a dictionary and locals can be any mapping,
    defaulting to the current globals and locals.
    If only globals is given, locals defaults to it.

>>> eval('1+7')
8
>>> eval('print('hello world')')
SyntaxError: invalid syntax
>>> eval('print("hello world")')
hello world
>>> code = '''
if 5 > 4:
    print('ok')
else:
    print('fail')'''
>>> eval(code)
Traceback (most recent call last):
  File "<pyshell#27>", line 1, in <module>
    eval(code)
  File "<string>", line 2
    if 5 > 4:
     ^
SyntaxError: invalid syntax
>>> 只能处理简单单行代码
Traceback (most recent call last):
  File "<pyshell#28>", line 1, in <module>
    只能处理简单单行代码
NameError: name '只能处理简单单行代码' is not defined
>>> exec(code)
ok
>>> help(exec)
Help on built-in function exec in module builtins:

exec(source, globals=None, locals=None, /)
    Execute the given source in the context of globals and locals.

    The source may be a string representing one or more Python statements
    or a code object as returned by compile().
    The globals must be a dictionary and locals can be any mapping,
    defaulting to the current globals and locals.
    If only globals is given, locals defaults to it.

>>> res = eval('1+3+4')
>>> res1 = exec('1+3+4')
>>> print('res',res,res1)
res 8 None
>>> exec可以执行多行但拿不到返回值
KeyboardInterrupt
>>> eval执行单行可以拿到返回值
KeyboardInterrupt
>>> ord('a')
97
>>> chr('')
Traceback (most recent call last):
  File "<pyshell#35>", line 1, in <module>
    chr('')
TypeError: an integer is required (got type str)
>>> chr(98)
'b'
>>> s = '元贞'
>>> s[0]
'元'
>>> s.encode('utf-8')
b'\xe5\x85\x83\xe8\xb4\x9e'
>>> s1 = bytearray(s)
Traceback (most recent call last):
  File "<pyshell#40>", line 1, in <module>
    s1 = bytearray(s)
TypeError: string argument without an encoding
>>> s1 = bytearray(s.encode (utf-8))
Traceback (most recent call last):
  File "<pyshell#41>", line 1, in <module>
    s1 = bytearray(s.encode (utf-8))
NameError: name 'utf' is not defined
>>> s1 = bytearray(s.encode ('utf-8'))
>>> s1[0] = 'a'
Traceback (most recent call last):
  File "<pyshell#43>", line 1, in <module>
    s1[0] = 'a'
TypeError: an integer is required
>>> s1[0] = 65
>>> s
'元贞'
>>> s1
bytearray(b'A\x85\x83\xe8\xb4\x9e')
>>> s1[4]
180
>>> id(s[0])
2456860022384
>>> 源地址修改
KeyboardInterrupt
>>> map(lambda x:x*x,[1,2,3])
<map object at 0x0000023C0842C908>
>>> list(map(lambda x:x*x,[1,2,3]))
[1, 4, 9]
>>> list(map(lambda x:x<3,[1,2,3]))
[True, True, False]
>>> list(filter(lambda x:x<3,[1,2,3]))
[1, 2]
>>> import functools
>>> functools.reduce
<built-in function reduce>
>>> bytes('')
Traceback (most recent call last):
  File "<pyshell#55>", line 1, in <module>
    bytes('')
TypeError: string argument without an encoding
>>> bytes('34)

SyntaxError: EOL while scanning string literal
>>> bytes(34)

b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
>>> bytes('你好')

Traceback (most recent call last):
  File "<pyshell#58>", line 1, in <module>
    bytes('你好')
TypeError: string argument without an encoding
>>> bytes(你好)

Traceback (most recent call last):
  File "<pyshell#59>", line 1, in <module>
    bytes(你好)
NameError: name '你好' is not defined
>>> print('python','linux',sep='->')

python->linux
>>> print('python','linux',end='->')

python linux->
>>> print('python','linux')

python linux
>>> print('python','linux',end='')

python linux
>>>

msg = 'linux is lady '
f = open('skill.txt','w')
print(msg,'python is yong woman',sep='|',end="",file=f)
print(msg,'python is yong woman',sep='|',end="",file=f)

>>> def f():
    pass

>>> a = [1,2,3]

>>> callable(f)

True
>>> callable(a)

False

>>> s = {1,2,3,4}

>>> s.discard(2)

>>> s

{1, 3, 4}
>>> s = frozenset(s)

>>> s

frozenset({1, 3, 4})
>>> 

>>> 

>>> s.discard(3)

Traceback (most recent call last):
  File "<pyshell#77>", line 1, in <module>
    s.discard(3)
AttributeError: 'frozenset' object has no attribute 'discard'
>>> s.remove(2)

Traceback (most recent call last):
  File "<pyshell#78>", line 1, in <module>
    s.remove(2)
AttributeError: 'frozenset' object has no attribute 'remove'

打印变量名和对应的值
>>> vars()

{'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, 'd': {0: -2, 1: -1, 2: 0, 3: 1, 4: 2, 5: 3, 6: 4, 7: 5, 8: 6, 9: 7, 10: 8, 11: 9, 12: 10, 13: 11, 14: 12, 15: 13, 16: 14, 17: 15, 18: 16, 19: 17}, 'i': 19, 'code': "\nif 5 > 4:\n    print('ok')\nelse:\n    print('fail')", 'res': 8, 'res1': None, 's': frozenset({1, 3, 4}), 's1': bytearray(b'A\x85\x83\xe8\xb4\x9e'), 'functools': <module 'functools' from 'C:\\Users\\Administrator\\AppData\\Local\\Programs\\Python\\Python36\\lib\\functools.py'>, 'f': <function f at 0x0000023C08464400>, 'a': [1, 2, 3]}

>>> def f():
    n = 3
    print(locals())

>>> f()

{'n': 3}
>>> globals()

{'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, 'd': {0: -2, 1: -1, 2: 0, 3: 1, 4: 2, 5: 3, 6: 4, 7: 5, 8: 6, 9: 7, 10: 8, 11: 9, 12: 10, 13: 11, 14: 12, 15: 13, 16: 14, 17: 15, 18: 16, 19: 17}, 'i': 19, 'code': "\nif 5 > 4:\n    print('ok')\nelse:\n    print('fail')", 'res': 8, 'res1': None, 's': frozenset({1, 3, 4}), 's1': bytearray(b'A\x85\x83\xe8\xb4\x9e'), 'functools': <module 'functools' from 'C:\\Users\\Administrator\\AppData\\Local\\Programs\\Python\\Python36\\lib\\functools.py'>, 'f': <function f at 0x0000023C084646A8>, 'a': [1, 2, 3]}
>>>

>>> a

[1, 2, 3]
>>> repr(a)

'[1, 2, 3]'

>>> a

[1, 2, 3]
>>> b = [6,7,8]

>>> zip(a,b)()

Traceback (most recent call last):
  File "<pyshell#96>", line 1, in <module>
    zip(a,b)()
TypeError: 'zip' object is not callable
>>> list(zip(a,b))

[(1, 6), (2, 7), (3, 8)]
>>>