Python中函数参数设置及使用的学习笔记
一、参数和共享引用:
In [56]: def changer(a,b): ....: a=2 ....: b[0]='spam' ....: In [57]: X=1 In [59]: L=[1,2] In [60]: changer(X,L) In [61]: X,L Out[61]: (1, ['spam', 2])
函数参数是赋值得来，在调用时通过变量实现共享对象，函数中对可变对象 参数的在远处修能够影响调用者。
避免可变参数修改:
In [67]: X=1 In [68]: a=X In [69]: a=2 In [70]: print(X) 1 In [71]: L=[1,2] In [72]: b=L In [73]: b[0]='spam' In [74]: print(L) ['spam', 2] In [75]: changer(X,L[:]) #不想要函数内部在原处的修改影响传递给它的对象，可以创建一个对象的拷贝 In [77]: changer(a,b) In [78]: def changer(a,b): ....: b=b[:] #如果不想改变传入对象，无论函数怎么调用，同样可以在函数内部进行拷贝。 ....: In [79]: a=2 In [80]: b[0]='spam'
二、特定参数匹配模型:
函数匹配语法:
例子:
关键字参数:
In [2]: def f(a,b,c):print (a,b,c) In [3]: f(1,2,3) #位置参数调用 (1, 2, 3) In [4]: f(c=3,b=2,a=1) #关键字参数调用 (1, 2, 3)
默认参数:
In [5]: def f(a,b=2,c=3):print (a,b,c) In [6]: f(1) #给a赋值，b,c使用默认赋值 (1, 2, 3) In [7]: f(a=1) (1, 2, 3) In [8]: f(1,4) (1, 4, 3) In [9]: f(1,4,5) #不适用默认值 (1, 4, 5) In [10]: f(1,c=6) #a通过位置得到1，b使用默认值，c通过关键字得到6 (1, 2, 6)
三、任意参数:
1、收集参数:
#*和**出现在函数定义或函数调用中。
In [11]: def f(*args):print (args)
In [12]: f() #将所有位置相关的参数收集到一个新的元祖中
()
In [13]: f(1)
(1,)
In [14]: f(1,2,3,4)
(1, 2, 3, 4)
In [15]: def f(**args):print (args)
In [16]: f()
{}
In [17]: f(a=1,b=2) #**只对关键字参数有效
{'a': 1, 'b': 2}
In [19]: def f(a, *pargs,**kargs):print(a,pargs,kargs)
In [20]: f(1,2,3,4,5,6,x=1,y=2,z=3)
(1, (2, 3, 4, 5, 6), {'y': 2, 'x': 1, 'z': 3})
2、解包参数:
注意:不要混淆函数头部或函数调用时*/**的语法:在头部意味着收集任意数量的参数，而在调用时，它接驳任意数量的参数。
In [21]: def func(a,b,c,d):print(a,b,c,d)
In [22]: args=(1,2)
In [23]: args += (3,4)
In [24]: func(*args)
(1, 2, 3, 4)
In [25]: args={'a':1,'b':2,'c':3}
In [26]: args['d']=4
In [27]: func(**args)
(1, 2, 3, 4)
In [28]: func(*(1,2),**{'d':4,'c':4})
(1, 2, 4, 4)
In [30]: func(1,*(2,3),**{'d':4})
(1, 2, 3, 4)
In [31]: func(1,c=3,*(2,),**{'d':4})
(1, 2, 3, 4)
In [32]: func(1,*(2,3,),d=4)
(1, 2, 3, 4)
In [33]: func(1,*(2,),c=3,**{'d':4})
(1, 2, 3, 4)
3、应用函数通用性:
In [34]: def tracer(func,*pargs,**kargs):
....: print ('calling:',func.__name__)
....: return func(*pargs,**kargs)
....:
In [35]: def func(a,b,c,d):
....: return a+b+c+d
....: print (tracer(func,1,2,c=3,d=4))
....:
('calling:', 'func')
10
4、python3.X中废弃apply内置函数
In [36]: pargs=(1,2)
In [37]: kargs={'a':3,'b':4}
In [41]: def echo(*args,**kargs):print (args,kargs)
In [42]: apply(echo,pargs,kargs)
((1, 2), {'a': 3, 'b': 4})
运用解包调用语法,替换:
In [43]: echo(*pargs,**kargs)
((1, 2), {'a': 3, 'b': 4})
In [44]: echo(0,c=5,*pargs,**kargs)
((0, 1, 2), {'a': 3, 'c': 5, 'b': 4})
四、python3.x中Keyword-only参数
python3.x把函数头部的排序规则通用化了，允许我们指定keyword-only参数，即按照关键字传递并且不会由一个位置参数来填充的参数；参数*args之后，必须调用关键字语法来传递。
In [1]: def kwonly(a,*b,c): ...: print(a,b,c) In [2]: kwonly(1,2,c=3) 1 (2,) 3 In [3]: kwonly(a=1,c=3) 1 () 3 In [4]: kwonly(1,2,3) #c必须按照关键字传递 TypeError: kwonly() missing 1 required keyword-only argument: 'c' In [6]: def kwonly(a,*,b,c):print(a,b,c) In [7]: kwonly(1,c=3,b=2) 1 2 3 In [8]: kwonly(c=3,b=2,a=1) 1 2 3 In [9]: kwonly(1,2,3) TypeError: kwonly() takes 1 positional argument but 3 were given
1、排序规则:
**不能独自出现在参数中,如下都是错误用法:
In [11]: def kwonly(a,**pargs,b,c): ....: File "<ipython-input-11-177c37879903>", line 1 def kwonly(a,**pargs,b,c): ^ SyntaxError: invalid syntax In [13]: def kwonly(a,**,b,c): ....: File "<ipython-input-13-46041ada2700>", line 1 def kwonly(a,**,b,c): ^ SyntaxError: invalid syntax
也就是说一个函数头部，keyword-only参数必须编写在*args任意关键字形式之前，或者出现在args之前或者之后，并且可能包含在**args中。
In [14]: def f(a,*b,**d,c=6):print(a,b,c,d)
File "<ipython-input-14-43c901fce151>", line 1
def f(a,*b,**d,c=6):print(a,b,c,d)
^
SyntaxError: invalid syntax
In [15]: def f(a,*b,c=6,**d):print(a,b,c,d) #keyword-only在*args之后，**args之前
In [16]: f(1,2,3,x=4,y=5)
1 (2, 3) 6 {'x': 4, 'y': 5}
In [20]: f(1,c=7,*(2,3),**dict(x=4,y=5)) #keyword-only在
1 (2, 3) 7 {'x': 4, 'y': 5}
In [21]: f(1,*(2,3),**dict(x=4,y=5,c=7))
1 (2, 3) 7 {'x': 4, 'y': 5}
2、为什么使用keyword-only参数?
很容易允许一个函数既接受任意多个要处理的位置参数，也接受作为关键字传递的配置选项， 可以减少代码，如果没有它的话，必须使用*args和**args，并且手动地检查关键字。
3、min调用
编写一个函数，能够计算任意参数集合和任意对象数据类型集合中的最小值。
方法一:使用切片
In [23]: def min(*args): ....: res=args[0] ....: for arg in args[1:]: ....: if arg < res: ....: res = arg ....: return res ....:
方法二:让python自动获取，避免切片。
In [28]: def min2(first,*rest): ....: for arg in rest: ....: if arg < first: ....: first = arg ....: return first ....:
方法三:调用内置函数list，将元祖转换为列表，然后调用list内置的sort方法实现。 注意:因为python sort列程是以C写出的，使用高度优化算法，运行速度要比前2中快很多。
In [32]: def min3(*args): ....: tmp=list(args) ....: tmp.sort() ....: return tmp[0] ....: In [29]: min2(3,*(1,2,3,4)) Out[29]: 1 In [31]: min(*(5,6,6,2,2,7)) Out[31]: 2 In [33]: min3(3,4,5,5,2) Out[33]: 2
五、例子:
1、模拟通用set函数:
编写一个函数返回两个序列的公共部分,编写inter2.py文件如下:
#!/usr/bin/python3
def intersect(*args):
res=[]
for x in args[0]:
for other in args[1:]:
if x not in other: break
else:
res.append(x)
return res
def union(*args):
res=[]
for seq in args:
for x in seq:
if not x in res:
res.append(x)
return res
测试:
In [3]: from inter2 import intersect,union In [4]: s1,s2,s3="SPAM","SCAM","SLAM" In [5]: intersect(s1,s2),union(s1,s2) Out[5]: (['S', 'A', 'M'], ['S', 'P', 'A', 'M', 'C']) In [6]: intersect([1,2,3],(1,4)) Out[6]: [1] In [7]: intersect(s1,s2,s3) Out[7]: ['S', 'A', 'M'] In [8]: union(s1,s2,s3) Out[8]: ['S', 'P', 'A', 'M', 'C', 'L']
2、模拟python 3.x print函数
编写文件python30.py
(1)使用*args和**args方法
环境python2.7
#!/usr/bin/python
import sys
def print30(*args,**kargs):
sep = kargs.get('sep',' ')
end = kargs.get('end','\n')
file = kargs.get('file',sys.stdout)
if kargs:raise TypeError('extra keywords: %s' %kargs)
output = ''
first = True
for arg in args:
output += ('' if first else sep)+str(arg)
first = False
file.write(output + end)
交互结果:
In [5]: print30(1,2,3) 1 2 3 In [6]: print30(1,2,3,sep='') 123 In [7]: print30(1,2,3,sep='...') 1...2...3 In [8]: print30(1,[2],(3,),sep='...') 1...[2]...(3,) In [9]: print30(4,5,6,sep='',end='') 456 In [11]: print30(1,2,3) 1 2 3 In [12]: print30()
(2)使用keyword-only方法，实现效果和方法一一样：
#!/usr/bin/python3
import sys
def print30(*args,sep=' ',end='\n',file=sys.stdout):
output = ''
first=True
for arg in args:
output += ('' if first else sep) + str(arg)
first = False
file.write(output + end)
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