03笨方法学Python|ex11-17

2018-01-14  本文已影响28人  ericazhan

ex11

1.raw_input 和 input 的区别

raw_input() ，无论什么输入内容，python都把内容看成一个字符串string.
input ，python会根据输入的内容，决定其类型。Shawn建议避免使用此命令。

代码错误。。MARK在此，待改。
print "what year is it this year?",
year = int(raw_input())
if year % 4 == 0   **??? invalid**
print "this is a leap year"
else print "not leap year"

leap year: 闰年

2. 附加练习：乘幂计算练习，熟悉数字类型int 和 float

print "Give me a base",
base = float(raw_input())
print "Give me then a exponent which is a integer.",
exponent = int(raw_input()) 
ans = base ** exponent
print "So the answer for %.f to the power %d is %f." % (base,exponent,ans)

以上代码运行后，如果在指数exponent 那里输入3.1，就会报错：

数字类型int 和 float

所以如果想变成整数，需要先转成浮点，再转成整数。

print "Les't do some exponentiation."
print "Give me a base",
base = float(raw_input())
print "Give me then a exponent which is a integer.",
exponent = int(float(raw_input()))
ans = base ** exponent
print "So the answer for %.2f to the power %d is %.2f." % (base,exponent,ans)

格式符 %.4f 表示保留4位小数。

查看帮助

python -m pydoc 命令名

pydoc 在线帮助系统 https://docs.python.org/2.7/library/pydoc.html

ex12

简化上面的乘幂计算例子

base = float(raw_input("Give me a base.            "       )) 
expo = int(float(raw_input("Give me a exponent."   )))
ans = base ** expo
print "So the answer for %d to the power %.2f is %.4f." %(base,expo,ans)

第一行，引号前面的空格，在powershell里才体现出是空格。

ex13

from sys import argv

script,first,second,third = argv

print "The script is called:",script
print "Your first variable is:",first
print "Your second variable is",second
print "Your third variable is:",third

如上按书中一字不差打完代码后，我一如既往在powershell里敲入

python ex13.py

然后就看到这样的错误提示：

Traceback (most recent call last):
  File "ex13.py", line 3, in <module>
    script,first,second,third = argv
ValueError: need more than 1 value to unpack

解释：代码没错，但是在powershell里要输入的东西多了 3个任意变量！正好三个，不可多不可少，以空格区分。
比如这样的，

python ex13.py aa bb cc

为什么呢？因为小白如我，根本没有理解这节课，没有理解sys.argv，看了stackoverflow上这位高手的回答，才算清晰。
这节课讲的还是如何给python“喂食”，输入变量。
sys模块，argv是其中的一个函数。argv表示一串参数(a tuple of arguments). 第一行 from sys import argv，表示从sys模块调出这么一串参数。
那这一串参数到底是啥呢？4个变量并排放置，暂时被命名script,first,second,third.
所以可以把argv看成一个行向量[a,b,c,d],然后具体计算再带入具体数值。在powershell中，我们敲进去的，在python后面的内容，按空格区分，分别被带入这个“向量”里。
比如这一课我们完全可以这么敲

from sys import argv
aa,bb,cc,dd = argv                #define what is this argv 
print "The first variable is called:",aa
print "Your 2nd  is:",bb
print "Your 3rd is",cc
print "Your 4th is:",aa

在powershell敲入 python ex13.py 11 22 33，看见以下结果

The first variable is called: ex13.py
Your 2nd  is: 11
Your 3rd is 22
Your 4th is: ex13.py

故意不用33，哈哈。

ex14

ex14和13大同小异。
不如再次理解sys.argv. How to use sys.argv in Python
sys 是System-specific parameters and functions的缩写，表示python内置的各种参数和方程。sys.argv表示一个清单，清单内容需要用命令行输入。
试试下面的代码，就更清楚了~
argv[0]表示代码文件的名字
函数len(sys.argv) 可以看这个argv清单中有几个参数。
函数str(),将括号里内容转换成字符串。

import sys
print "This is the name of the script: ",sys.argv[0]
print "Number of arguments: ",len(sys.argv)
print "The arguments are: ",str(sys.argv)

测试两次，看到的结果分别为：

$ PS D:\105-coding\learnpython> python ex14_sys.py
This is the name of the script:  ex14_sys.py
Number of arguments:  1
The arguments are:  ['ex14_sys.py']

$ PS D:\105-coding\learnpython> python ex14_sys.py 11 22 33
This is the name of the script:  ex14_sys.py
Number of arguments:  4
The arguments are:  ['ex14_sys.py', '11', '22', '33']

Zork 在线版本 zork online, 这个网站上全是类似的文字型游戏，好有爱啊~
Adventure没找到。

ex15

mode description
'r' Open a file for reading. (default)
'w' Open a file for writing. Creates a new file if it does not exist or truncates the file if it exists.
'r+' open a file for both reading and writing.
'a' open a file for adding a data to the end of file.
method description
close() Close an open file. It has no effect if the file is already closed.
read(n) Read atmost n characters form the file. Reads till end of file if it is negative or None.
readline(n=-1) Read and return one line from the file. Reads in at most n bytes if specified.
seek(offset,from=SEEK_SET) Change the file position to offset bytes, in reference to from (start, current, end).
truncate(size=None) Resize the file stream to size bytes. If size is not specified, resize to current location.
write(s) Write string s to the file and return the number of characters written.

摘自 programmiz/file-operation

>>> showme = open("ex15_sample.txt")
>>> print(showme.read())
This is stuff I typed into a file.
It is really cool stuff.
Lots and lots of fun to have in here.
>>> showme.close()
>>>

只用open,read命令，一次只能打开一个文件。如果要同时打开多个file,比较复杂，涉及另一个函数，解答在下面链接里，暂时略过。
https://stackoverflow.com/questions/4617034/how-can-i-open-multiple-files-using-with-open-in-python
https://stackoverflow.com/questions/30041837/how-to-open-multiple-files-one-by-one-and-print-the-contents-in-python

ex 16

target.write(line1,"\n",line2,"\n",line3,"\n")

报错。

 File "ex16.py", line 25, in <module>
    target.write(line1,"\n",line2,"\n",line3,"\n")
TypeError: function takes exactly 1 argument (6 given)

可行的命令：

ff = "%s\n%s\n%s\n" %(line1,line2,line3)
target.write(ff)

ex17

indata = open(from_file).read()

from sys import argv ;open(argv[2], 'w').write(open(argv[1]).read())

或者使用更高级的代码。

import shutil, sys ; shutil.copy(sys.argv[1], sys.argv[2])
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