Python编程基础详解-优快云博客

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1.print函数

完整语法：print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)

解释：

*Object：一个或多个要print的Object
sep：每个object间的间隔字符，默认为''
end：在最后打印的字符，默认为'\n'
file: 必须是一个写入方法的object，默认为sys.stdout，即objects会打印在屏幕上。
**flush** ：flush参数主要是刷新，flush=True时，会在print结束后立即将内存中的东西显示到屏幕上并清空缓存。(一般print都是将要输出的东西先存到内存中，再输出。

2. Escape Character

Escape Character	Meaning	Example (`print()`)	Output
\'	'	Don\'t do this	Don't do this
\"	"	Penny said \"hi\"	Penny said "hi"
\\	\	backslash: \\b	backslash: \
\n	linebreak	a\nb	a b
\t	TAB	a\tb	a b
\b	backspace	ab\bc	ac
\r	overriding	123456\rXX_XX	XX_XX6

3. 变量命名规则

只能是字母，数字或下划线的组合
只能以字母或下划线作为首字符
大小写区分
keywords 不能用作变量名

4.标量

1.int：only signed int；在32位系统里占32bits，64位系统里则占64bits。

2.float：有科学计数法e.g. $1.23\times 10^{23}$ ->1.23e23;占64bits/8bytes

3.complex：两种表示方法：<real part> + <imaginary part>j

<real part> + <imaginary part>J

4.type()函数返回变量的类型

5.Strings 字符串

using raw strings by adding r before the first quote without interpretation of escape characters

e.g.print(r'Don\'t fo this')

Don\'t do this

String 切片[a:b] means [a,b)

String Methods

Some of the daily used built-in string operations

strip()
- removes the whitespace at the beginning or the end
```
wd = 'my name is '
wd.strip()
```
len()
- returns the length of the string
```
wd = 'Python'
len(wd)
```
lower() returns a string with lower case
upper() returns a string with upper case
```
 wd = 'Python'
 wd.lower()
 wd.upper()
```
replace(old, new)
- return a copy of the string with all occurrences of substring old replaced by new
```
wd = 'Python'
wd.replace('P', 'p')
```

6.一些等价关系

>>> 1 == 1.0 
True 
>>> 1 is 1.0
False
>>> 1 !=1.0 
False 
>>> 1 is not 1.0 
True
>>> None == False
False

7.LEGB原则

查找变量名时的顺序：Local->Enclosing->Global->Built-in

8.函数参数

def example( required_parameter,default_parameter = '',*args,**kw):
    pass

参数种类：必选参数，默认参数，可选参数，关键字参数

note：默认参数有多个的话，可以不按顺序传参。

可选参数本质上传进去的是一个tuple；关键字参数本质上传进去的是一个dict。

9.Call by object

个人认为可以这么理解，传进去的是对象的地址的复制，所以在list，dict这样的mutable对象上进行操作，实际上是会改变其本来的值的；而在str，tuple，int这样的对象上进行操作，因为其本身immutable的属性，所以大多进行的是赋值的操作，这个时候，参数不再指向原来的对象，而是指向一个新的对象，原先对象的值没有发生任何改变，所以这相当于call by value，不会改变其本身的值。

10. Working with files

1.使用open()函数

open(filename, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)

returns 一个文件对象
```
file_obj = open(filename, mode)
```

Note：文件名要保留后缀

使用close()函数关闭文件
f.close()

mode	details
'w': write	used when files need to be changed, changed from the very beginning of the file, file is erased
'r': read	only read the file, do not change the file, file pointer is placed at the very beginning of the file
'a': append	adds information to the end of the file, file pointer is placed at the end of the file
'r+': read and write	file pointer is placed at the very beginning of the file
'a+': append and read	file is opened and allows program to read information and new information to be added at the end of the file
't': text mode	by default
'b': binary mode	only used for data
'x': exclusive creation	exclusively creat a file, if a file of the same name already exists, it will fail

files are opened in text and read mode by default

it means you read and write strings from and to the file, which are encoded in a specific encoding. keyword argument encoding can be set
open file in binary mode, append 'b' to the mode argument, e.g., 'rb'
写入文件用‘w’mode，使用write函数

f.open('text.txt','w')
f.write('Hello')
f.close()

读取文件

使用 read(size)
- size specifies the amount of data to be read
- when size omitted, read the entire file
using the readline() function to read line by line
using readlines() to get a list of remaining lines of the file
Note：readline()和read()函数会读取换行符

11.Class

最好通读lecture7，都蛮重要的。

12.List.__mul__(self,n)

__mul__(self, n) : implementation of * operator

 lis.__mul__(n), or lis * n

n must be an integer, otherwise TypeError
if n <= 0, returns an empty list
if n = 1, returns a new list that has the same reference of the object lis
otherwise, returns a new list that contains n times of references of the object lis

lis1 = [[1, 2], [3, 4]]
lis2 = lis1 * 2
print(lis2)

#output:[[1, 2], [3, 4], [1, 2], [3, 4]]

13.map(function,Iterable,......)

简单说，就是把function应用到每个Iterable上，返回一个Iterator

x = map(lambda x : x ** 2, range(3))


next(x)
>>> 0
next(x)
>>> 1
next(x)
>>> 4
next(x)
>>> Stop Iteration

14 Generator

例

def foo(): 
    print("begin")
    for i in range(3): 
        print("before yield", i) 
        yield i 
        print("after yield", i) 
    print("end")

每次yield之后，Generator都会保存yield之前的数据。

>>> import os 
>>> os.chdir("D:\\Test") 
>>> from foo import foo 
>>> f = foo() 
>>> print(f) 
<generator object foo at 0x035A7470> 
>>> next(f) 
begin 
before yield 0 
0 
>>> next(f) 
after yield 0 
before yield 1 
1

当Generator终止后，会自动调用StopIteration

Generator有更紧凑的表达方式

x = (i+1 for i in range(5))

(注意与列表生成式区分！！！！）

x = [i+1 for i in range(5)]#列表生成式

15.默认参数的值

lecture9 P41-51

SI100b期末复习