> For the complete documentation index, see [llms.txt](https://louisazhou.gitbook.io/notes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://louisazhou.gitbook.io/notes/chapter1/python-good-to-knows.md).

# Python Good to knows

1. 在Google Colaboratory 可以使用一些Linux命令，比如

```bash
!pip install pandasql 
```

安装一些Colab上没有的包

2\. 在Colab上可以用#@title +Title做标题

3\. 在Python里可以 print(1 or False)这样的语句，它返回的是 第一个不是false的值

4\. 对于string的一些简便操作

```python
.capitalize()     #大写首字母
.upper()          #所有字母大写
.count('o')       #数o有几个
.index('te')      #给出te的index
```

5\. Python List的elements可以不相同，但是np array就不行；比如可以有一个

```python
lst3 = ['one', 2, 'three', 4]
```

6\. Python List是mutable的

7\. +号可以用来连接字符串，也可以用来连接两个list

```python
lst2 = [1, 2, 3, 4] 

print(lst2 + lst3)

s1 = 'single quote' # use single quotes
s2= "double quote" # use double quotes 
x = 100
s3 = s1 + ' huozhe ' + s2 + str(x) #如果是str的加，要把数值型转成字符型才能+
print(s3)
```

8\. list2=list1\[:]是shallow copy，lst4 = lst2.copy()是hard copy，lst5 = lst2是copy reference

9\. Python list或string的切片（slicing）的规矩是\[start(inclusive) : end(exclusive) : step] 

10\. lst\[::-1]可以reverse a list

11\. slicing的过程也可以改变数值，比如`lst[2:4] = [3, 2]`

12\. Python的Dictionary

如果我们想要索引一个成绩，用两个list的话效果是这样的 时间复杂度是O(n)

```python
skills = ['ML', 'Stat', 'Coding', 'Business']
scores = [90, 80, 1, 59]

print(scores[skills.index('Coding')])
```

用dic就是这样的效果 {key:value} key必须unique，但是value可以是任意一个东西

```python
score_dict = {'ML': 90, 'Stat': 80, 'Coding': 1, 'Business': 59} # create a dictionary
print(score_dict['ML']) # get ML score
print('Stat' in score_dict) # check if 'Stat' in the dictionary

#如果想要loop over dictionary
for skill in score_dict.values():
  print(skill)
```

13\. 不想看那些warnings

```python
import warnings
warnings.filterwarnings('ignore')
```

14\. numpy的random seed和python的random seed不是一个东西，所以 random.seed = xxx 和np.random.seed=xxx 也是两个东西

15\. \t \n \r 这些C语言的东西在Python里照样能用，\t可以自动对齐

16\. 在Python中使用非ASICC字符，比如 中文

```python
#!usr/bin/python
# -*- coding: UTF-8 -*-
print "现在可以中文了"
```

17\. Python的list支持各种类型，也支持混着，所以使用的时候要留意判断

18\. remove list by index 用的是del; O(n)的操作，因为后面的要往左移，但是假如本来要删除的就是最后一个，那O(1)

而list.remove() 是remove by value，而且是删除第一个出现的，O(n)的操作

```python
del list[1]
list.remove(1)
```

19\. 看懂python doc\
\- \[, xxx]代表变量可忽略

\*arg 代编不确定个数的参数，进入函数之后是一个tuple

\*\*args 更灵活，通过var\_name=var\_value调用，进入函数后是dict

[链接](https://stackoverflow.com/questions/400739/what-does-asterisk-mean-in-python)

20\. 在函数括号传的值里可以放default value，在没有值传进来的时候就用default，有值传进来的时候就用传入的值

21\. 在python2里 range()占O(n)的空间，xrange()占O(1)的空间 

22\. 如果有一些column在打印的时候被...了 想看到全部，用

```python
np.set_printoptions(threshold = np.nan) 
```

或者用

```python
pd.set_option('display.max_columns', 500)
```

23\. 针对整个Jupiter notebook的打印格式，可以用

```python
pd.set_option('display.float_format', lambda x: '%.3f' % x)
```

24\. Python有个内置函数，enumerate(), 可以帮助遍历，它可以获取index和value

```python
list1 = ["1", "2", "3", "4"]
for index, item in enumerate(list1):
    print index, item
>>>
0 1
1 2
2 3
3 4
```

enumerate还可以接收第二个参数，用于指定索引起始值

```python
list1 = ["1", "2", "3", "4"]
for index, item in enumerate(list1, 1):
    print index, item
>>>
1 1
2 2
3 3
4 4
```

还可以用enumerate统计文件的行数

```python
ount = 0
for index, line in enumerate(open(filepath,'r'))： 
    count += 1
```

25\. 面向对象和面向过程

面向过程：sequential 把问题分成多个步骤，按顺序执行；打开冰箱，放食物，关上冰箱

面向对象OOP：封装，冰箱的操作：打开；存储；关闭 食物的操作：名字；大小

面试Amazon考过，OOP，停车场

练习方式：sorting之类的改写成oop的形式

26\.  class内部如果有多个def，那么顺序可以不和实现一样，但是如果是class外部的，要先定义先使用的

27\. if \_\_name\_\_="\_\_main\_\_" Python的启动函数，这个文件直接被执行的时候 会执行main后面的 ；如果被作为外部文件导入（比如 from \*\*.py import \*） 就不执行main的

\
意味着 只有在这个文件内部的才会执行

27\. 程序运行时，stack存的是reference，heap存的是value

28\. 单双下划线：

\_\_foo\_\_ 定义的是特殊方法，一般是系统builtin的名字，比如\_\_init\_\_() 或者\_\_name\_\_

\_foo 表示protected的变量，只允许其本身和子类访问，不能外部调用，不能用于 from module import \* 

\_\_foo 表示private的变量，只能允许本身的访问，不能被子类访问，不能继承，不能被外部访问

So there it is, you just call the global function `len()` on the `list` or `tuple` and you get its size / length.

Did you expect to have something like `list.len()` or `tuple.len()` instead? Actually there is something like that, but it is called `list.__len__()` or `tuple.__len__()`. And what `len()` really does is, it takes the object and tries to call the objects's `__len__()` method. So essentially, `len()` works only on objects that has a `__len__()` method. If you have a custom object that has the `__len__()` method, you can call `len()` even on that.

So you learnt: to get the size or length of a `list` or `tuple` in Python (you probably didn't mean Python `array`), you just call the `len()` function on the object.


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