CS50P: 4. Libraries¶

libraries, modules¶

libraries are bits of code written by you or others we can use in our program

Python allows us to share functions or features with others as "modules"

random¶

python: random

random is a library that comes with Python that we could import into our own project

we need keyword import

import¶

作用¶

import the contents of the functions from some module

使用random模块中的函数¶

import random
coin = random.choice(["heads", "tails"])
print(coin)

random is the module we are importing. Inside that module, there is the choice function -- random.choice(seq)

参数：list

返回值：list中的随机一个元素

from¶

from: allows us to be very specific about what we’d like to import

import: bringing the entire contents of the functions of a module

from random import choice
coin = choice(["heads", "tails"])

注：使用时直接用函数名 (因为只 import 了一部分)

functions¶

random.randint(a, b)¶

return random int in [a,b] 区间

random.shuffle(x)¶

shuffle a list into a random order

注意：没有返回，而是直接作用于list

statistics¶

python: statistics

mean function¶

print(statistics.mean([100, 90]))

接收列表，返回平均值

Command-Line Arguments¶

take input from the command-line命令行参数

sys¶

a module that allows us to take arguments at the command line

python: sys

argv¶

a function within the sys module

import sys
print("hello, my name is", sys.argv[1])

sys.argv是一个list
命令行输入 python3 sys.py chase
sys.argv[0] 是 sys.py
"chase tsai" 视作一个命令（加了引号）

IndexError¶

如果使用命令 python3 sys.py，返回 IndexError: list index out of range

改进¶

way 1

try:
    print("hello, my name is", sys.argv[1])
except IndexError:
    print("Too few arguments")

way 2

if len(sys.argv) < 2:
    print("Too few arguments")
elif len(sys.argv) > 2:
    print("Too many arguments")
else:
    print("hello, my name is", sys.argv[1])

sys.exit()¶

exit the program if an error was introduced by the user

if len(sys.argv) < 2:
    sys.exit("Too few arguments")

常常用来结束整个程序

slice¶

To take a slice of a list means to take a subset of it

for arg in sys.argv[1:]:
    print("hello, my name is", arg)

: 必须有
sys.argv[1:-2] 表示取左一到右二之间的子序列

packages¶

One of the reasons Python is so popular is that there are numerous powerful third-party libraries that add functionality. We call these third-party libraries, implemented as a folder, "packages".

find other third-party packages at PyPI

learn more on PyPI’s entry for cowsay: a well-known package that allows a cow to talk to the user

pip¶

install packages quickly onto your system

APIs¶

APIs or “application program interfaces” allow you to connect to the code of others

requests¶

python: requests.

requests is a package that allows our program to behave as a web browser would

import requests
import sys

if len(sys.argv) != 2:
    sys.exit()

"""
looking for a song, with a limit of one result, that relates to the term called weezer
"""
response = requests.get("https://itunes.apple.com/search?entity=song&limit=1&term=" + sys.argv[1])  
print(response.json())

第10行是把python连接到浏览器，就像我们在Safari中输入urls，按enter键

Apple documentation about this API: what is returned is a JSON file. Running python3 itunes.py weezer, you will see the JSON file returned by Apple.

However, the JSON response is converted by Python into a dictionary!!!

JSON¶

库¶

python的一个库

功能：manipulate JSON data, nicely printing on the screen

python: JSON

应用样例¶

修改上例代码

import requests
import sys
import json

if len(sys.argv) != 2:
    sys.exit()
response = requests.get("https://itunes.apple.com/search?entity=song&limit=1&term=" + sys.argv[1])
print(json.dumps(response.json(), indent=2))

进一步观察，删除 &limit=1 并插入代码：

o = response.json()
for result in o["results"]:
    print(result["trackName"])

会得到所有的trackName：

Making Our Own Libraries¶

sayings.py¶

def main():
    hello("world")
    goodbye("world")

def hello(name):
    print(f"hello, {name}")

def goodbye(name):
    print(f"goodbye, {name}")

main()

say.py¶

import sys
from sayings import hello
if len(sys.argv) == 2:
    hello(sys.argv[1])

sayings.py是我们自己写的module，用 from sayings import hello ，python会知道是在saying.py中找hello函数，然后执行它以下的所有代码

运行¶

命令行 python3 say.py chase：

1
2
3

hello, world
goodbye, world
hello, chase

name¶

只想输出 hello, xxx ，解决方法：

if __name__ == "__main__":
    main()

__name__ 是特殊变量，python将它自动设置成 "__main__" 当在command line运行该程序，本例中即 python3 sayings.py 。而我们在command line只运行了say.py程序，sayings.py程序只是import的，所以 __name__ 没有自动初始化，不会被执行