CS50P: 6. File I/O

File I/O

list相关函数

python: list

append()

• 向列表末尾添加元素

list.append(element)

sorted(*, key=None, reverse=False)

• Python: sorted

• sorts the list in place, using only < comparisons between items

• key: eg. key=str.lower() 说明用小写来排序，但是列表还是不变
• 稳定排序

程序

names = []
for _ in range(3):
    names.append(input("What's your name? "))
for name in sorted(names):
    print(f"hello, {name}")

程序一旦结束，所有信息都会消失，File I/O可以存储信息，便于后续使用

open

open files

Python: open

file = open("name.txt", "w") 

open 以写入的方式打开name.txt文件。如果不存在，就创建一个文件；如果存在，会清空

返回“文件指针”

写入文件

name = input("What's your name? ")
file = open("name.txt", "w")
file.write(name)
file.close()

每次运行程序都从头重写name.txt文件

追加内容

"w" 替换成 "a" (append)

with

自动关闭文件

with open("name.txt", "a") as file:
    file.write(f"{name}\n")

file得到open的返回值

读文件

全部读

with open("name.txt", "r") as file:
    lines = file.readlines()
for line in lines:
    print("hello,", line.rstrip())	#去掉文件换行符

read所有行，存入lines，是一个list

"r" 可省，是默认值

逐行读

with open("name.txt", "r") as file:
    for line in file:
        print("hello,", line.rstrip())

不需要先读所有行，for line in file 可以一行一行地读

排序

names = []
with open("name.txt", "r") as file:
    for line in file:
        names.append(line.rstrip())
for name in sorted(names):
    print(f"hello, {name}")

CSV

CSV stands for "comma separated values"

students.csv文件

Hermione,Gryffindor
Harry,Gryffindor
Ron,Gryffindor
Draco,Slytherin

一列名字，一列房屋，用 , 隔开

读取信息

with open("students.csv") as file:
    for line in file:
        row = line.rstrip().split(',')
        print(f"{row[0]} is in {row[1]}")

split()返回list

设置两个变量接收信息，而非一个list：

name, house = line.rstrip().split(',')
print(f"{name} is in {house}")

排序

存成元素为 dict 的 list

students = []

with open("students.csv") as file:
    for line in file:
        name, house = line.rstrip().split(',')
        student = {}    #student是一个dict
        student["name"] = name
        student["house"] = house
        students.append(student) 

for student in students:
    print(f"{student['name']} is in {student['house']}")

注意：

1. line 12 中 'name' 用单引号括起来，因为已经使用过双引号

2. line 6~8 可以换成 student = {"name": name, "house": house}

sort keys

def get_name(student):      #student is a dict
    return student["name"]
for student in sorted(students, key=get_name):
    print(f"{student['name']} is in {student['house']}")

line 3 key=get_name ：sorted函数会用list中的每个字典自动帮我们call get_name(student)，可以理解为key指示如何排序

另一种写法：

for student in sorted(students, key=lambda student: student["name"]):

lambda function: 未名函数，"Hey Python, here is a function that has no name: Given a student (参数), access their name and return that to the key "

CSV Library

CSV是Python的一个库，Python: CSV

csv.reader

假设csv文件某行有不止一个 , ，可以用 "" 括起来，再调用库

Harry,"Number Four, Privet Drive"
Ron,The Burrow
Draco,Malfoy Manor

import csv
students = []
with open("students.csv") as file:
    reader = csv.reader(file)
    for row in reader:
        students.append({"name": row[0], "home": row[1]})

A reader works in a for loop, where each iteration the reader gives us another row from our CSV file. This row itself is a list, where each value in the list corresponds to an element in that row

因为 row 是一个 list，所以 line 5~6 可写为：

for name, home in reader:
    students.append({"name": name, "home": home})

csv.DictReader

通常csv文件头部是描述 (header information)，比如：

name,home

with open("students.csv") as file:
    reader = csv.DictReader(file)
    for row in reader:	#row是字典
        students.append({"name": row["name"], "home": row["home"]})

DictReader 从头到尾，把每一行看作一个 dict 并返回

the compiler will directly access the row dictionary, getting the name and home of each student

row["name"] 需要 .csv 文件中有 name 一栏，并且不要求 name 和 home 有序

csv.writer

import csv
name = input("What's your name? ")
home = input("Where's your home? ")
with open("students.csv", "a") as file:
    writer = csv.writer(file)
    writer.writerow([name, home])

csv.DictWriter

with open("students.csv", "a") as file:
    writer = csv.DictWriter(file, fieldnames=["name", "home"])
    writer.writerow({"home": home, "name": name})

fieldnames 参数决定了 csv 文件里 name 在前，home 在后，即 line 3 的输入同样会得到先名字再房子

PIL

a python library

Pillow’s documentation of: PIL

import sys
from PIL import Image

images = []

for arg in sys.argv[1:]:
    image = Image.open(arg)
    images.append(image)

images[0].save(
    "costumes.gif", save_all=True, append_images=[images[1]], duration=200, loop=0
)

The last lines of code saves the first image and also appends a second image to it as well, creating an animated gif

loop=0 循环无限次