Python Programming

Lecture 9 Closures, Decorators, File

9.1 Nested Functions

Nested Functions (嵌套函数)


def outer():
    def inner():
        num = 100
        print(num)
    inner()
outer()




num = 3           # Global（全局）
def outer():
    num = 10      # Enclosing（外层）
    def inner():
        num = 100 # Local（局部）
        print(num)
    inner()
    print(num)
outer()

  • （引用顺序）Local$\rightarrow$Enclosing$\rightarrow$Global$\rightarrow$ Build-in (LEGB principle)


          
num = 3
def outer():
    num = 10
    def inner():
        print(num)
    inner()
outer()

# output？

nonlocal keyword


def outer():
    num = 10
    def inner():
        nonlocal num   
        num = 100
        print(num)
    inner()
    print(num)
outer()


100
100








def outer():
    num = 10
    def inner():
        global num  
        num = 100
        print(num)
    inner()
    print(num)
outer()


100 
10








def outer():
    global num
    num = 10
    def inner():
        global num   
        num = 100
        print(num)
    inner()
    print(num)
outer()


100
100









def outer():
    global num
    num = 10
    def inner():  
        num = 100
        print(num)
    inner()
    print(num)
outer()


100 
10








num = 3                            
def outer():
    global num
    num = 10
    def inner():  
        num = 100
        print(num)
    inner()
    print(num)
outer()
print(num)


100
10
10









num = 3                            
def outer():
    num = 10
    def inner(): 
        global num 
        num = 100
        print(num)
    inner()
    print(num)
outer()
print(num)


100
10
100









num = 3                            
def outer():
    num = 10
    def inner(): 
        nonlocal num 
        num = 100
        print(num)
    inner()
    print(num)
outer()
print(num)


100
100
3









num = 3                            
def outer():
    global num
    num = 10
    def inner(): 
        global num 
        num = 100
        print(num)
    inner()
    print(num)
outer()
print(num)


100
100
100

9.2 Closures & Decorators

Closure Function (闭包函数)


def print_msg(msg):
    def printer():
        print(msg)
    return printer

another = print_msg("zen of python")
print(another())

  • We must have a nested function (function inside a function).

  • The nested function must refer to a value defined in the enclosing function.

  • The enclosing function must return the nested function.

  • When to use closures? Closures can avoid the use of global values and provides some form of data hiding. It can also provide an object oriented solution to the problem.


def make_multiplier_of(n):
    def multiplier(x):
        return x * n
    return multiplier

times3 = make_multiplier_of(3)
times5 = make_multiplier_of(5)

print(times3(9))
print(times5(3))
print(times5(times3(2)))

Example: Delivery Platform Order System 1.0 (外卖平台订单系统)

Each city should keep its own order data. Orders in one city should not affect another city.


def create_city_order_system(city):
    user_data = {}

    def process_order(user, price):
        nonlocal user_data

        if user not in user_data:
            user_data[user] = {"count": 0, "total": 0}

        user_data[user]["count"] += 1
        user_data[user]["total"] += price

        print(f"[{city}站] {user} 本次消费 {price} 元")
        print(f"[{city}站] 累计订单：{user_data[user]['count']}")
        print(f"[{city}站] 累计消费：{user_data[user]['total']} 元\n")

    return process_order
  • Why should the variable user_data be in the enclosing?
  • If it is inside the inner function, it will reset every time. (每次运行内层函数，数据清零)
  • If it is global, all systems will share the same data. (上海站，北京站等等，数据相同)
  • A closure works because the data lives in the enclosing function, not inside the function that runs every time.

Decorator (装饰器)

  • Decorators are "wrappers", which means that they let you execute code before and after the function they decorate without modifying the function itself.


def new_decorator(a_function_to_decorate):
    def the_wrapper():
        print("Before the function runs") 
        a_function_to_decorate()
        print("After the function runs")
    return the_wrapper

def a_function():
    print("I am a stand alone function.")


a_function_decorated = new_decorator(a_function)
a_function_decorated()


Before the function runs
I am a stand alone function.
After the function runs


@new_decorator
def another_stand_alone_function():
    print("Leave me alone")

another_stand_alone_function()  


Before the function runs
Leave me alone
After the function runs


def bread(func):
    def wrapper():
        print("")
        func()
        print("<\______/>")
    return wrapper

def ingredients(func):
    def wrapper():
        print("#tomatoes#")
        func()
        print("~salad~")
    return wrapper

def sandwich(food="--ham--"):
    print(food)


sandwich()



--ham--


@bread
@ingredients # The order matters here.
def sandwich(food="--ham--"):
    print(food)

sandwich()



 #tomatoes#
 --ham--
 ~salad~
<\______/>

  • Taking decorators to the next level

  • Passing arguments to the decorated function


def passing_arguments(function_to_decorate):
    def wrapper(arg1, arg2):
        print(f"I got args! Look: {arg1}, {arg2}")
        function_to_decorate(arg1, arg2)
    return wrapper

@passing_arguments
def print_name(first_name, last_name):
    print(f"My name is {first_name}, {last_name}")

print_name("Peter", "Venkman")


I got args! Look: Peter, Venkman
My name is Peter, Venkman

  • If you're making general-purpose decorator--one you'll apply to any function or method, no matter its arguments--then just use *args, **kwargs:


def passing_arbitrary_arguments(function_to_decorate):
    def wrapper(*args, **kwargs):
        print("Do I have args?:")
        print(args)
        print(kwargs)
        function_to_decorate(*args, **kwargs)
    return wrapper

@passing_arbitrary_arguments
def function_with_no_argument():
    print("Python is cool, no argument here.")

function_with_no_argument()


Do I have args?:
()
{}
Python is cool, no argument here.


@passing_arbitrary_arguments
def function_with_arguments(a, b, c, d):
    print(a, b, c, d)

function_with_arguments(1,2,3,4)


Do I have args?:
(1, 2, 3, 4)
{}
1 2 3 4



function_with_arguments(1,2,3,d = "banana")


Do I have args?:
(1, 2, 3)
{'d': 'banana'}
1 2 3 banana

Delivery Platform Order System 2.0

Every time an order is processed, the system should automatically print a log message.


def log_order(func):
    def wrapper(user, price):
        print(f"[日志] 收到订单：用户={user}, 金额={price}")
        return func(user, price)
    return wrapper


def create_city_order_system(city):
    user_data = {}

    @log_order
    def process_order(user, price):
        nonlocal user_data

        if user not in user_data:
            user_data[user] = {"count": 0, "total": 0}

        user_data[user]["count"] += 1
        user_data[user]["total"] += price

        print(f"[{city}站] {user} 本次消费 {price} 元")
        print(f"[{city}站] 累计订单：{user_data[user]['count']}")
        print(f"[{city}站] 累计消费：{user_data[user]['total']} 元\n")

    return process_order

Exercise: 修改下面的代码，创建健身打卡系统

    
def exercise(minutes):
    print(f"运动了 {minutes} 分钟")
  1. 用闭包写 create_tracker(name)，记录某人累计运动时间。提示：上面的代码可用于内层函数，create_tracker(name) 是外层函数。
  2. 加装饰器 @warm_up，每次运动前自动输出：
    3. 
先热身 3 分钟！

9.3 File

Reading from a File

  • pi_digits.txt


3.1415926535
8979323846
2643383279


from pathlib import Path

path = Path('pi_digits.txt')
contents = path.read_text()
contents = contents.rstrip()
print(contents)


3.1415926535
8979323846
2643383279

File Path

  • relative path


path = Path('text_files/filename.txt')

  • absolute path


path = Path('/home/eric/data_files/text_files/filename.txt') 

path = Path('c:/text_files/filename.txt') 
path = Path(r'c:\text_files\filename.txt')
path = Path('c:\\text_files\\filename.txt') 

path = Path('c:\text_files\filename.txt')  #error

  • Making a List of Lines from a File


from pathlib import Path

path = Path('pi_digits.txt')
contents = path.read_text()

lines = contents.splitlines()
print(lines)

  • Working with a File's Contents


pi_string = ''
for line in lines:
    pi_string = pi_string + line
print(pi_string)
print(len(pi_string))


['3.1415926535', '8979323846', '2643383279']
3.141592653589793238462643383279 # string
32

Writing to a File


from pathlib import Path

path = Path('programming.txt')

path.write_text("I love programming.")

  • Python can only write strings to a text file. If you want to store numerical data in a text file, you'll have to convert the data to string format first using the str() function.


from pathlib import Path


contents = "I love programming.\n"
contents += "I love creating new games.\n"
contents += "I also love working with data.\n"

path = Path('programming.txt')
path.write_text(contents)


I love programming.
I love creating new games.
I also love working with data.

  • Handling the FileNotFoundError Exception


from pathlib import Path

path = Path('alice.txt')
contents = path.read_text(encoding='utf-8') #Error


from pathlib import Path

path = Path('alice.txt')
try:
    contents = path.read_text(encoding='utf-8')
except FileNotFoundError:
    print(f"Sorry, the file {path} does not exist.")


Sorry, the file alice.txt does not exist.

  • ZeroDivisionError

  • try-except-else 

from pathlib import Path

path = Path('alice.txt')
try:
    contents = path.read_text(encoding='utf-8')
except FileNotFoundError:
    print(f"Sorry, the file {path} does not exist.")
else:
    # Count the approximate number of words in the file:
    words = contents.split()
    num_words = len(words)
    print(f"The file {path} has about {num_words} words.")

JSON

  • JSON (JavaScript Object Notation, pronounced /ˈdʒeɪsən/; also /ˈdʒeɪˌsɒn/) is an open standard file format and data interchange format that uses human-readable text to store and transmit data objects consisting of attribute–value pairs and arrays (or other serializable values). It is a common data format with diverse uses in electronic data interchange, including that of web applications with servers.
  • JSON is a language-independent data format. It was derived from JavaScript, but many modern programming languages include code to generate and parse JSON-format data. JSON filenames use the extension .json.
  • json.dumps() and json.loads()

from pathlib import Path
import json


numbers = [2, 3, 5, 7, 11, 13]

path = Path('numbers.json')
contents = json.dumps(numbers)
path.write_text(contents)


from pathlib import Path
import json

path = Path('numbers.json')
contents = path.read_text()
numbers = json.loads(contents)

print(numbers)


[2, 3, 5, 7, 11, 13]

  • Saving and Reading User-Generated Data


from pathlib import Path
import json

username = input("What is your name? ")

path = Path('username.json')
contents = json.dumps(username)
path.write_text(contents)

print(f"We'll remember you when you come back, {username}!")


from pathlib import Path
import json

path = Path('username.json')
contents = path.read_text()
username = json.loads(contents)

print(f"Welcome back, {username}!")


from pathlib import Path
import json


def get_stored_username(path):
    """Get stored username if available."""
    if path.exists():
        contents = path.read_text()
        username = json.loads(contents)
        return username
    else:
        return None


def get_new_username(path):
    """Prompt for a new username."""
    username = input("What is your name? ")
    contents = json.dumps(username)
    path.write_text(contents)
    return username


def greet_user():
    """Greet the user by name."""
    path = Path('username.json')
    username = get_stored_username(path)
    if username:
        print(f"Welcome back, {username}!")
    else:
        username = get_new_username(path)
        print(f"We'll remember you when you come back, {username}!")

greet_user()


What is your name? Eric #First time
We'll remember you when you come back, Eric!


Welcome back, Eric!

9.4 Data Reading Examples

Reading Multiple Lines in JSON file


from pathlib import Path
import json

path = Path('yelp_sample.json')
contents = path.read_text()
lines = contents.splitlines()
yelp = []
for line in lines:
    yelp.append(json.loads(line))
    
print(len(yelp))
print(yelp[100])

Reading Multiple text file


import os
from pathlib import Path
from textblob import TextBlob #pip install

folder_path = "marvel/"
summary = []


for filename in os.listdir(folder_path):  #列出所有文件 
    # 组成相对路径
    path = Path(os.path.join(folder_path, filename))
    contents = path.read_text(errors="ignore")
    lines = contents.splitlines()
    scores = []

    for line in lines:
        line = line.strip()
        if line:
            # 看看这一行话的情绪是正面、负面，还是中性（+1到-1）
            polarity = TextBlob(line).sentiment.polarity
            scores.append(polarity) 
    if scores:
        average = sum(scores) / len(scores)
        summary.append((average, filename))


summary.sort()
for score, movie in summary:
    print(movie, score)

Summary

  • File
    • Reading: Python Crash Course, Chapter 10