Summary of techniques for object iteration and anti-iteration in Python

1. How to implement iterable objects and iterator objects?

Actual case

A certain software requires to capture the smell information of each city from the network and display it next:

北京： 15 ~ 20 天津： 17 ~ 22 长春： 12 ~ 18 ......

If we capture the weather of all cities at once and then display it, there will be a high delay and a waste of storage space when displaying the temperature of the first city. We expect to access it in a time-consuming manner. Strategy, and encapsulates all city temperatures into an object. You can use the for statement to iterate. How to solve it?

Solution

Implement an iterator objectWeatherlterator,nextThe method returns a city temperature each time, implement an Iterable objectWeatherlterable,————iter__ method returns an iterator object

import requests from collections import Iterable, Iterator # 气温迭代器 class WeatherIterator(Iterator): def __init__(self, cities): self.cities = cities self.index = 0 def getWeather(self, city): r = requests.get('http://wthrcdn.etouch.cn/weather_mini?city=' + city) data = r.json()['data']['forecast'][0] return '%s：%s , %s' % (city, data['low'], data['high']) def __next__(self): if self.index == len(self.cities): raise StopIteration city = self.cities[self.index] self.index += 1 return self.getWeather(city) # 可迭代对象 class WeatherIterable(Iterable): def __init__(self, cities): self.cities = cities def __iter__(self): return WeatherIterator(self.cities) for x in WeatherIterable(['北京', '上海', '广州', '深圳']): print(x)

The execution result is as follows:

C:\Python\Python35\python.exe E:/python-intensive-training/s2.py 北京：低温 21℃ , 高温 30℃ 上海：低温 23℃ , 高温 26℃ 广州：低温 26℃ , 高温 34℃ 深圳：低温 27℃ , 高温 33℃ Process finished with exit code 0

2. How to use generator functions to implement iterable objects?

Actual case

Implement a class of iterable objects that can iterate out all prime numbers in a given range:

python pn = PrimeNumbers(1, 30) for k in pn: print(k) `` 输出结果text
2 3 5 7 11 13 17 19 23 29
“`

Solution

- Implement the generator function in the __iter__ method of this class, each TimesyieldReturns a prime number

class PrimeNumbers: def __init__(self, start, stop): self.start = start self.stop = stop def isPrimeNum(self, k): if k < 2: return False for i in range(2, k): if k % i == 0: return False return True def __iter__(self): for k in range(self.start, self.stop + 1): if self.isPrimeNum(k): yield k for x in PrimeNumbers(1, 20): print(x)

Run result

C:\Python\Python35\python.exe E:/python-intensive-training/s3.py 2 3 5 7 11 13 17 19 Process finished with exit code 0

3. How to perform reverse iteration and how to implement reverse iteration?

Actual case

Implementing a continuous floating point number generatorFloatRange(and rrangeSimilar), generate a series of continuous floating point numbers based on the given range (start, stop) and step value (step), such as iterationFloatRange(3.0,4.0,0.2)Can generate sequence:

正向：3.0 > 3.2 > 3.4 > 3.6 > 3.8 > 4.0 反向：4.0 > 3.8 > 3.6 > 3.4 > 3.2 > 3.0

Solution

Implementation The __reversed__ method of the reverse iteration protocol, which returns a reverse iterator

class FloatRange: def __init__(self, start, stop, step=0.1): self.start = start self.stop = stop self.step = step def __iter__(self): t = self.start while t <= self.stop: yield t t += self.step def __reversed__(self): t = self.stop while t >= self.start: yield t t -= self.step print("正相迭代-----") for n in FloatRange(1.0, 4.0, 0.5): print(n) print("反迭代-----") for x in reversed(FloatRange(1.0, 4.0, 0.5)): print(x)

output result

C:\Python\Python35\python.exe E:/python-intensive-training/s4.py 正相迭代----- 1.0 1.5 2.0 2.5 3.0 3.5 4.0 反迭代----- 4.0 3.5 3.0 2.5 2.0 1.5 1.0 Process finished with exit code 0

4. How to perform slicing operations on iterators?

Actual case

There is a certain text file, and we want to remove a certain range of content, such as 100~300 lines The content between text files in python is an iterable object. Can we use a method similar to list slicing to get a generator of file content between 100 and 300 lines?

Solution

Use itertools.islice from the standard library, which returns a generator for slices of iterator objects

from itertools import islice f = open('access.log') # # 前500行 # islice(f, 500) # # 100行以后的 # islice(f, 100, None) for line in islice(f,100,300): print(line)

Every training session of islice will consume the previous iteration object

l = range(20) t = iter(l) for x in islice(t, 5, 10): print(x) print('第二次迭代') for x in t: print(x)

Output result

C:\Python\Python35\python.exe E:/python-intensive-training/s5.py 5 6 7 8 9 第二次迭代 10 11 12 13 14 15 16 17 18 19 Process finished with exit code 0

5. How to iterate multiple iterable objects in a for statement?

Actual case

1. The final exam results of a certain class of students, Chinese, mathematics, and English are stored in 3 lists respectively. , iterate three lists at the same time, and calculate the total score of each student (parallel)

2. There are four classes in a certain age, and the English scores of each class in a certain test are stored in four lists respectively. Iterate through each list in turn and count the number of people whose scores are higher than 90 points in the entire school year (serial)

Solution

Parallel: Use built-in functions zip, it can merge multiple iterable objects, and each iteration returns a tuple

from random import randint # 申城语文成绩，# 40人，分数再60-100之间 chinese = [randint(60, 100) for _ in range(40)] math = [randint(60, 100) for _ in range(40)] # 数学 english = [randint(60, 100) for _ in range(40)] # 英语 total = [] for c, m, e in zip(chinese, math, english): total.append(c + m + e) print(total)

The execution results are as follows :

C:\Python\Python35\python.exe E:/python-intensive-training/s6.py [232, 234, 259, 248, 241, 236, 245, 253, 275, 238, 240, 239, 283, 256, 232, 224, 201, 255, 206, 239, 254, 216, 287, 268, 235, 223, 289, 221, 266, 222, 231, 240, 226, 235, 255, 232, 235, 250, 241, 225] Process finished with exit code 0

Serial: Use itertools.chain from the standard library, which can connect multiple iterable objects

from random import randint from itertools import chain # 生成四个班的随机成绩 e1 = [randint(60, 100) for _ in range(40)] e2 = [randint(60, 100) for _ in range(42)] e3 = [randint(60, 100) for _ in range(45)] e4 = [randint(60, 100) for _ in range(50)] # 默认人数=1 count = 0 for s in chain(e1, e2, e3, e4): # 如果当前分数大于90，就让count+1 if s > 90: count += 1 print(count)

Output result

C:\Python\Python35\python.exe E:/python-intensive-training/s6.py 48 Process finished with exit code 0

Summary

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