Catalog
List generator
List generated case 1
List generated case 2
List generated case 3
List generated case 4
List generated case 5
iterator （Iterator）
Iteratable object ：Iterable
iterator ：Iterator
iter() function
Summary
List generation is List Comprehensions：
yes Python Built in very simple but powerful can be used to create list Generative form of .
To generate list [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]：
It can be used list(range(1, 11))：
list(range(1, 11))The result is ：[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
But if you want to generate [1x1, 2x2, 3x3, ..., 10x10] How do you do it? ？
The first method is circulation ：
L = []
for x in range(1, 11):
L.append(x * x)The result is ：
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
But the cycle is too complicated ,
List generation can use a single line of statements instead of loop generation list：
[x * x for x in range(1, 11)]The result is ：[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
When writing list generation , Put the elements to be generated x * x Put it in front , Followed by for loop , You can put the list created , Simple 、 convenient .
for You can also add if Judge ：
So we can filter out only even squares ：
[x * x for x in range(1, 11) if x % 2 == 0]The result is ：[4, 16, 36, 64, 100]
Can generate a full permutation ：
[m + n for m in 'ABC' for n in 'XYZ']The result is ：['AX', 'AY', 'AZ', 'BX', 'BY', 'BZ', 'CX', 'CY', 'CZ']
Use list generation , Can write very simple code .
for example , List all files and directory names in the current directory , It can be implemented in one line of code ：
Be careful ：os.listdir You can list files and directories
import os # Import os modular , The concept of modules is discussed later
[d for d in os.listdir('.')]for Loops can actually use two or more variables at the same time , such as dict Of items() You can iterate at the same time key and value：
d = {'x': 'A', 'y': 'B', 'z': 'C' }
for k, v in d.items():
print(k, '=', v)
The result is ：y = B x = A z = C
therefore , List generation can also use two variables to generate list：
d = {'x': 'A', 'y': 'B', 'z': 'C' }
[k + '=' + v for k, v in d.items()]The result is ：['y=B', 'x=A', 'z=C']
Put one list All strings in are lowercase ：
L = ['Hello', 'World', 'IBM', 'Apple']
[s.lower() for s in L]The result is ：['hello', 'world', 'ibm', 'apple']
Can act directly on for The data types of the loop are as follows ：
One is set data type , Such as list、tuple、dict、set、str etc. ; One is generator, Including generator and belt yield Of generator function.
These can act directly on for Circular objects are collectively called iteratable objects ：Iterable.
have access to isinstance() Determine whether an object is Iterable object ：
And generators can not only for loop , Can also be next() Function calls continuously and returns the next value , Until the last throw StopIteration An error indicates that the next value cannot be returned .
Can be next() The object that function calls and returns the next value continuously is called an iterator ：Iterator.
have access to isinstance() Determine whether an object is Iterator object ,list、dict、str Although it is Iterable, But not Iterator.
hold list、dict、str etc. Iterable become Iterator have access to iter() function ：
isinstance(iter('abc'), Iterator)The result is ：True
Iterator It can even represent an infinite data flow , For example, all natural numbers . While using list It's never possible to store all natural numbers .
What works for The objects of the loop are all Iterable type ; What works next() Function objects are Iterator type , They represent a sequence of lazy calculations ; The collection data type is as follows list、dict、str Is such as Iterable, But it's not Iterator, But it can go through iter() Function to get a Iterator object .
