An iterator is a data stream object or container. When using the data in it, it takes one data from a data stream at a time until the data is fetched, and the data will not be reused;
1. Custom iterator
Implementing a custom iterator requires implementing the following methods:
- __iter__() : The method returns the object itself, which is a requirement for the for statement to use the iterator;
- __next()__ : The method returns the next element or data for the container. Raise StopIteration exception when data in container is exhausted
Custom exception example:
#依次遍历X的n次方，知道大于100时结束迭代
class MyIterator:
def __init__(self, x=2, max=100):
self.__x = x
self.__mul = x
self.__max = max
# 定义iter函数，返回自身
def __iter__(self):
return self
# 定义next函数，制定迭代器的返回值
def __next__(self):
if self.__x != 0 and self.__x != 1:
self.__mul *= self.__x
if self.__mul <= self.__max:
return self.__mul
else:
raise StopIteration
else:
raise StopIteration
if __name__ == '__main__':
myItor = MyIterator()
for i in myItor:
print(i)Running result:
4
8
16
32
64
The iterator class must raise a StopIteration error under certain conditions to end the traversal loop, otherwise an infinite loop will occur.
2. Built-in iteration tools
(1), the built-in iteration function iter()
- iter(iterable) , the parameter is an iterable type, such as various sequences;
- iter(callable, sentinel) , the first parameter is a callable type, usually a function; the second parameter is called "sentinel", that is, when the return value of the first parameter call is equal to the value of the second parameter, iterative or traversal stop;
Example of built-in function iter():
def function():
global x
x += 2
return x
if __name__ == '__main__':
lst = [1, 2, 3, 4]
for i in iter(lst): #iter()函数一个参数
print(i, end=' ')
print()
x = 0
for i in iter(function, 12): #iter()函数两个参数
print(i, end=' ')Running result:
1 2 3 4
2 4 6 8 10
(2) Common iterative tools in the itertools module
Common iteration tools in the itertools module are shown in the following table:
| Classification | function name | describe |
|---|---|---|
| infinite iteration | count(start[, step]) | Start from start, iterate with step as the step count |
| infinite iteration | cycle(seq) | infinite loop iteration seq |
| infinite iteration | repeat(elem[, n]) | loop iteration elem |
| Iterate over short sequences | chain(p, q, …) | chain iterations, linking p,q... to iterate as if iterating from a sequence |
| Iterate over short sequences | compress(data, selectors) | Select the values in the iterative data sequence based on the values in selectors |
| Iterate over short sequences | dropwhile(pred, seq) | When the result of pred processing the sequence element is false, the value after iterating seq starts |
| Iterate over short sequences | filterfalse (pred, seq) | Elements when pred handles false |
| Iterate over short sequences | takewhile(pred, seq) | Opposite to dropwhile |
| Iterate over short sequences | tee(it, n) | Repeat it n times to iterate |
| Combining iterative sequences | product(p, q, …[, n]) | Iterate through all sequences |
| Combining iterative sequences | permutations(p, r) | permutation of r elements in iterative sequence |
| Combining iterative sequences | combinations(p, r) | iterate over combinations of r elements in a sequence |