习近平两会新语之“干”字篇
iter() is used to obtain the iterator object, and next() is used to obtain the next element; 1. Use iterator() to convert iterable objects such as lists into iterators; 2. Call next() to obtain elements one by one, and trigger StopIteration exception when the elements are exhausted; 3. Use next(iterator, default) to avoid exceptions; 4. Custom iterators need to implement the __iter__() and __next__() methods to control iterative logic; using default values is a common way to safe traversal, and the entire mechanism is concise and practical.
In Python, iter() and next() are the core functions of operating iterators. The following is a simple and easy-to-understand example to illustrate their usage.
What is an iterator?
An iterator is an object that can remember the traversal location, which implements the iterator protocol (i.e. __iter__() and __next__() methods). iter() is used to obtain an iterator object, and next() is used to obtain elements one by one.
Basic usage examples
# Create a list my_list = [10, 20, 30, 40] # Use iter() to get the iterator my_iter = iter(my_list) # Use next() to get the element print(next(my_iter)) one by one # Output: 10 print(next(my_iter)) # Output: 20 print(next(my_iter)) # Output: 30 print(next(my_iter)) # Output: 40 # Call next again will throw StopIteration print(next(my_iter)) # Error: StopIteration
When all elements are taken, continuing to call
next()will triggerStopIterationexception, indicating that the iteration is over.
How to use next() safely
To avoid StopIteration errors, you can provide the default value to next() :
my_list = [1, 2, 3] my_iter = iter(my_list) print(next(my_iter, 'nothing')) # 1 print(next(my_iter, 'nothing')) # 2 print(next(my_iter, 'nothing')) # 3 print(next(my_iter, 'nothing')) # Nothing (nothing will be reported)
This is very useful when dealing with iterations of uncertain lengths.
Custom iterator class
You can also implement an iterator class yourself:
class CountUpTo:
def __init__(self, max_val):
self.max_val = max_val
self.current = 1
def __iter__(self):
Return self
def __next__(self):
if self.current > self.max_val:
raise StopIteration
else:
value = self.current
self.current = 1
Return value
# Use custom iterator counter = CountUpTo(3)
it = iter(counter)
print(next(it)) # 1
print(next(it)) # 2
print(next(it)) # 3
print(next(it, "end")) # End (avoid exceptions)summary
-
iter()turns iterable objects (such as lists, strings, generators) into iterators. -
next()takes a value from the iterator. - After taking it, continuing to call
next()will throwStopIteration. -
next(iterator, default)can be used to avoid exceptions.
Basically all this is not complicated but very practical.
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