Iterators
循环的iterator，结构化输出
Iterators vs. iterables
Iterable
Examples: lists, strings, dictionaries, file connections
An object with an associated
iter()methodApplying
iter()to an iterable creates an iterator
Iterator
Produces next value with
next()
An iterable is an object that can return an iterator, while an iterator is an object that keeps state and produces the next value when you call next() on it.
Iterating over iterables (list)
# Create a list of strings: flash
flash = ['jay garrick', 'barry allen', 'wally west', 'bart allen']
# Print each list item in flash using a for loop
for person in flash:
print (person)
# Create an iterator for flash: superhero
superhero=iter(flash)
# Print each item from the iterator
print(next(superhero))
print(next(superhero))
print(next(superhero))
print(next(superhero))Iterating over dictionaries
Iterating over file connections
Iterating over iterables (range)
Not all iterables are actual lists. You can use range() in a for loop as if it's a list to be iterated over:
Recall that range() doesn't actually create the list; instead, it creates a range object with an iterator that produces the values until it reaches the limit (in the example, until the value 4).
Iterators as function arguments
There are also functions that take iterators and iterables as arguments. For example, the list() and sum() functions return a list and the sum of elements, respectively.
enumerate()
enumerate() returns an enumerate object that produces a sequence of tuples, and each of the tuples is an index-value pair.
Then, we can use list to turn this enumerate object into a list of tuples and print to see what it contains
enumerate() and unpack
zip()
zip() takes any number of iterables and returns a zip object that is an iterator of tuples.
zip() and unpack
zip(*object)
use * in a call to zip() to unpack the tuples produced by zip().
Print zip with *
* operator (splat operator)
Use iterator to load data in chunks
也可以用%d的占位符，就有点C的写法...
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