Most can use for Keyword traversal container All are iterator
for element in [1, 2, 3]:
print(element)
for element in (1, 2, 3):
print(element)
for key in {'one':1, 'two':2}:
print(key)
for char in "123":
print(char)
for line in open("myfile.txt"):
print(line, end='')This access is clear 、 concise 、 convenient .
The principle behind it is ,for Statement on a container object iter(). This function returns an iterator object , This object defines __next__() Method , This method accesses the elements in one container at a time . When there are no more elements ,__next__() Will trigger a StopIteration abnormal , It tells for Cycle termination .
You can use built-in functions next() call __next__() Method
>>> s = 'abc' >>> it = iter(s) >>> it <iterator object at 0x00A1DB50> >>> next(it) 'a' >>> next(it) 'b' >>> next(it) 'c' >>> next(it) Traceback (most recent call last): File "<stdin>", line 1, in <module> next(it) StopIteration
Add to class iterator act , You can do this by defining __iter__() Method realization , This method returns a containing __next__() The object of ; If the class is already defined __next__(), __iter__() Can return self
class Reverse: """Iterator for looping over a sequence backwards.""" def __init__(self, data): self.data = data self.index = len(data) def __iter__(self): return self def __next__(self): if self.index == 0: raise StopIteration self.index = self.index - 1 return self.data[self.index]
>>> rev = Reverse('spam')
>>> iter(rev)
<__main__.Reverse object at 0x00A1DB50>
>>> for char in rev:
... print(char)
...
m
a
p
sPython Of Iterator Object represents a Data flow ,Iterator Objects can be next() Function call and continue to return the next data , Thrown until no data StopIteration error . You can think of this data flow as an ordered sequence , But we can't know the length of the sequence in advance , It can only be passed continuously next() Function to calculate the next data on demand , therefore Iterator The calculation of is inert , It only calculates if it needs to return the next data .
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 .
According to this feature , We know iterator It can represent some infinite sequences , for example Fibonacci（ Fibonacci ） The sequence , All prime numbers .
Be careful ：list、dict、str Equal data type is not Iterator, But it can go through iter(obj) Convert to an object
>>> from collections.abc import Iterable >>> isinstance([], Iterable) True
>>> from collections.abc import Iterator >>> isinstance((x for x in range(10)), Iterator) True
You build a custom container object , It contains a list 、 Tuples or other iteratable objects . You want to iterate directly on your new container object .
class Node:
def __init__(self, value):
self._value = value
self._children = []
def __repr__(self):
return 'Node({!r})'.format(self._value)
def add_child(self, node):
self._children.append(node)
def __iter__(self):
return iter(self._children)
# Example
if __name__ == '__main__':
root = Node(0)
child1 = Node(1)
child2 = Node(2)
root.add_child(child1)
root.add_child(child2)
# Outputs Node(1), Node(2)
for ch in root:
print(ch)The depth first traversal is realized through iteration
class Node:
def __init__(self, value):
self._value = value
self._children = []
def __repr__(self):
return 'Node({!r})'.format(self._value)
def add_child(self, node):
self._children.append(node)
def __iter__(self):
return iter(self._children)
def depth_first(self):
yield self
for c in self:
yield from c.depth_first()
# Example
if __name__ == '__main__':
root = Node(0)
child1 = Node(1)
child2 = Node(2)
root.add_child(child1)
root.add_child(child2)
child1.add_child(Node(3))
child1.add_child(Node(4))
child2.add_child(Node(5))
for ch in root.depth_first():
print(ch)
# Outputs Node(0), Node(1), Node(3), Node(4), Node(2), Node(5) In this code ,depth_first() The method is simple and intuitive . It first returns itself and iterates over each child node By calling the... Of the child node depth_first() Method ( Use yield from sentence ) Return the corresponding element .
