Stop generator from inside a block in Python

Question

I have a generator that gives nodes from a Directed Acyclic Graph (DAG), first depth:

def depth_first_search(self):
    yield self, 0 # root
    for child in self.get_child_nodes():
        for node, depth in child.depth_first_search():
            yield node, depth+1

I can iterate over nodes like this

for node, depth in graph.depth_first_search():
    # do something

I would like to be able to tell the generator from the for loop to stop going deeper in the graph if any condition is met.

I came up with the following solution using an external function.

def depth_first_search(self, stop_crit=lambda n,d: False):
    yield self, 0 # root
    for child in self.get_child_nodes():
        for node, depth in child.depth_first_search():
            yield node, depth+1
            if stop_crit(node, depth): break

This solution forces me to declare the variables that I need before stop_crit is defined so that I can access them.

In Ruby, income returns the last expression from a block, so it’s convenient to use it to make the generator continue or stop.

What is the best way to implement this function in Python?

Answer 1

:

def depth_first_search(self):
    yield self, 0 # root
    for child in self.get_child_nodes():
        for node, depth in child.depth_first_search():
            if(yield node, depth+1):
                yield None # for .send
                return

, , :

it = graph.depth_first_search()
for node, depth in it: #this is why there should be pronouns for loop iterables
    stuff(node,depth)
    if quit: it.send(1) 
    # it.next() should raise StopIteration on the next for iteration

, .

Answer 2

Python . . ( , )

, generator.close(), , .

:

>>> def gen():
...     try: 
...         for i in range(10):
...             yield i
...     finally:
...         print "gen cleanup"
...         
>>> g = gen()
>>> next(g)
0
>>> for x in g:
...     print x
...     if x > 3:
...         g.close()
...         break
...        
1
2
3
4
gen cleanup
>>> g = gen()
>>> h = g
>>> next(g)
0
>>> del g
>>> del h   # last reference to generator code frame gets lost
gen cleanup

Answer 3

( ) , . , , .

class Node(object):
    def __init__(self, val, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

    # the producing coroutine, it sends data to the consumer
    def depth_first_search(self, consumer, depth=0):
        """ `consumer` is a started coroutine that yields True to continue a branch """
        if consumer.send((self, depth)): # continue this branch?
            for child in self.get_child_nodes():
                child.depth_first_search(consumer, depth+1)

    def get_child_nodes(self):
        for node in (self.left, self.right):
            if node is not None:
                yield node

    def __repr__(self):
        return "Node(val=%d)" % self.val

def coroutine(func):
    """ decorator that declares `func` as a coroutine and starts it """
    def starter(*args, **kwargs):
        co = func(*args, **kwargs)
        next(co) # corotines need to be started/advanced to the first yield
        return co
    return starter

# the consumer takes data and yields if it wants to continue
@coroutine
def consumer( continue_branch=lambda n,d:True ):
    node, depth = (yield True) # first node
    while True:
        print node, depth # do stuff
        node, depth = (yield continue_branch(node, depth))


# testing
tree = Node(5, Node(2, Node(3), Node(4)), Node(6, Node(7), Node(8))) # 
cons = consumer()
tree.depth_first_search(cons)# yields all

print
stopper = consumer(lambda n,d: n.val > 2) # skips the children of Node 2
tree.depth_first_search(stopper)

, , depth_first_search , ... .

Python support for coroutines is a bit inconvenient ( @coroutinefor rescue). There is a beautiful, good tutorial for Python and many resources for languages ​​that depend on coroutines, such as Lua. In any case, this is a very cool concept, which is worth exploring :-)

Answer 4

As a rule, you do not specify your iterative for checking conditions, you do this in the body of the loop:

for node, depth in graph.depth_first_search():
    if node meets condition:
        # do something with node 
        break
# do something with node, its still referencing what you breaked on

This code has the advantage that it is not surprising and does not bother anyone.