Question

parameters confuse me a lot when i using defer function with twisted.

i consider if the function’s parameter is a INT-VALUE, or other basic types besides of list and dict, it should pass-by value not reference.

from twisted.internet import defer, reactor

def deferFunc(x):
    print "11111 %d" % x
    d = defer.Deferred()
    reactor.callLater(1, d.callback, x)
    return d

@defer.inlineCallbacks
def inlineDeferFunc(x):
    print "11111 %d" % x
    d = defer.Deferred()
    reactor.callLater(1, d.callback, x)
    result = yield d
    defer.returnValue(result)

def loop():
    x = [1,2,3,4]
    d = defer.succeed(0)
    for i in x:
        d.addCallback(lambda _ : inlineDeferFunc(i))

if __name__ == '__main__':
    loop()

    try:
        reactor.run()
    except:
        reactor.stop()

the result is :

11111 1
11111 4
11111 4
11111 4

and the result confused me.

how can i get the right output like 1,2,3,4

Answer 1

how can i get the right output like 1,2,3,4

like this:

def loop():
    x = [1,2,3,4]
    d = defer.succeed(0)
    for i in x:
        d.addCallback(lambda _, i = i : inlineDeferFunc(i))



11111 1
11111 2
11111 3
11111 4

note that we are copying the values in the function definition i = i.
the problem was that the lambda grabs the parent environment to access the variable i after all how it would know what value it is, for performance reasons I don’t think it copies it during definition, but the parent environment keeps being updated by the for loop.

here is a simple test.

>>> def test_closures():
...     i = 2
...     def test():
...         return i
...     i = 3
...     return test
... 
>>> test_closures()()
3