Bugs item #1339045, was opened at 2005-10-26 17:21
Message generated for change (Comment added) made by josiahcarlson
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Category: Threads
Group: Python 2.4
Status: Deleted
Resolution: None
Priority: 5
Submitted By: Maciek Fijalkowski (fijal)
Assigned to: Nobody/Anonymous (nobody)
Summary: Threading misbehavior with lambdas
Initial Comment:
suppose i write:
def f(x):
print x()
for i in range(3):
f ( lambda : i )
I got 0,1,2
But when I write
for i in range(3):
thread . start_new_thread ( f , ( lambda : i ) )
I got 2,2,2
Probably I don't get well design principles, but isn't
it against thread consistency? (as long as threads does
not interact with each other, interlace doesn't matter).
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Comment By: Josiah Carlson (josiahcarlson)
Date: 2005-10-27 12:32
Message:
Logged In: YES
user_id=341410
This is a bug in your understanding of lambdas, not in how
threads work. More specifically, lambdas do late binding.
By the time the threads have actually started executing and
call the lambda, the name 'i' is bound to the value 2.
If you need early binding, then you should bind early:
for i in xrange(3):
thread.start_new_thread(f, (lambda i=i:i))
----------------------------------------------------------------------
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