"for item in list" - where does the None item come from?

Question

i got this list of objects with one item in it.

    print self.parameters
    print len(self.parameters)
    for p in self.parameters:
        print p

when i print out the list and len of the lsit i see the expected: one item. But when looping over the lsit i also get a None item...!?

[<__main__.Parameter object at 0x00000000022D4828>]
1
<__main__.Parameter object at 0x00000000022D4828>
None

what is going on here? (yes im sure, that the "None" output is from this print statement)

EDIT: i was manipulating the list i was looping over:

print self.parameters
print len(self.parameters)
for p in self.parameters:
    print p
    (...)
    self.parameters.append(<something that returned None>)

is this inside a function ... ? are you saying `print self.doSomething()` somewhere else? — Joran Beasley, Jul 27 '12 at 22:55
There's no other prints in the program after this loop that could be doing this? — Doug T., Jul 27 '12 at 22:58
Also is it possible to create a full, minimal program that demonstrates the issue? — Doug T., Jul 27 '12 at 22:58

score 1 · Accepted Answer · edited May 23 '17 at 12:13

1

Don't do this.

for p in self.parameters:
    print p
    ...
    self.parameters.append(...) # No.

See Modifying list while iterating -- basically, you shouldn't modify something while iterating over it. You can make a copy if you want:

for p in list(self.parameters):
    print p
    ...
    self.parameters.append(...) # Okay

edited May 23 '17 at 12:13

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answered Jul 27 '12 at 23:29

Dietrich Epp

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"for item in list" - where does the None item come from?

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