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Question about 'print' in a loop

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  Question about 'print' in a loop fl <rxjwg98@gmail.com> - 2015-11-18 18:55 -0800
    Re: Question about 'print' in a loop Chris Angelico <rosuav@gmail.com> - 2015-11-19 14:11 +1100
      Re: Question about 'print' in a loop fl <rxjwg98@gmail.com> - 2015-11-18 19:20 -0800
    Re: Question about 'print' in a loop dieter <dieter@handshake.de> - 2015-11-19 08:31 +0100

#99029 — Question about 'print' in a loop

Hi,

From previous post, I get many helpful replies. Now I have a new question
when I run this example code:


---------
sq=[]
for xx in range(5):
    print 'here'
    sq.append(lambda:xx**2)
    
here
here
here
here
here

xx
Out[150]: 4

sq[2]()
Out[151]: 16

sq[3]()
Out[152]: 16
/////

There are only one time 5 'here' printed out, but there is no 'here' print
out in thereafter call sq[2]() etc. How to understand this phenomenon?

Thanks,

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#99030

On Thu, Nov 19, 2015 at 1:55 PM, fl <rxjwg98@gmail.com> wrote:
> There are only one time 5 'here' printed out, but there is no 'here' print
> out in thereafter call sq[2]() etc. How to understand this phenomenon?

Code does what code should.

Before you ask for comprehension of "this phenomenon", why don't you
tell us what you expect your code to do, and why? I just asked my
non-programmer sister and she was completely unsurprised by what
Python did here.

ChrisA

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#99031

On Wednesday, November 18, 2015 at 10:11:24 PM UTC-5, Chris Angelico wrote:
> On Thu, Nov 19, 2015 at 1:55 PM, fl  wrote:
> > There are only one time 5 'here' printed out, but there is no 'here' print
> > out in thereafter call sq[2]() etc. How to understand this phenomenon?
> 
> Code does what code should.
> 
> Before you ask for comprehension of "this phenomenon", why don't you
> tell us what you expect your code to do, and why? I just asked my
> non-programmer sister and she was completely unsurprised by what
> Python did here.
> 
> ChrisA

Excuse me. That is my unclear question.
I expect that each call sq will have one print 'here', as I suppose that
print 'here' is inside the loop.

Oh, I just realize that it is not a function. It constructs list sq.
When the loop (function as a constructor (not necessarily correct called),
it print 5 'here'.

Thanks for you feedback.

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#99034

fl <rxjwg98@gmail.com> writes:

> Hi,
>
> From previous post, I get many helpful replies. Now I have a new question
> when I run this example code:
>
>
> ---------
> sq=[]
> for xx in range(5):
>     print 'here'
>     sq.append(lambda:xx**2)
> ...
> sq[2]()
> Out[151]: 16
>
> sq[3]()
> Out[152]: 16
> /////

Same reason as in your previous thread: variables in (the body of) function
definitions (and "lambda"s, which are functions definitions, too) are
resolved/dereferenced at call time, not at function definition time.

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