[ python-Bugs-1048870 ] call arg of lambda not updating

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Bugs item #1048870, was opened at 2004-10-17 16:55
Message generated for change (Comment added) made by rhettinger
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1048870&group_id=5470

Category: Python Interpreter Core
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Kevin Quick (kquick)
Assigned to: Nobody/Anonymous (nobody)
Summary: call arg of lambda not updating

Initial Comment:
The attachment contains a script intended to do performance testing
on various types of dictionary keys.  Lambda functions are used to
"generate" a sequence of keys with bound arguments to arrays that are
modified during execution.

Script fails with pop() on empty list error, but examination shows that 
the lambda function from the *previous* call is being used for the 
current call; if proper lambda was used, list would not be empty and
operation should succeed.

Script is large and has been "grown", so it's not very elegant, but either 
I'm missing somewhere where I'm making a stupid mistake or else the
lambda argument to the function call isn't being set properly.  Don't be 
put off by the size of the script; the problem is easily demonstrated and 
fairly clearly understood in just a few minutes of looking at the 
generated stack trace relative to the code.

As it exists, script will fail immediately due to the bug.  If it worked as 
expected, at least 3-4 lines of output should be generated before 
encountering new code that hasn't yet been fully debugged.  New code 
could be removed, but it was left in because some of the complexity of 
core code is due to this latter code, and it doesn't obstruct the primary 
bug, so it can be ignored unless of interest.

python
Python 2.3.3 (#1, May 27 2004, 20:44:16) 
[GCC 3.3.2 20031218 (Gentoo Linux 3.3.2-r5, propolice-3.3-7)] on 
linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 

Thanks for looking at this!


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>Comment By: Raymond Hettinger (rhettinger)
Date: 2004-10-17 18:04

Message:
Logged In: YES 
user_id=80475

Please resubmit after boiling this down to the simplest
possible thing that doesn't work according to your
expectations. 

Submitting a whole application and playing the "find the
bug" game is not good use of developer time.  Often, when a
submitter starts taking away the unnecessary pieces and
isolates the issue, they find that there was a bug in their
own code or a bug in their own understanding of how the
language works.

When it comes to lambdas, the most frequent misunderstanding
is knowing that default arguments bind only once as the time
of the lambda declaration and that free variables bind
whenever the resulting function is invoked.  Here's an
example of code that looks similar but behaves differently:
 
def once(x): return x
def twice(x): return 2*x
def thrice(x): return 3*x
funcs = [once, twice, thrice]

flim = [lambda x:funcs[0](x), lambda x:funcs[1](x), lambda
x:funcs[2](x)]
flam = [lambda x:f(x) for f in funcs]

print flim[0](1), flim[1](1), flim[2](1)
print flam[0](1), flam[1](1), flam[2](1)

The difference in output is because nested scopes bind
names, while argument binding binds values.

Hope this helps.  If not, please resubmit with the smallest
case that demonstrates the problem.




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