[issue6921] recursion wierdness ... related to variable scoping?

[David Hanney]

New submission from David Hanney <spam2008 at nney.com>:

I was playing about with ideas behind b-plus-trees
and found i python bug

the important bit of code is:

            


            for p, k in enumerate(self.keys):
                self.ptrs[p].dump(indent+1, kmin=lk, kmax=k)
                print sindent + "(" + str(k) + ")"
                lk = k

            #
            # python bug?
            #
            show_python_bug = len(sys.argv)>1
            if show_python_bug:
                #
                # why not this?
                assert p == len(self.keys)-1
            else:
                #
                # why do I need this instead?
                p = len(self.keys)-1


i'm expecting k to still be in scope and equal len(self.keys)-1
sometimes it is and sometimes it isn't (depending on recursion depth)

you can try it for yourselves as I attach the full program:

./btree.py
runs fine
./btree.py  show_python_bug
eventually breaks like this:

Traceback (most recent call last):
  File "./btree.py", line 235, in <module>
    page.dump()
  File "./btree.py", line 223, in dump
    self.ptrs[p+1].dump(indent+1, kmin=lk)
  File "./btree.py", line 223, in dump
    self.ptrs[p+1].dump(indent+1, kmin=lk)
  File "./btree.py", line 223, in dump
    self.ptrs[p+1].dump(indent+1, kmin=lk)
  File "./btree.py", line 217, in dump
    assert p == len(self.keys)-1
UnboundLocalError: local variable 'p' referenced before assignment

... despite executing that code many times successfully before this
happens ... strange!

I hope you can figure it out and that this report proves helpful.

----------
components: Interpreter Core
files: btree.py
messages: 92691
nosy: mrdiskodave
severity: normal
status: open
title: recursion wierdness ... related to variable scoping?
type: behavior
versions: Python 2.6
Added file: http://bugs.python.org/file14897/btree.py

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<http://bugs.python.org/issue6921>
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