Help with changes in traceback stack from Python 2.7 to Python 3.x

[Ned Batchelder]

On 4/26/14 1:50 AM, Andrew Konstantaras wrote:
> I wrote the following code that works in Python 2.7 that takes the
> variables passed to the function into a dictionary.  The following call:
>
>       strA = 'a'
>       intA = 1
>       dctA = makeDict(strA, intA)
>
> produces the following dictionary:
>
>        {'strA':'a', 'intA':1}
>
> To access the names passed into the function, I had to find the
> information by parsing through the stack.  The code that used to work is:
>
> from traceback import extract_stack
>
> def makeDict(*args):
>      strAllStack = str(extract_stack())
>      intNumLevels = len(extract_stack())
>      intLevel = 0
>      blnFinished = False
>      while not blnFinished:
>          strStack = str(extract_stack()[intLevel])
>          if strStack.find("makeDict(")>0:
>              blnFinished = True
>          intLevel += 1
>          if intLevel >= intNumLevels:
>              blnFinished = True
>      strStartText = "= makeDict("
>      intLen = len(strStartText)
>      intOpenParenLoc = strStack.find(strStartText)
>      intCloseParenLoc = strStack.find(")", intOpenParenLoc)
>      strArgs = strStack[ intOpenParenLoc+intLen : intCloseParenLoc ].strip()
>      lstVarNames = strArgs.split(",")
>      lstVarNames = [ s.strip() for s in lstVarNames ]
>      if len(lstVarNames) == len(args):
>          tplArgs = map(None, lstVarNames, args)
>          newDict = dict(tplArgs)
>          return newDict
>      else:
> return "Error, argument name-value mismatch in function 'makeDict'.
> lstVarNames: " + str(lstVarNames) + "\n args: " + str(args), strAllStack
>
> The same code does not work in Python 3.3.4.  I have tried parsing
> through the stack information and frames and I can't find any reference
> to the names of the arguments passed to the function.  I have tried
> inspecting the function and other functions in the standard modules, but
> I can't seem to find anything that will provide this information.

1) This is a very strange function. Instead of going to all this 
trouble, why not use:

     dctA = dict(strA=strA, intA=intA)

Yes, you have to repeat the names, but you'd be done by now, it works on 
both versions of Python, and people reading your code would understand 
what it does.

2) Why is your code examining the entire stack?  The frame you want is 
the one just above you.  Why are you stringifying the tuple produced by 
extract_stack when the source line you want is the fourth element?  Why 
are you using a while loop and a counter to iterate over elements of a list?

3) In the error case you return a tuple when the caller is expecting a 
dict? Why not raise an exception?

4) Your code only works if makeDict is assigned to a name.  When I first 
tried it, I used "print(makeDict(...))", and it failed completely.

5) You haven't mentioned what goes wrong on Python 3, but when I tried 
it, I got:

     Traceback (most recent call last):
       File "foo3.py", line 34, in <module>
         d = makeDict(a, b)
       File "foo3.py", line 27, in makeDict
         newDict = dict(list(tplArgs))
     TypeError: 'NoneType' object is not callable

Looking at your code, I see:

     tplArgs = map(None, lstVarNames, args)

I didn't realize map accepted a callable of None (TIL!), but it no 
longer does in Python 3.  You'll have to do this a different way.

But seriously: just use dict() and be done with it.  There's a lot here 
that can be much simpler if you learn to use Python as it was meant to 
be used.  You are swimming upstream, there are easier ways.

>
> Can anyone point me in the direction to find this information?  Any help
> is appreciated.
>
> ---Andrew
>
>
>


-- 
Ned Batchelder, http://nedbatchelder.com