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Date: Sun, 17 Mar 2013 17:22:39 -0400
Subject: Re: PyWart: NameError trackbacks are superfluous
From: Jason Swails <jason.swails@gmail.com>
To: Rick Johnson <rantingrickjohnson@gmail.com>
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On Sat, Mar 16, 2013 at 2:27 PM, Rick Johnson
<rantingrickjohnson@gmail.com>wrote:

>
> [snip junk]
> We don't need multiple layers of traces for NameErrors. Python does not
> have *real* global variables; and thank Guido for that! All we need to know
> is which module the error occurred in AND which line of that module
> contains the offensive lookup of a name that does not exist.
> [snip more junk]


2 comments here.

1) Where's the consistency??  NameError is an exception.  All other
exceptions get full tracebacks.  A NameError is not special enough to
deserve special treatment (zen of Python? PEP 8?  I don't remember).  If
you like the full traceback (like I do), it's there. If you just want the
error, look at the last frame only.

==== Example where you want a full traceback ====

Pyflakes doesn't catch all NameErrors.  If you don't set all of an object's
possible attributes inside its constructor, but some are applied later-on
(i.e., when they are needed, like in some sort of specialized setup routine
that you only call in certain circumstances), then you can benefit from a
full traceback even for a NameError. Consider the DataSet class below:

import numpy as np
from scipy.stats.kde import gaussian_kde as kde
class DataSet(object):

   def __init__(self, data):
      self._dataset = np.asarray(data)
   def setup_kde(self):
      "Set up KDE. Do not do by default since it is expensive for large
data sets"
      self.kde = kde(self._dataset)
   def resampled(self):
      return DataSet(self.kde.resample())

In this case generating the KDE can become time- and RAM-consuming for
large data sets, so it's worthwhile to only generate the KDE if you
actually plan on doing something with that requires it.  If you call
resampled before setup_kde, you get a NameError, and it would be helpful to
have the full traceback when debugging.  Yes, you can get around having the
'non-trivial' NameError, but having the full traceback if someone _did_
write code like this makes your job a hell of a lot easier to debug if you
don't have to go stack tracing yourself. And for a large project with
multiple coders that is built in stages and has functionality added as it's
needed, this type of situation is not at all unusual.  While one approach
is to take to the (email) streets and proclaim how incompetent all coders
that came before you truly were and that such a project is not worth
modifying in view of their ineptitude, the approach that does _not_ lead to
your firing would benefit from a full NameError traceback. Go ahead and
fire away about how stupid I am for the suggestion anyway, though.

2) Fine.  You don't like long tracebacks for NameErrors, write a 8-line
module that you import at the top of every program (like division from
__future__ if you work with Python 2 like I have to):

--- file ricksstupididea.py ---
import sys, traceback
def excepthook(exctype, value, tb):
   if exctype is NameError:
      print 'Traceback (only printing last frame):'
      print '  File "%s", line %d, in %s\n    %s' %
traceback.extract_tb(tb).pop()
      print 'NameError: %s' % value
   else:
      sys.__excepthook__(exctype, value, tb)
sys.excepthook = excepthook
--- end ricksstupididea.py ---

Once you do this, you get your special behavior for NameErrors like you
want.  Observe, padawan:

-- begin nameerror.py --
import ricksstupididea
def func1(x):
   return func2(x)

def func2(x):
   return func3(x)

def func3(x):
   return func4(x)

def func4(x):
   return noname

func1(1)

$ python noname.py
Traceback (only printing last frame):
  File "noname.py", line 12, in func4
    return noname
NameError: global name 'noname' is not defined

-- begin divzero.py --
import ricksstupididea
def func1(x):
   return func2(x)

def func2(x):
   return func3(x)

def func3(x):
   return func4(x)

def func4(x):
   return x / 0

func1(1)

$ python divzero.py
Traceback (most recent call last):
  File "divzero.py", line 14, in <module>
    func1(1)
  File "divzero.py", line 3, in func1
    return func2(x)
  File "divzero.py", line 6, in func2
    return func3(x)
  File "divzero.py", line 9, in func3
    return func4(x)
  File "divzero.py", line 12, in func4
    return x / 0
ZeroDivisionError: integer division or modulo by zero

GvR's time machine strikes again.  The only bad thing about this approach
is that you have to actually write that extra 8 lines of code, and I get to
keep the behavior I like rather than being forced into the (inconsistent)
behavior you would prefer I use instead.  But since the 8 lines has been
provided to you here free of charge, the only real cost for you is how I
like my NameErrors given to me.

I hereby leave you enlightened.

</rant>
Jason

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<font color=3D"#000000"><br></font><br><div class=3D"gmail_quote">On Sat, M=
ar 16, 2013 at 2:27 PM, Rick Johnson <span dir=3D"ltr">&lt;<a href=3D"mailt=
o:rantingrickjohnson@gmail.com" target=3D"_blank">rantingrickjohnson@gmail.=
com</a>&gt;</span> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><br>[snip junk]<br>
We don&#39;t need multiple layers of traces for NameErrors. Python does not=
 have *real* global variables; and thank Guido for that! All we need to kno=
w is which module the error occurred in AND which line of that module conta=
ins the offensive lookup of a name that does not exist.<br>

[snip more junk]</blockquote><div><br></div><div>2 comments here.</div><div=
><br></div><div>1) Where&#39;s the consistency?? =A0NameError is an excepti=
on. =A0All other exceptions get full tracebacks. =A0A NameError is not spec=
ial enough to deserve special treatment (zen of Python? PEP 8? =A0I don&#39=
;t remember). =A0If you like the full traceback (like I do), it&#39;s there=
. If you just want the error, look at the last frame only.</div>
<div><br></div><div>=3D=3D=3D=3D Example where you want a full traceback =
=3D=3D=3D=3D</div><div><br></div><div>Pyflakes doesn&#39;t catch all NameEr=
rors. =A0If you don&#39;t set all of an object&#39;s possible attributes in=
side its constructor, but some are applied later-on (i.e., when they are ne=
eded, like in some sort of specialized setup routine that you only call in =
certain circumstances), then you can benefit from a full traceback even for=
 a NameError. Consider the DataSet class below:</div>
<div><br></div><div>import numpy as np</div><div>from scipy.stats.kde impor=
t gaussian_kde as kde</div><div>class DataSet(object):</div><div><br></div>=
<div>=A0 =A0def __init__(self, data):</div><div>=A0 =A0 =A0 self._dataset =
=3D np.asarray(data)</div>
<div>=A0 =A0def setup_kde(self):</div><div>=A0 =A0 =A0 &quot;Set up KDE. Do=
 not do by default since it is expensive for large data sets&quot;</div><di=
v>=A0 =A0 =A0 self.kde =3D kde(self._dataset)</div><div>=A0 =A0def resample=
d(self):</div><div>
=A0 =A0 =A0 return DataSet(self.kde.resample())</div><div><br></div><div>In=
 this case generating the KDE can become time- and RAM-consuming for large =
data sets, so it&#39;s worthwhile to only generate the KDE if you actually =
plan on doing something with that requires it. =A0If you call resampled bef=
ore setup_kde, you get a NameError, and it would be helpful to have the ful=
l traceback when debugging. =A0Yes, you can get around having the &#39;non-=
trivial&#39; NameError, but having the full traceback if someone _did_ writ=
e code like this makes your job a hell of a lot easier to debug if you don&=
#39;t have to go stack tracing yourself. And for a large project with multi=
ple coders that is built in stages and has functionality added as it&#39;s =
needed, this type of situation is not at all unusual. =A0While one approach=
 is to take to the (email) streets and proclaim how incompetent all coders =
that came before you truly were and that such a project is not worth modify=
ing in view of their ineptitude, the approach that does _not_ lead to your =
firing would benefit from a full NameError traceback. Go ahead and fire awa=
y about how stupid I am for the suggestion anyway, though.</div>
<div><br></div><div>2) Fine. =A0You don&#39;t like long tracebacks for Name=
Errors, write a 8-line module that you import at the top of every program (=
like division from __future__ if you work with Python 2 like I have to):</d=
iv>
<div><br></div><div>--- file ricksstupididea.py ---</div><div>import sys, t=
raceback</div><div>def excepthook(exctype, value, tb):</div><div><div>=A0 =
=A0if exctype is NameError:</div><div>=A0 =A0 =A0 print &#39;Traceback (onl=
y printing last frame):&#39;</div>
<div>=A0 =A0 =A0 print &#39; =A0File &quot;%s&quot;, line %d, in %s\n =A0 =
=A0%s&#39; % traceback.extract_tb(tb).pop()</div><div>=A0 =A0 =A0 print =
9;NameError: %s&#39; % value</div><div>=A0 =A0else:</div><div>=A0 =A0 =A0 s=
ys.__excepthook__(exctype, value, tb)</div>
<div>sys.excepthook =3D excepthook</div></div><div>--- end ricksstupididea.=
py ---</div><div><br></div><div>Once you do this, you get your special beha=
vior for NameErrors like you want. =A0Observe, padawan:</div><div><br></div=
>
<div>-- begin nameerror.py --</div><div><div>import ricksstupididea</div><d=
iv>def func1(x):</div><div>=A0 =A0return func2(x)</div><div><br></div><div>=
def func2(x):</div><div>=A0 =A0return func3(x)</div><div><br></div><div>def=
 func3(x):</div>
<div>=A0 =A0return func4(x)</div><div><br></div><div>def func4(x):</div><di=
v>=A0 =A0return noname</div><div><br></div><div>func1(1)</div></div><div><b=
r></div><div><div>$ python noname.py=A0</div><div>Traceback (only printing =
last frame):</div>
<div>=A0 File &quot;noname.py&quot;, line 12, in func4</div><div>=A0 =A0 re=
turn noname</div><div>NameError: global name &#39;noname&#39; is not define=
d</div></div><div><br></div><div>-- begin divzero.py --</div><div><div><div=
>import ricksstupididea</div>
<div>def func1(x):</div><div>=A0 =A0return func2(x)</div><div><br></div><di=
v>def func2(x):</div><div>=A0 =A0return func3(x)</div><div><br></div><div>d=
ef func3(x):</div><div>=A0 =A0return func4(x)</div><div><br></div><div>def =
func4(x):</div>
<div>=A0 =A0return x / 0</div><div><br></div><div>func1(1)</div></div></div=
><div><br></div><div><div>$ python divzero.py=A0</div><div>Traceback (most =
recent call last):</div><div>=A0 File &quot;divzero.py&quot;, line 14, in &=
lt;module&gt;</div>
<div>=A0 =A0 func1(1)</div><div>=A0 File &quot;divzero.py&quot;, line 3, in=
 func1</div><div>=A0 =A0 return func2(x)</div><div>=A0 File &quot;divzero.p=
y&quot;, line 6, in func2</div><div>=A0 =A0 return func3(x)</div><div>=A0 F=
ile &quot;divzero.py&quot;, line 9, in func3</div>
<div>=A0 =A0 return func4(x)</div><div>=A0 File &quot;divzero.py&quot;, lin=
e 12, in func4</div><div>=A0 =A0 return x / 0</div><div>ZeroDivisionError: =
integer division or modulo by zero</div></div><div><br></div><div>GvR&#39;s=
 time machine strikes again. =A0The only bad thing about this approach is t=
hat you have to actually write that extra 8 lines of code, and I get to kee=
p the behavior I like rather than being forced into the (inconsistent) beha=
vior you would prefer I use instead. =A0But since the 8 lines has been prov=
ided to you here free of charge, the only real cost for you is how I like m=
y NameErrors given to me.</div>
<div><br></div><div>I hereby leave you enlightened.</div><div><br></div><di=
v>&lt;/rant&gt;</div><div>Jason</div></div>

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