[New-bugs-announce] [issue22563] namedtuple: raise an exception when the given class name would override an existing name

[Leo]

New submission from Leo:

The nametuple function creates classes dynamically. The caller must provide the class name thus overriding any existing object name. 
An overridden object may be garbage-collected
or survive depending on its reference count.

While this behavior is not new to Python, I am unaware of a simple function with such an awkward side effect. 
One might consider this a feature rather than a bug
thus shifting responsibility for carefully passing suitable class names to nametuple. 
However, I believe nametuple should raise an exception
if the provided name would override an existing name. If needed, this behavior could be made 
customizable by a keyword argument override defaulting to False. 

Consequently, the caller would have to delete the object under that 
name explicitly before calling nametuple. This behavior would increase code clarity and eliminate´ a source of subtle
errors. 


The code example given below shows the survival of 
a pre-existing class due to an instance linked to it. As a consequence, two different classes with the same name exist. Only the posterior is, 
however, bound to the
__main__ namespace. The prior is no longer known to that namespace potentially breaking
remote parts of the code.
 

In [1]: from collections import namedtuple

In [2]: AB=namedtuple('AB', ('a','b'))

In [3]: ab=AB(4,9)

In [4]: type(ab)
Out[4]: __main__.AB

In [6]: AB=namedtuple('AB', ('c','d'))

In [7]: type(ab)
Out[7]: __main__.AB

In [8]: ab2=AB(16,25)

In [9]: type(ab2)
Out[9]: __main__.AB

In [10]: type(ab)
Out[10]: __main__.AB

In [11]: ab
Out[11]: AB(a=4, b=9)

In [12]: ab2
Out[12]: AB(c=16, d=25)

----------
components: Library (Lib)
messages: 228641
nosy: fhaxbox66 at googlemail.com
priority: normal
severity: normal
status: open
title: namedtuple: raise an exception when the given class name would override an existing name
type: behavior
versions: Python 3.4

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue22563>
_______________________________________