Why doesn't the namedtuple module use a metaclass to create nt class objects?
There are some hints in the issue 3974. The author proposed a new way to create named tuples, which was rejected with the following comments:
It seems the benefit of the original version is that it's faster, thanks to hardcoding critical methods. - Antoine Pitrou
There is nothing unholy about using exec. Earlier versions used other approaches and they proved unnecessarily complex and had unexpected problems. It is a key feature for named tuples that they are exactly equivalent to a hand-written class. - Raymond Hettinger
Additionally, here is the part of the description of the original namedtuple recipe:
... the recipe has evolved to its current exec-style where we get all of Python's high-speed builtin argument checking for free. The new style of building and exec-ing a template made both the __new__ and __repr__ functions faster and cleaner than in previous versions of this recipe.
If you're looking for some alternative implementations:
abstract base class + mix-in for named tuples recipe by Jan Kaliszewski
metaclass-based implementation by Aaron Iles (see his blog post)
As a sidenote: The other objection I see most often against using exec is that some locations (read companies) disable it for security reasons.
Besides an advanced Enum and NamedConstant, the aenum library* also has NamedTuple which is metaclass-based.
* aenum is written by the author of enum and the enum34 backport.