In Python, can I define a named tuple using typename?

Question

I wonder why the third line in Snippet B would trigger an error. My understanding is in the second line in Snippet B (and A), I created a class variable (not a class instance) cls_obj whose type/class name is Duck. It's like

class Duck(...):
    ...Code goes here

cls_obj=Duck

So I expected snippet A and B would both work, however, snippet B failed! What has gone wrong?

# Snippet A
from collections import namedtuple
cls_obj=namedtuple('Duck', 'bill tail')
duck = cls_obj(bill='wide orange', tail='long')

# Snippet B
from collections import namedtuple
cls_obj=namedtuple('Duck', 'bill tail')
duck = Duck(bill='wide orange', tail='long')

`namedtuple()` is an ordinary function. It can't define names in the enclosing scope. — Dietrich Epp, Jul 14 '16 at 04:38
The fact that the class is named `Duck` doesn't mean that the `Duck` variable refers to that class. The `class` statement has to both create the class and assign it to a variable; `namedtuple` can only create the class. — user2357112, Jul 14 '16 at 04:43
Hmm, but how can a function define a class and hence a class instance without defining the class name? @DietrichEpp — Nicholas, Jul 14 '16 at 08:31
I wonder what do you mean by `Duck` variable. Are you saying the string 'Duck'? I can't see where I created a `Duck` variable.@user2357112 — Nicholas, Jul 14 '16 at 08:35
@NicholasLiu: You didn't create a `Duck` variable, and that's why you get a `NameError` when you try to do `Duck(...)`. — user2357112, Jul 14 '16 at 18:57

score 1 · Accepted Answer · answered Jul 14 '16 at 13:49

In Python, a class is just a special kind of value.

class Duck(object):
    pass

# 'Duck' is just a variable, you can change it
Duck = 3

x = Duck() # Fails!

You can do things like this:

>>> class Goat(object):
...     def __repr__(self):
...         return 'Goat()'
...
>>> Cow = Goat
>>> del Goat
>>> Goat()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'Goat' is not defined
>>> Cow()
Goat()
>>> Cow.__name__
'Goat'
>>> Cow().__class__
<class '__main__.Goat'>

Now that you understand that a class is just a value, everything will start to make more sense. The namedtuple('Goat', ...) isn't the same as class Goat. It defines a class but does not assign the resulting value (the class itself) to a variable at global scope. Functions can't do that, and namedtuple() is an ordinary function.

While functions can't do that other constructs like a class definition **can** do that. — syntonym, Jul 14 '16 at 13:52
@syntonym: Right, because class definitions are not functions. — Dietrich Epp, Jul 14 '16 at 14:17

In Python, can I define a named tuple using typename?

1 Answers1