namedtuple._replace() doesn't work as described in the documentation

Question

I was having trouble implementing namedtuple._replace(), so I copied the code right off of the documentation:

Point = namedtuple('Point', 'x,y')

p = Point(x=11, y=22)

p._replace(x=33)

print p

and I got:

Point(x=11, y=22)

instead of:

Point(x=33, y=22)

as is shown in the doc.

I'm using Python 2.6 on Windows 7

What's going on?

Arkhangelsk

This works the same way as str.replace() does. Both strings and named tuples are immutable, so their replacement methods produce new values while leaving the old value unchanged. The solution to your problem is to save the new result to a variable. – Asta

Great question if for no other reason than that everybody wanting to "modify" a namedtuple is going to trip over this at some point. – Selfdelusion

Answer 1

Yes it does, it works exactly as documented.

._replace returns a new namedtuple, it does not modify the original, so you need to write this:

p = p._replace(x=33)

See here: somenamedtuple._replace(kwargs) for more information.

Steelworker

Good thing to note from the documentation: I was wary of using an underscore-prefixed method since that's typically done to indicate the method is private. However in this case it's done to avoid conflicts with the namedtuple's fields (which might include something called replace), so this is safe to use. There's an ._asdict() method which works similarly. – Ambience

Answer 2

A tuple is immutable. _replace() returns a new tuple with your modifications:

p = p._replace(x=33)

Gantlet

Answer 3

namedtuple._replace() returns a new tuple; the original is unchanged.

Symonds

Answer 4

It looks to me as if namedtuple is immutable, like its forebear, tuple.

>>> from collections import namedtuple
>>> Point = namedtuple('Point', 'x,y')
>>>
>>> p = Point(x=11, y=22)
>>>
>>> p._replace(x=33)
Point(x=33, y=22)
>>> print(p)
Point(x=11, y=22)
>>> p = p._replace(x=33)
>>> print(p)
Point(x=33, y=22)

NamedTuple._replace returns a new NamedTuple of the same type but with values changed.

Napoleon

FWIW, the method name was inspired by str.replace() which has a remarkably similar pattern of use :-) s='the quick brown fox'; s=s.replace('quick', 'slow') – Asta

Answer 5

But YES, you are right: in the 'official' documentation, they forgot to assign the replaced tuple to a variable: https://docs.python.org/3/library/collections.html?highlight=collections#collections.namedtuple

p._replace(x=33)

instead of

p1 = p._replace(x33)

Gondola

The official documentation shows that when you run p._replace(x=33) on the interactive Python prompt, it outputs the new tuple to screen. They didn't forget anything, this is perfectly fine interactive use. The next line shows a more complex bit of code where the return value is assigned to something. – Crass