[issue46450] namedtuple leaks data between instances when field's default value is empty list

[maximus733]

maximus733 <3ed7qja...@liamekaens.com> added the comment:

Thanks for the clarification.

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[issue46450] namedtuple leaks data between instances when field's default value is empty list

[Eric V. Smith]

Eric V. Smith  added the comment:

Also see 
https://docs.python.org/3/faq/programming.html#why-are-default-values-shared-between-objects

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nosy: +eric.smith
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue46450] namedtuple leaks data between instances when field's default value is empty list

[Dennis Sweeney]

Dennis Sweeney  added the comment:

This seems to be the standard confusion people have with mutable defaults, just 
with namedtuple defaults rather than function defaults.

Similar behavior elsewhere in Python:

>>> def f(x, y=[]):
... y.append(x)
... print(y)
... 
... 
>>> f(1)
[1]
>>> f(2)
[1, 2]
>>> f(15)
[1, 2, 15]

Or even

>>> a = []
>>> b = a
>>> b.append(4)
>>> a
[4]

This happens because the values you provide as defaults are *objects*, not 
*expressions* to be re-evaluated. If you make one of the defaults a list, you 
ask the namedtuple to "give me this particular list every time", and it will 
give you that same list, even if it has been modified.

There is currently no support for "construct a brand new list every time" in 
namedtuples. You could look to the dataclasses module for such a feature, where 
you can specify default_factory=list, and it will make a new list for each 
instance.

https://docs.python.org/3/library/dataclasses.html#dataclasses.field

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[issue46450] namedtuple leaks data between instances when field's default value is empty list

[Curtis M]

New submission from Curtis M <3ed7qja...@liamekaens.com>:

Example code to replicate issue on python 3.10.2 is attached.

How to replicate issue:
1. Define a namedtuple where all fields have default values. At least one 
field's default value will be an empty list: []
2. Instantiate 2 instances of the namedtuple.
3. In the first instance, use namedtuple._replace() to add a replacement list 
into a field that defaults to [].
4. Now look at the contents of both namedtuples. *Both* of them are modified, 
even though the code only modified the first object.


***

Output from attached example code:

g.P5 1: myNamedTuple(P1='hello', P2='world', P3=None, P4='', P5=[], P6=[])
h.P5 1: myNamedTuple(P1='good', P2='morning', P3=None, P4='', P5=[], P6=[])
Expected: g.P5: []
Actual:   g.P5: []
Expected: h.P5: []
Actual:   h.P5: []
g.P5 2: myNamedTuple(P1='hello', P2='world', P3=None, P4='', P5=['a', 'b'], 
P6=[])
h.P5 2: myNamedTuple(P1='good', P2='morning', P3=None, P4='', P5=['a', 'b'], 
P6=[])
Expected: g.P5: ['a', 'b']
Actual:   g.P5: ['a', 'b']
Expected: h.P5: []
Actual:   h.P5: ['a', 'b']
g.P5 3: myNamedTuple(P1='hello', P2='world', P3=None, P4='', P5=['a', 'b', 'c', 
'd'], P6=[])
h.P5 3: myNamedTuple(P1='good', P2='morning', P3=None, P4='', P5=['a', 'b', 
'c', 'd'], P6=[])
Expected: g.P5: ['a', 'b', 'c', 'd']
Actual:   g.P5: ['a', 'b', 'c', 'd']
Expected: h.P5: []
Actual:   h.P5: ['a', 'b', 'c', 'd']

--
components: Library (Lib)
files: namedtuple_example.py
messages: 411079
nosy: user027.7
priority: normal
severity: normal
status: open
title: namedtuple leaks data between instances when field's default value is 
empty list
type: behavior
versions: Python 3.10
Added file: https://bugs.python.org/file50568/namedtuple_example.py

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