GenConverter unstructures tuple to list if in union type.

[aha79]

• cattrs version: 1.10
• Python version: 3.9.7
• Operating System: Windows 10

Description

GenConverter incorrectly unstructures tuples when in union (or optional). Converter does do it correctly.

Example:

from typing import Optional, Tuple
import cattr

orig = (1,'2')

c = cattr.Converter()
print( c.unstructure(orig, Tuple[int,str] ) )
# -> (1, '2')
print( c.unstructure(orig, Optional[Tuple[int,str]] ) )
# -> (1, '2')

c = cattr.GenConverter()
print( c.unstructure(orig, Tuple[int,str] ) )
# -> (1, '2')
print( c.unstructure(orig, Optional[Tuple[int,str]] ) )
# -> [1, '2']    <<< incorrect, no more tuple

[Tinche]

I don't think it'd be too difficult to fix, but why is it a problem?

The code essentially boils down to:

>>> from cattrs import GenConverter
>>> c = GenConverter()
>>> c.unstructure((1,2))  # equivalent to: unstructure((1,2), tuple)
[1, 2]

Lists are better as the default here because:

• they are actually faster, [x for x in y] is faster than tuple(x for x in y)
• more serialization libraries support them (actually, all of them), while not all support tuples

[Tinche]

As a small tip, if you really need them to stay tuples:

>>> c = GenConverter(unstruct_collection_overrides={tuple: tuple})
>>> c.unstructure((1,2))
(1, 2)
>>> c.unstructure((1,2), Optional[Tuple[int, int]])
(1, 2)

[aha79]

I see. I am fine with tuples always being structured into lists. And now that I think about it, I see your point with favoring list instead of tuple.

Perhaps the real issue is that the behavior is inconsistent.

When Tuple is unstructured directly we get tuple, but Tuple inside Optional we get list. And it is not the nesting that causes this (Tuple inside of List generates tuple), it is only inside Union (or Optional).

I dont see any reason for this behavior. Naturally I would expect that if I put Optional around something I get the same unstructuring behavior. The difference would just be that None is also allowed. But that does not make much of a difference for unstructuring as it does not validate either.

[Tinche]

Hm I think you're right in that our Optional handling is inconsistent. I can add a test for this.

[bibajz]

Hi, this issue is also fixed by #227 - both cases are now unstructured to list