Question on mutable default values

[function2-llx]

Description

Some codes in the repository use mutable default value (mostly, default value for norm and act). E.g., in the following example, although a tuple of ("instance", {"affine": True}) is immutable, the dict inside the tuple is still mutable.

MONAI/monai/networks/nets/basic_unet.py

Lines 178 to 190 in 782c1e6

	class BasicUNet(nn.Module):
	def __init__(
	self,
	spatial_dims: int = 3,
	in_channels: int = 1,
	out_channels: int = 2,
	features: Sequence[int] = (32, 32, 64, 128, 256, 32),
	act: str | tuple = ("LeakyReLU", {"negative_slope": 0.1, "inplace": True}),
	norm: str | tuple = ("instance", {"affine": True}),
	bias: bool = True,
	dropout: float | tuple = 0.0,
	upsample: str = "deconv",
	):

Using mutable default is usually discouraged (e.g., official documentation). However, most of the time it actually works as expected, maybe it is just a choice of coding style.

Possible solution: change every dict in the default to MappingProxyType, but I personally think that this will make code ugly.