Default values and subcast

[tobiasmboelz]

As far as I can tell the default value can either be of the cast type or a string. So env.bool('FOO', True), env.bool('FOO', 'true'), env.bool('FOO', '1') and env.bool('FOO', 'YES') all are basically the same.

>>> os.environ.get('FOO')
>>> env.bool('FOO', True)
True
>>> env.bool('FOO', 'true')
True
>>> env.bool('FOO', '1')
True
>>> env.bool('FOO', 'YES')
True

The same goes for lists with a subcast to a basic type. env.list('FOO', '1, 2, 42', subcast=float) and env.list('FOO', [1.0, 2.0, 42.0], subcast=float) both do work.

>>> env.list('FOO', '1, 2, 42', subcast=float)
[1.0, 2.0, 42.0]
>>> env.list('FOO', [1.0, 2.0, 42.0], subcast=float)
[1.0, 2.0, 42.0]

But, using the target type as default only works as long as the subcast type/callable can be called with the cast type itself. It breaks when the callable expects a string and returns something else. For example:

>>> env.list('FOO', 'a:b, b:c', subcast=lambda s: tuple(s.split(':')))
[('a', 'b'), (' b', 'c')]
>>> env.list('FOO', ['a:b', 'b:c'], subcast=lambda s: tuple(s.split(':')))
[('a', 'b'), ('b', 'c')]
>>> env.list('FOO', [('a', 'b'), ('b', 'c')], subcast=lambda s: tuple(s.split(':')))
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "[…]\lib\site-packages\environs\__init__.py", line 123, in method
    value = field.deserialize(value)
  File "[…]\lib\site-packages\marshmallow\fields.py", line 368, in deserialize
    output = self._deserialize(value, attr, data, **kwargs)
  File "[…]\lib\site-packages\marshmallow\fields.py", line 784, in _deserialize
    result.append(self.inner.deserialize(each, **kwargs))
  File "[…]\lib\site-packages\marshmallow\fields.py", line 368, in deserialize
    output = self._deserialize(value, attr, data, **kwargs)
  File "[…]\lib\site-packages\environs\__init__.py", line 188, in _deserialize
    return func(value)
  File "<console>", line 1, in <lambda>
AttributeError: 'tuple' object has no attribute 'split'

If this is expected behaviour, I’d be happy to provide a pull request with a supplement to the documentation (well, README.md).

[BroFromSpace]

Am I correct in understanding that the subcast is applied to the default value? If so, does this make sense, since a default value is supposed to be predictable and shouldn't require any transformations? Wouldn't it be better to simply check if the default value is of the correct type or None?

[tobiasmboelz]

The subcast is applied to elements of the list after splitting.

Env.list has special handling for the case that a list (or any other iterable except string) is passed in¹, but subcast has not. I'd suggest that, for consistency, the subcast should only be applied if the value is a string.

As for the suggestion to check if the default value is of the correct type: That would only work as long as the subcast argument is a type. For (other) callables, that would require either another argument or a mandatory return annotation. IMO, both would make usage of the package unnecessarily complex.

Footnotes

1. So does Env.dict. ↩