Add default fallback for missing keys

[sachinsachdeva]

Fixes #74.

This adds an opt-in _default constructor argument for callers who want missing attributes to return a fallback value instead of autovivifying an empty DotMap. Existing behavior remains unchanged unless _default is provided.

Example:

m = DotMap({'city': 'abc', 'CountryCode': 101}, _default='')
assert m.zipCode == ''

Tests:

python3 -m unittest dotmap.test

[sachinsachdeva]

@drgrib take a look when you get a chance

[FelixSchwarz]

Thank you for your PR. I'll try to have a closer look this week. Unfortunately time is pretty limited in this crazy time.

[FelixSchwarz]

Thank you very much - solid PR which needs just a few changes.

[FelixSchwarz]

Oh, and if you rebase to/merge in the latest master branch, you should also get github actions running again.

[sachinsachdeva]

@FelixSchwarz I think I got all your comments.

[FelixSchwarz]

I'm sorry that it took so long but I thought about the PR a bit and something felt "off" after my first review. After pondering this a bit more I think I now have a better picture in my mind.

This is what annoyed me:

>>> from dotmap import DotMap
>>> m = DotMap({}, _default=[])
>>> m.x
[]
>>> m.y
[]
>>> m.x.append(42)
>>> m.y
[42]

That is clearly wrong.

After a lot of back and forth I came up with two options:

• return deepcopy(self._default) on each miss
• replace _default with _default_factory, called once per miss. Similar to defaultdict, e.g. _default_factory=str instead of _default=''

The latter version is a bit more explicit but on the other hand causes a bit longer lines for each user of that feature. But using a _default_factory just simplify the internal code of dotmap and it fits an established pattern in Python to avoid silently shared mutable values.

It prevents the problem by construction, and it actually simplifies the diff. _default_provided can go away since None simply means "not set". A factory must be callable, we can raise a TypeError for non-callables.

What do you think?

[sachinsachdeva]

I'm sorry that it took so long but I thought about the PR a bit and something felt "off" after my first review. After pondering this a bit more I think I now have a better picture in my mind.

This is what annoyed me:

>>> from dotmap import DotMap
>>> m = DotMap({}, _default=[])
>>> m.x
[]
>>> m.y
[]
>>> m.x.append(42)
>>> m.y
[42]

That is clearly wrong.

After a lot of back and forth I came up with two options:

• return deepcopy(self._default) on each miss
• replace _default with _default_factory, called once per miss. Similar to defaultdict, e.g. _default_factory=str instead of _default=''

The latter version is a bit more explicit but on the other hand causes a bit longer lines for each user of that feature. But using a _default_factory just simplify the internal code of dotmap and it fits an established pattern in Python to avoid silently shared mutable values.

It prevents the problem by construction, and it actually simplifies the diff. _default_provided can go away since None simply means "not set". A factory must be callable, we can raise a TypeError for non-callables.

What do you think?

I and Claude both like the default factory approach :-)