Confusing behavior Machine(model=None) vs Machine()

[patrickwolf]

Describe the bug
Based on the readme I thought that Machine() and Machine(model=None) would show the same behavior which is that the states are created on the machine itself.

transitions/README.md

Lines 228 to 236 in 7ac5112

	An alternative is to not explicitly pass a model to the `Machine` initializer:
	```python
	machine = Machine(states=['solid', 'liquid', 'gas', 'plasma'], initial='solid')
	# The machine instance itself now acts as a model
	machine.state
	>>> 'solid'

But this is not the case...

Minimal working example

from transitions import Machine

machine = Machine(states=['solid', 'liquid', 'gas', 'plasma'], initial='solid')
# The machine instance itself now acts as a model
print(machine.state) # -> solid


machine = Machine(model=None, states=['solid', 'liquid', 'gas', 'plasma'], initial='solid')
# The machine instance itself does not act as a model
print(machine.state) # -> raise AttributeError: 'state' does not exist on <Machine@124164572018320>

Expected behavior
I would expect Machine(model=None) to have the same behavior as Machine() ie that the attributes are created on the Machine itself

[aleneum]

Hello @patrickwolf,

I would expect Machine(model=None) to have the same behavior as Machine() ie that the attributes are created on the Machine itself

it used to be that way until the issue arose where some users wanted to initialize a machine without any model. We have chosen to use an explicit None for that use case and a string literal ('self') as the default value which will cause the machine to add itself as a model (see this issue). In my opinion, model=None implies 'no model at all' but I understand that one could also interpret it as 'use default behavior'. The documentation mentions that the default value is a string literal/'self' though (code and README).

That being said, nothing is set in stone, and I am open to discussion if you have any suggestions on how to make the interface more intuitive.

[patrickwolf]

I have thought very deeply about it but for me it's clear that Machine () and Machine(model=None) need to be the same as it's standard behavior that people just expected...

So using an enum
Machine (model=Machine.NoModel) is seemingly an easy and explicit fix and would be extensible going forward as well.