Jeff Hinrichs: Mutant Registration: Implicit or Explicit

Since Python is call-by-object(*1), a function that mutates a mutable argument, changes that object in the caller’s scope. some code to illustrate:

>>> mobject = {'magic': True}
>>> id(mobject)
140330485577440
>>> 
>>> def toggle(mdata):
...    '''flip the state of magic'''
...    mdata['magic'] = not mdata['magic']
... 
>>> 
>>> toggle(mobject)
>>> mobject
{'magic': False}
>>> id(mobject)
140330485577440

So hopefully this does not surprise you. If it does, please see the two links in the footnotes(*1) as they explain it quiet well.

My question deals with the implicit nature of the mutation. Not that Python is somehow wrong, it is the fact that the function usage does not convey the mutation to the reader as pointedly as I want. Coming from other languages that are call by value, a function that wanted to mutate an argument and get it back into the caller’s scope had to return the mutated value.

>>> def toggle_explicit(mdata):                                                 
...    '''flip the state of magic and return'''                                
...    mdata['magic'] = not mdata['magic']
...    return mdata
... 
>>> mobject = toggle_explicit(mobject)
>>> mobject
{'magic': True}
>>> id(mobject)
140330485577440
>>> 

Now I know that the above code is most definitely NOT call by value, but I do feel that it is more explicit about my intention. Even though the assignment is not needed for the actual effect. i.e.:

>>> toggle_explicit(mobject)
{'magic': False}
>>> mobject
{'magic': False}

So why does toggle_explicit() give me the warm fuzzies? Where as toggle() requires the reader to know what is going on. Is it just me shaking the cruft of call-by-value languages off? What do you do, when you mutate state within a function? Is the toggle_explicit() form not/less Pythonic?

— Footnotes —

(*1) Raymond Hettinger in this SO article references a post by Fredrik Lundh’s on “Call By Object“