[Python-ideas] Mitigating 'self.' Method Pollution

Sat Jul 11 12:02:23 CEST 2015

On Jul 11, 2015, at 00:49, Michael Hewitt <michael at hewitts.us> wrote:
> 
> Let me state my argument more succinctly.  If 'self.' is a good thing, then *all* Python variable references should always be explicitly prefixed by their scope -- 'global.', 'local.', and 'self.'. 

If I were designing a new pythonesque language, I think I might well add something like that for when you need to disambiguate from the default, and then get rid of the global statement. In fact, if it weren't for the obvious huge backward compat problems, I might even go for a suggestion to do that in Python. Even for closure variables, I kind of like the look of "nonlocal.x = 3", although it's not _as_ compelling as "global.x = 3" to me.

But the idea that self is a more important default than local, and we should therefore have SLEGB instead of LEGB for unqualified lookups, and self for unqualified assignments instead of local? Definitely not. Creating new locals is something I do in half my functions and methods; creating new attributes is something I rarely do outside of __init__ methods.

I've used plenty of languages that don't require declarations, and I've never found one whose default rules made me as happy as Python's LEGB/L. It sort of works in languages where classes have a predefined set of attributes, but even there I don't like it as much; I always end up naming one set or the other with an underscore prefix or suffix or some other marker which is just "self." in a disguise that fools my tools.

> Why does it make more sense to not require explicit scoping of global variables, which are far more dangerous, while requiring explicit class scoping when nothing is more natural than for a method to refer to variables within its own class?
> 
> This is totally backwards.  If anything, the opposite should be true.  'global.' should be required for *all* global references, of which there should be very few in well-written, modular code, and 'self.' should not be required for class references because of course methods are going to naturally refer to variables within their class.
> 
> Keep in mind that the proposed design explicitly specifies the class variable scoping at the top of each method, so that anyone reading the method can simply look at the top of the method to find out which variables are class variables.
> 
> 
>> On Sat, Jul 11, 2015 at 12:25 AM, Michael Hewitt <michael at hewitts.us> wrote:
>> Let's compare two versions of a method taken from some code that my 11 year old son wrote yesterday:
>> 
>> Current
>> 
>> global height
>> def keep_moving_gravity(self):
>>     self.y += self.gravity
>>     self.y = max(self.y, 0)
>>     self.y = min(self.y, height - 1)
>> 
>> Proposed
>> 
>> global height
>> def keep_moving_gravity(self):
>>     self y, gravity
>>     y += gravity
>>     y = max(y, 0)
>>     y = min(y, height - 1)
>> 
>> Is anyone actually going to argue that the first version is cleaner and more readable than the second?  All I see when I read the first is 'self', 'self', 'self' -- my son's exact words to me last night.
>> 
>> As far as maintainability, the author of the first version must repeatedly make the same decisions over and over regarding how to scope each variable reference as he/she types 'y', 'gravity', and 'height'. The author of the second code makes these decisions exactly once at the top of the method and then is free to refer to each variable naturally without the mental overhead of prefixing each 'y' and 'gravity' with 'self.', but God forbid - not 'height'.  I can tell you that the mental overhead of this is taxing my son & is the cause of many painful mistakes -- just forgetting a single 'self.' prefix on one of the above field references can waste a significant amount of time.
>> 
>> As far as static analysis tools, this should honestly not be a lot of extra work, since the tools must already handle 'global' in a very similar fashion.
>> 
>> If the 'self.' prefix really does make code clearer, then we should do away with the 'global' scope declaration as well as automatic local scoping and require prefixing of all Python variables with 'self.', 'global.' or 'local.'.  My mind becomes numb thinking about writing such code.  To me, the existence of the keyword 'global' for automatically scoping subsequent variable references is a strong argument that a similar 'self' scoping mechanism is called for as well.
>> 
>> And, for folks who still prefer to prefix all their field references with 'self.', the proposal in no way prevents them from doing so.  It merely allows the rest of us to be a bit less wordy and more pithy in our code.
>> 
>> Mike
>> 
>> 
>>> On Friday, July 10, 2015, Mark Lawrence <breamoreboy at yahoo.co.uk> wrote:
>>>> On 10/07/2015 23:31, Michael Hewitt wrote:
>>>> Last night I made a post to the neopythonic blog proposing a Python 3.x
>>>> feature that Guido asked me to forward to this alias.  For the full
>>>> background, see the link to my post below.  For brevity, I will simply
>>>> submit the proposal here.  The specific problem I am addressing is the
>>>> pollution of Python methods by 'self.' to reference fields.  Here is the
>>>> proposal:
>>>> 
>>>>     The name of the first parameter to a method can be used to scope
>>>>     subsequent variable references similar to the behavior of 'global'.
>>>> 
>>>> 
>>>> Here are some examples:
>>>> 
>>>>     class Foo:
>>>> 
>>>>         def method_a(self)
>>>> 
>>>>             self x # subsequent 'x' references are scoped to 'self'
>>>> 
>>>>             x = 5 # same as self.x = 5
>>>> 
>>>>         def method_b(this)
>>>> 
>>>>             this x, y # subsequent 'x' & 'y' refs are scoped to 'this'
>>>> 
>>>>             x = y # same as this.x = this.y
>>>> 
>>>>         def method_c(its)
>>>> 
>>>>             its.x = 5 # still works just like it used to
>>>> 
>>>> 
>>>> This suggestion is fully backward compatible with existing Python code,
>>>> but would eliminate the need to pollute future Python methods with
>>>> copious 'self.' prefixes, thereby improving both readability and
>>>> maintainabilty.
>>> 
>>> I disagree completely.  When I see:-
>>> 
>>> self.x = 1
>>> 
>>> I currently know exactly what I'm looking at.  All I see with this proposal is more work for my MKI eyeballs, which are already knackered, and more work for the people who maintain our static analysis tools, as you can still forget to properly scope your variables.  So -1.
>>> 
>>>> 
>>>> Thank you for your consideration.
>>>> 
>>>> Michael Hewitt
>>>> 
>>>> Original Post:
>>>> http://neopythonic.blogspot.com/2008/10/why-explicit-self-has-to-stay.html
>>> 
>>> 
>>> 
>>> -- 
>>> My fellow Pythonistas, ask not what our language can do for you, ask
>>> what you can do for our language.
>>> 
>>> Mark Lawrence
>>> 
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