Re: [PATCH v3 3/4] migrate-bitmaps-test: Fix pylint warnings

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

14.05.2021 18:43, Max Reitz wrote:
> There are a couple of things pylint takes issue with:
> - The "time" import is unused
> - The import order (iotests should come last)
> - get_bitmap_hash() doesn't use @self and so should be a function
> - Semicolons at the end of some lines
> - Parentheses after "if"
> - Some lines are too long (80 characters instead of 79)
> - inject_test_case()'s @name parameter shadows a top-level @name
>    variable
> - "lambda self: mc(self)" were equivalent to just "mc", but in
>    inject_test_case(), it is not equivalent, so add a comment and disable
>    the warning locally
> - Always put two empty lines after a function
> - f'exec: cat > /dev/null' does not need to be an f-string
> 
> Fix them.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---

[..]

> -def inject_test_case(klass, name, method, *args, **kwargs):
> +def inject_test_case(klass, suffix, method, *args, **kwargs):
>       mc = operator.methodcaller(method, *args, **kwargs)
> -    setattr(klass, 'test_' + method + name, lambda self: mc(self))
> +    # The lambda is required to enforce the `self` parameter.  Without it,
> +    # `mc` would be called without any arguments, and then complain.
> +    # pylint: disable=unnecessary-lambda
> +    setattr(klass, 'test_' + method + suffix, lambda self: mc(self))
> +
>   

Interesting... I decided to experiment a bit, and what I can say now:

The actual reason is that class attrubute of type <class 'function'>, becomes a <class 'method'> of the class instance on instantiation.

lambda is a function, so on instantiation we'll have "method", and method can be called as obj.method(), and original function will get "self" first argument automatically.

mc is not a function, it's <class 'operator.methodcaller'>, so there is no magic, instance of the class doesn't get own method but just a refence to class variable instead.

So, let's modify comment to something like:

We want to add function attribute to class, so that it correctly converted to method on instantiation. lamba is necessary to "convert" methodcaller object (which is callable, but not a function) to function.


with it:
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

==== my expirements =====

# cat x.py
import operator

class X:
     def hello(self, arg):
         print("hello", arg)


mc = operator.methodcaller("hello", "Vova")
lmd = lambda self: mc(self)

print('mc:', type(mc))
print('lmd:', type(lmd))

setattr(X, "test_hello_direct", mc)
setattr(X, "test_hello_lambda", lmd)
X.simply_assigned = lmd

x = X()

x.assigned_to_instance = lmd

print('mc attached:', type(x.test_hello_direct))
print('lmd attached:', type(x.test_hello_lambda))
print('lmd simply assigned:', type(x.simply_assigned))
print('lmd assigned to instance:', type(x.assigned_to_instance))

x.test_hello_lambda()
x.simply_assigned()

print("x.test_hello_lambda is x.simply_assigned", x.test_hello_lambda is x.simply_assigned)
print("x.test_hello_lambda is X.test_hello_lambda", x.test_hello_lambda is X.test_hello_lambda)
print("x.test_hello_direct is X.test_hello_direct", x.test_hello_direct is X.test_hello_direct)
print("X.test_hello_lambda is X.simply_assigned", X.test_hello_lambda is X.simply_assigned)
print("X.test_hello_lambda type:", type(X.test_hello_lambda))

try:
     x.assigned_to_instance()
except Exception as e:
     print("assigned to instance call failed:", e)

try:
     x.test_hello_direct()
except Exception as e:
     print("direct call failed:", e)



# python3 x.py
mc: <class 'operator.methodcaller'>
lmd: <class 'function'>
mc attached: <class 'operator.methodcaller'>
lmd attached: <class 'method'>
lmd simply assigned: <class 'method'>
lmd assigned to instance: <class 'function'>
hello Vova
hello Vova
x.test_hello_lambda is x.simply_assigned False
x.test_hello_lambda is X.test_hello_lambda False
x.test_hello_direct is X.test_hello_direct True
X.test_hello_lambda is X.simply_assigned True
X.test_hello_lambda type: <class 'function'>
assigned to instance call failed: <lambda>() missing 1 required positional argument: 'self'
direct call failed: methodcaller expected 1 argument, got 0


-- 
Best regards,
Vladimir