Creating a local variable scope.

Steven D'Aprano steve at REMOVE-THIS-cybersource.com.au
Sat Sep 12 03:29:55 CEST 2009


On Fri, 11 Sep 2009 19:36:14 +0200, Johan Grönqvist wrote:

> Hi All,
> 
> I find several places in my code where I would like to have a variable
> scope that is smaller than the enclosing function/class/module
> definition.
...
> The essence is that for readability, I want spam and eggs in separate
> definitions, but for clarity, I would like to express the fact that they
> are "local to the definition of ham", i.e., they are not used outside of
>   the definition of ham.

Personally, I don't think your use-case is the least bit convincing, and 
I think that introducing a new scope would hurt readability and clarity 
rather than help it, but if you really want this, there are a couple of 
approaches:

(1) Use comments to give your intention.

spam = 'abc'  # Used only in definition of ham.
eggs = 'def'  # Likewise.
ham = (a[eggs], b[spam])


(2) Delete the local names afterwards.

spam = 'abc'
eggs = 'def'
ham = (a[eggs], b[spam])
del spam, eggs


(3) Create an inner function, then call that.

def outer(*args):
    a = parrot()
    b = spanish_inquistion()
    def inner():
        spam = 'abc'
        eggs = 'def'
        return a[eggs], b[spam]
    ham = inner()
    return do_something_with(ham)


(4) Create a "do nothing" context manager allowing you to visually indent 
the block, but otherwise have no effect:

class do_nothing:
    def __enter__(self):
        pass
    def __exit__(self, type, value, traceback):
        pass

ham = ()
with do_nothing() as imaginary_local_scope:
    spam = 'abc'
    eggs = 'def'
    ham = a[eggs], b[spam]
    del spam, eggs, imaginary_local_scope


I think the fourth is an abomination and I only mention it for completion.

My personal opinion is that if you really need a local scope inside a 
function, the function is doing too much and should be split up.


-- 
Steven



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