[Python-Dev] Intricacies of calling eq (original) (raw)

Terry Reedy tjreedy at udel.edu
Wed Mar 19 02:29:05 CET 2014

On 3/18/2014 9:09 PM, Steven D'Aprano wrote:

Currently, the code:

if key in dict: return dict[key]

performs two dictionary lookups. If you read the code, you can see the two lookups: "key in dict" performs a lookup, and "dict[key]" performs a lookup.

The doc says that

@deco def f(): pass

is equivalent to

def f(): pass f = deco(f)

That clearly implies two assignments to the containing namespace. But CPython only does one. It actually does something like

def (): pass f = deco()

This can be detected by defining deco to have the side-effect of checking the local namespace for a binding to 'f' (actually, .name) and reporting via print() before returning. When someone did detect the difference and filed a bug report, Guido declared that the difference did not matter and that the current behavior is close enough to meeting the as-if rule.

Optimizing away a dict lookup strikes me as similar to optimizing away a namespace binding, which is typically a dict setting, which would involve a dict lookup. In fact, I imagine that your test class with a eq method that reports calls would be an alternate way to detect the @deco optimization.

-- Terry Jan Reedy

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