Question

Suppose that for administrative reasons I do not have write access to module xxx.

I want to do something like:

from xxx import yyy

@myDeco
yyy

which of course fails.

I think I can do

yyy = myDeco(yyy)

but is there a way to use the @myDeco” notation ? Or is this only permitted immediately before a def?

Answer 1

In the Python reference documentation, Compound Statements section, it says:

decorated      ::=  decorators (classdef | funcdef)
decorators     ::=  decorator+
decorator      ::=  "@" dotted_name ["(" [argument_list [","]] ")"] NEWLINE
funcdef        ::=  "def" funcname "(" [parameter_list] ")" ":" suite

This is part of the syntax rules. As you can see, decorators are only applied before a function or class definition, which is what def <funcname> starts. It goes on, noting:

A function definition may be wrapped by one or more decorator
expressions. Decorator expressions are evaluated when the function is
defined, in the scope that contains the function definition.

That said, keep in mind that decorators are really just syntactic sugar, and before they were made available the form:

func = decorate_func(func)

Was used instead. So if all you have is the function object in some variable, you can’t use the decorator syntactic sugar and must revert to the second method.