Function Attributes

Function objects are not limited to the system-defined attributes listed in the prior section, though. As we learned in Chapter 17, it’s possible to attach arbitrary user-defined attributes to them as well:

>>> func
<function func at 0x0257C738>
>>> func.count = 0
>>> func.count += 1
>>> func.count
1
>>> func.handles = 'Button-Press'
>>> func.handles
'Button-Press'
>>> dir(func)
['__annotations__', '__call__', '__class__', '__closure__', '__code__',
...more omitted...
__str__', '__subclasshook__', 'count', 'handles']

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As we saw in that chapter, such attributes can be used to attach state information to function objects directly, instead of using other techniques such as globals, nonlocals, and classes. Unlike nonlocals, such attributes are accessible anywhere the function itself is. In a sense, this is also a way to emulate “static locals” in other languages—variables whose names are local to a function, but whose values are retained after a function exits. Attributes are related to objects instead of scopes, but the net effect is similar.