A situation where Python has undefined values

August 17, 2019

In most of Python, either a name has a value or it doesn't exist and attempts to access it will fail with some variation of 'that's not defined'. You get NameError for globals and AttributeError for attributes of objects, classes, and interestingly also for modules. Similarly, accessing a nonexistent key in a dictionary gets you a KeyError, also saying that 'this doesn't exist'.

(This means that code inside a module gets a different error for a nonexistent module variable than code outside it. I think this is just an artifact of how the name is accessed.)

But local variables in functions are different and special:

>>> def afunc():
...   print(a)
...   a = 10
>>> afunc()
UnboundLocalError: local variable 'a' referenced before assignment

When we do the print(), the name a exists as a local variable (at least in some sense), but its value is undefined (and an error) instead of being, say, None. If a was not even a local variable, we should get either some variant of 'name not defined' or we'd access a global a if it existed.

(I say that a exists in some sense because it doesn't fully exist; for example, it is not in the dictionary that locals() will return.)

At one level this is a straightforward consequence of how local variables are implemented in CPython. All references to local variables within a function use the same fast access method, whether or not a value has been bound to the local variable. When no value has been set, you get an error.

At another level, this is a sensible language design decision regardless of the specifics of the implementation. Python has decided that it has lexically scoped local variables, and this opens up the possibility of accessing a local variable before it's had a value set (unlike globals and attributes). When this happens, you have three choices; you can invent an arbitrary 'unset' value, such as None, you can generate a 'name does not exist' error, or you can generate a unique error. Python doesn't have zero values in the way that a language like Go does (fundamentally because the meaning of variables is different in the two languages), so the first choice would be unusual. The second choice would be a confusing pretense, because the name actually does exist and is in fact blocking you from accessing a global version of the name. That leaves the third choice of a unique error, which is at least clear even if it's unusual.

(This sprung from a Twitter thread.)

Written on 17 August 2019.
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Last modified: Sat Aug 17 23:31:36 2019
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