Understanding a Python closure oddity

June 16, 2024

Recently, Glyph pointed out a Python oddity on the Fediverse and I had to stare at it for a bit to understand what was going on, partly because my mind is partly thinking in Go these days, and Go has a different issue in similar code. So let's start with the code:

def loop():
    for number in range(10):
        def closure():
            return number
        yield closure

eagerly = [each() for each in loop()]
lazily = [each() for each in list(loop())]

The oddity is that 'eagerly' and 'lazily' wind up different, and why.

The first thing that is going on in this Python code is that while 'number' is only used in the for loop, it is an ordinary function local variable. We could set it before the loop and look at it after the loop if we wanted to, and if we did, it would be '9' at the end of the for loop. The consequence and the corollary is that every closure returned in the 'for' loop is using the same 'number' local variable.

(In some languages and in some circumstances, each closure would close over a different instance of 'number'; see for example this Go 1.22 change.)

Since all of the closures are using the same 'number' local variable, what matters for what value they return is when they are called. When you call any of them, it will return the value of 'number' that is in effect in the 'loop' function as of that moment. And if you call any of them after the 'loop' function has finished, 'number' has the value of '9'.

This also means that if you call a single 'each' function more than once, the value it returns can be different. For example:

>>> g = loop()
>>> each0 = g.__next__()
>>> each0()
>>> each1 = g.__next__()
>>> each0()

(What the 'loop()' call actually returns is a generator. I'm directly calling its magic method to be explicit, rather than using the more general next().)

And in a way this is the difference between 'eagerly' and 'lazily'. For 'eagerly', the list comprehension iterates through the results of 'loop()' and immediately calls each version of 'each' that it obtains, which gets the value of 'number' that is in effect right then. For 'lazily', the 'list(loop())' first collects all of the 'each' closures, which ends the 'for' loop in the 'loop' function and means 'number' is now '9', and then calls all of the 'each' closures, which all return the final value of 'number'.

The 'eagerly' and 'lazily' names may be a bit confusing (they were to me). What they refer to is whether we eagerly or lazily call each closure as it is returned by 'loop()'. In 'eagerly', we call the closures immediately; in 'lazily', we call them only later, after the 'for' loop is done and 'number' has taken on its final value. As Glyph said on the Fediverse, there is another level of eagerness or laziness, which is how aggressively we iterate the generator from 'loop()', and this is actually backward from the names; in 'eagerly' we lazily iterate the generator, while in 'lazily' we eagerly iterate the generator (that's what the 'list()' does).

(I'm writing this entry partly for myself, because someday I may run into an issue like this in my own Python code. If you only use a generator with code patterns like the 'eagerly' case, an issue like this could lurk undetected for some time.)

Written on 16 June 2024.
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Last modified: Sun Jun 16 23:31:25 2024
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