Comment by mharig
12 days ago
No. The code is completely consistent: the closures are different instances of a function that returns the value of 'number' at the moment it gets called. Not the value of 'number' when it gets created. And it gets called differently when a list comprehension calls it each step of the loop or when the list function loops completely through and then the list comprehension calls the instances. Nothing to see here.
> No. The code is completely consistent: the closures are different instances of a function that returns the value of 'number' at the moment it gets called. Not the value of 'number' when it gets created.
Isn't that exactly what GP said? And gave an example how to create that effect explicitly in functional languages, using refs?
Yes. But I found the explanations of the GP a little misleading, especially the eager/lazy part.
Naively I would expect that each iteration of a for loop creates a new loop variable (all with the same name, but effectively each in their own scope) and so each closure holds a reference to a variable named number whose value never changes, and there is a distinct one of these number variables per closure .
That is not how variables in Python work. Variable assignment is just name binding. This is clearly mentioned in every Python beginner tutorial I know of. Python code is a script. Follow the script and you know what Python does (with only a few exceptions, see e.g. below).
About the scope: Python knows in principal only 2 scopes: local and global. If you define a function (or a class) Python creates a new emtpy/filled with the function arguments local scope (system table). That gets filled successively by local variable assignments etc. There happens a little bit of magic: if you use a variable of the next higher scope, it gets copied (!) to the local scope. You can follow that easily by print(locals()). (You can also use the global scope, but that is another topic).
Edit: had to clarify some points
It's consistent, but unexpected. Especially if you don't know python well enough. And it leads to ugly effect sometimes, even for experienced pythonistas.
Yes, this bit me last week. I stumble over it about once a year. Recently encountered it in an open source project and filed an issue. I don't know where I fall on the "experienced" spectrum, but I've written a lot of python in the last 10 years.
No.