Cell-var-from-loop warning from Pylint

Question

For the following code:

for sort_key, order in query_data['sort']:
    results.sort(key=lambda k: get_from_dot_path(k, sort_key),
                 reverse=(order == -1))

Pylint reported an error:

Cell variable sort_key defined in loop (cell-var-from-loop)

Could anyone give a hint what is happening here? From pylint source code the description is:

A variable used in a closure is defined in a loop. This will result in all closures using the same value for the closed-over variable.

But I do not have a clue what it means. Could anyone give an example of the problem?

What kind of object is `results`? Ordinary list? Something else? — Kevin, Aug 14 '14 at 17:59
Ok. In that case, I agree with chepner that you don't need to worry about the warning here. — Kevin, Aug 14 '14 at 18:19

score 127 · Accepted Answer · edited Jun 13 '19 at 17:36

127

The name sort_key in the body of the lambda will be looked up when the function is actually called, so it will see the value sort_key had most recently. Since you are calling sort immediately, the value of sort_key will not change before the resulting function object is used, so you can safely ignore the warning. To silence it, you can make sort_key the default value of a parameter to the lambda:

results.sort(key=lambda k, sk=sort_key: get_from_dot_path(k, sk),
             reverse=(order == -1))

edited Jun 13 '19 at 17:36

drew

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answered Aug 14 '14 at 18:04

chepner

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I'd err on the side of fixing the issue instead of ignoring the warning. If possible, I'd use `key=partial(get_from_dot_path, foo=sort_key)` instead of the lambda expression (assuming there is some parameter name `foo` defined by `get_from_dot_path` that you can use for a keyword argument; `partial` only allows filling in positional parameters exclusively from the left). – chepner Dec 13 '18 at 15:17
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Ah I didn't realise this would fix it, I thought they were equivalent; in that case I agree. – Tim Diels Dec 14 '18 at 14:42
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be aware that currently the trick does not always work https://github.com/PyCQA/pylint/issues/3107 – Daniel Pinyol Sep 16 '19 at 13:39
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Using [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal) did _not_ help, I had to use [partial()](https://docs.python.org/3/library/functools.html#functools.partial) as suggested by @chepner. – Jens Feb 16 '21 at 10:49

score 7 · Answer 2 · answered Jun 10 '21 at 20:46

7

Use functools.partial():

import functools
results.sort(key=functools.partial(get_from_dot_path, foo=sort_key),
             reverse=(order == -1))

answered Jun 10 '21 at 20:46

mejdev

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can confirm this not only gets rid of the warning, but really fixes the problem. Try for example making a list of lambdas print the range 1,10 - with lambdas, they all print 9, with functools.partial they print each value – TamaMcGlinn Jun 21 '21 at 12:11

Cell-var-from-loop warning from Pylint

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