multiple python functions in one line causing problems

Question

Why would these two lines of code do anything differently from each other?

visited_with_path.append(deep_copy(get_path_to_point(from_point)).append(point))

and

    x = deep_copy(get_path_to_point(from_point))
    x.append(point)
    visited_with_path.append(x)

I just spent a whole lot of time debugging until I tried making the above change (from first to second), and although it fixed the problem, I have no idea why. They seem like they should have identical functionality.

If it helps, visited_with_path is a list of lists of points, deep_copy makes a deep copy of this list of lists, get_path_to_point inputs a point and returns a list of points, and from_point and point are just points.

Any thoughts?

Even if append returned a reference to the list being appended to, that one-liner is far too unreadable to be recommended. Plus, different people might assume different things: I would assume append would return a reference to the list so that you could append multiple items at a time; you assume it returns a reference to the time being appended. — chepner
– chepner, Commented Sep 26, 2014 at 18:12

a p · Accepted Answer · 2014-09-26 18:02:10Z

3

.append doesn't return anything - it modifies the list in place, so you're basically appending deep_copy(get_path_to_point(from_point)) to visited_with_path, which returns None, and then trying to .append(point) to None, which doesn't work.

answered Sep 26, 2014 at 18:02

a p

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1 Comment

aruisdante Over a year ago

The fact that functions with no explicit return return None rather than raising some sort of continuity error has tripped up many a person when debugging.

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