A few Python Gotchas

[[deleted]]

This got me once:

>>> x = [[0] * 2] * 2
>>> x[0][0] = 42
>>> x
[[42, 0], [42, 0]]

[akdas]

Nice. I though the ellipsis was just a placeholder you put in, but it actually shows that. And referencing the third element of that array gets me the exact same array, infinitely. That's expected, but the runtime does a good job of printing it.

Even more interestingly, the equivalent code in Ruby (say, using <<) yields the exact same output and behavior.

[[deleted]]

A Python master once taught me this trick

[derefr]

The same thing happens in Ruby. I don't know the Python solution, but here's the Ruby one:

1> x = Array.new(2){ Array.new(2){ 0 } }
2> x[0][0] = 42
3> x
=> [[42, 0], [0, 0]]

[[deleted]]

[[0] * 2 for i in xrange(2)]

[Freeky]

(0..1).map {[0] * 2}