41 lines
1.3 KiB
Plaintext
41 lines
1.3 KiB
Plaintext
From: dacut at kanga.org (David Cuthbert)
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Date: Fri, 23 Apr 1999 20:32:41 -0400
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Subject: Bug or Feature?
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References: <37208E69.4B022E0C@mediaone.net>
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Message-ID: <7fr3eg$bqr@world1.bellatlantic.net>
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X-UID: 152
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Fuming Wang <fmwang at mediaone.net> wrote:
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> I found this little surprise with Python 1.5.1:
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> >list = [[0]*2]*4
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> >list
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> [[0, 0], [0, 0], [0, 0], [0, 0]]
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> >list[0][1] = 9
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> >list
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> [[0, 9], [0, 9], [0, 9], [0, 9]]
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> Is this a bug or a feature that I don't know about?
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Most definitely a feature. You're getting the same reference for all four
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list elements. To break it apart:
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list = [[0] * 2] * 4 # let a = 0, an integer.
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[[a] * 2] * 4 # integers are immutable, so a copy is made
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# for [a] * 2; let the copy be b.
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[[a, b]] * 4 # ah, but [a, b] is a list, q.
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[ q ] * 4 # lists are mutable, so we're just expanding
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# references that all refer to the same
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# q object.
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[ q, q, q, q ] # Ta da! All four elements have the same
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# object.
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If this were not the case, lists would be completely unnecessary since they
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would be the same as tuples!
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