使用串联：
x = ([1] if conditional else []) + [3, 4]
演示：
>>> conditional = False
>>> ([1] if conditional else []) + [3, 4]
[3, 4]
>>> conditional = True
>>> ([1] if conditional else []) + [3, 4]
[1, 3, 4]
当然，这个概念也适用于更多元素：
x = ([1, 2, 3] if conditional else []) + [4, 5, 6]
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