(1)删除列表元素引起的下标变化错误
演示代码:
x = list(range(10)) for i in range(len(x)): if x[i]%2 == 0: del x[i]
出错信息:
Traceback (most recent call last): File "C:\Python36\bar.py", line 3, in <module> if x[i]%2 == 0: IndexError: list index out of range
出错原因分析:
Python列表在增加或删除元素时,会自动进行内存的扩展和收缩,从而保证元素连续。这会导致插入和删除位置之后的元素下标发生变化。上面的代码不仅会抛出异常,还会因为下标的变化而导致有些元素被跳过没有处理。例如:
x = list(range(10)) + [10]*3 for i in range(len(x)): print(x) if x[i]%2 == 0: del x[i]
运行结果为:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10] [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10] [1, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10] [1, 3, 5, 6, 7, 8, 9, 10, 10, 10] [1, 3, 5, 7, 8, 9, 10, 10, 10] [1, 3, 5, 7, 9, 10, 10, 10] [1, 3, 5, 7, 9, 10, 10] [1, 3, 5, 7, 9, 10] Traceback (most recent call last): File "C:\Python36\bar.py", line 4, in <module> if x[i]%2 == 0: IndexError: list index out of range
解决方案:
在涉及列表部分元素删除之类的操作时,应从后往前删除,避免因为下标变化导致的问题。例如:
x = list(range(10)) + [10]*3 for i in range(len(x)-1, -1, -1): print(x) if x[i]%2 == 0: del x[i] print(x)
运行结果:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [0, 1, 2, 3, 4, 5, 6, 7, 9] [0, 1, 2, 3, 4, 5, 6, 7, 9] [0, 1, 2, 3, 4, 5, 7, 9] [0, 1, 2, 3, 4, 5, 7, 9] [0, 1, 2, 3, 5, 7, 9] [0, 1, 2, 3, 5, 7, 9] [0, 1, 3, 5, 7, 9] [0, 1, 3, 5, 7, 9] [1, 3, 5, 7, 9]
(2)不可下标错误
演示代码:
>>> x = map(str, range(20)) >>> x[3] Traceback (most recent call last): File "<pyshell#28>", line 1, in <module> x[3] TypeError: 'map' object is not subscriptable >>> x = enumerate('hello world') >>> x[3] Traceback (most recent call last): File "<pyshell#30>", line 1, in <module> x[3] TypeError: 'enumerate' object is not subscriptable >>> x = set(range(20)) >>> x[5] Traceback (most recent call last): File "<pyshell#32>", line 1, in <module> x[5] TypeError: 'set' object does not support indexing >>> x = zip(range(5), range(5)) >>> x[3:5] Traceback (most recent call last): File "<pyshell#34>", line 1, in <module> x[3:5] TypeError: 'zip' object is not subscriptable
出错原因分析:
Python列表、元组、字符串等有序非惰性序列支持按位置随机访问,而字典、集合这样的无序序列不支持按位置随机访问其中的元素。map、zip、enumerate、filter、生成器对象等类似的有序、惰性对象只能按序访问,在前面的元素访问之前不支持按位置直接访问后面的元素,也不支持切片操作,并且已访问过的元素不可以再次访问。例如:
>>> x = map(str, range(20)) >>> next(x) '0' >>> next(x) '1' >>> '3' in x True >>> '3' in x False >>> '5' in x False