带有 If Else 的 Python 嵌套列表理解 [英] Python Nested List Comprehension with If Else

问题描述

我试图使用列表理解来替换值列表中的多个可能的字符串值.

I was trying to use a list comprehension to replace multiple possible string values in a list of values.

我有一个列名列表，这些列名取自 cursor.description ;

I have a list of column names which are taken from a cursor.description;

['UNIX_Time', 'col1_MCA', 'col2_MCA', 'col3_MCA', 'col1_MCB', 'col2_MCB', 'col3_MCB']

然后我有 header_replace ;

{'MCB': 'SourceA', 'MCA': 'SourceB'}

我想用这些值替换在列名中找到的 header_replace.keys()的字符串值.

I would like to replace the string values for header_replace.keys() found within the column names with the values.

我不得不使用以下循环；

I have had to use the following loop;

headers = []
for header in cursor.description:
    replaced = False
    for key in header_replace.keys():
        if key in header[0]:
            headers.append(str.replace(header[0], key, header_replace[key]))
            replaced = True
            break

    if not replaced:
        headers.append(header[0])

哪个给我正确的输出；

Which gives me the correct output;

['UNIX_Time', 'col1_SourceA', 'col2_SourceA', 'col3_SourceA', 'col1_SourceB', 'col2_SourceB', 'col3_SourceB']

我尝试使用此列表理解；

I tried using this list comprehension;

[str.replace(i[0],k,header_replace[k]) if k in i[0] else i[0] for k in header_replace.keys() for i in cursor.description]

但这意味着对于不匹配的密钥重复了一些项目，我会得到;

But it meant that items were duplicated for the unmatched keys and I would get;

['UNIX_Time', 'col1_MCA', 'col2_MCA', 'col3_MCA', 'col1_SourceA', 'col2_SourceA', 'col3_SourceA', 
'UNIX_Time', 'col1_SourceB', 'col2_SourceB', 'col3_SourceB', 'col1_MCB', 'col2_MCB', 'col3_MCB']

但是，如果我使用；

[str.replace(i[0],k,header_replace[k]) for k in header_replace.keys() for i in cursor.description if k in i[0]]

@Bakuriu 固定语法

我会得到正确的替换，但然后放掉不需要替换字符串的所有项目.

I would get the correct replacement but then loose any items that didn't need to have an string replacement.

['col1_SourceA', 'col2_SourceA', 'col3_SourceA', 'col1_SourceB', 'col2_SourceB', 'col3_SourceB']

是否有执行此操作的pythonesque方式?我是否在扩展列表理解范围?我当然觉得它们很难阅读.

Is there a pythonesque way of doing this or am I over stretching list comprehensions? I certainly find them hard to read.

推荐答案

[str.replace(i[0],k,header_replace[k]) if k in i[0] for k in header_replace.keys() for i in cursor.description]

这是一个 SyntaxError ，因为 if 表达式必须包含 else 部分.你可能的意思是:

this is a SyntaxError, because if expressions must contain the else part. You probably meant:

[i[0].replace(k, header_replace[k]) for k in header_replace for i in cursor.description if k in i[0]]

以 if 结尾.但是我必须说，嵌套循环的列表理解通常不是要走的路.我将使用扩展的 for 循环.实际上，我会改进它以除去 replaced 标志:

With the if at the end. However I must say that list-comprehension with nested loops aren't usually the way to go. I would use the expanded for loop. In fact I'd improve it removing the replaced flag:

headers = []
for header in cursor.description:
    for key, repl in header_replace.items():
        if key in header[0]:
            headers.append(header[0].replace(key, repl))
            break
    else:
        headers.append(header[0])

for 循环的 else 在迭代过程中未触发任何 break 时执行.

The else of the for loop is executed when no break is triggered during the iterations.

我不明白为什么在您的代码中使用 str.replace(字符串，子字符串，替换)而不是 string.replace(子字符串，替换).字符串具有 instance 方法，因此就这样使用它们，而不是像它们是该类的静态方法一样.

I don't understand why in your code you use str.replace(string, substring, replacement) instead of string.replace(substring, replacement). Strings have instance methods, so you them as such and not as if they were static methods of the class.

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