python - 如何在python中创建字典列表

Question

我对 Python 比较陌生，实际上是对整个编程而言。不幸的是，我还没有在论坛上找到我的问题的答案。

我有一个包含不同文件扩展名的列表，文件扩展名出现多次。请参阅此处的示例:

extensions = ["JPG", "XLSX", "MP3", "PDF", "EXE", "PY", "XLSX", "DOCX", "JPG", "PPTX"]

我想使用上面的列表创建一个新的字典列表。它应该是这样的:

dicts = [{"Extension": "py", "Count": 1}, {"Extension": "docx", "Count": 1}]

我的计划是遍历列表并将文件扩展名作为新字典附加到新列表中，如上面的代码行所示。如果扩展已作为字典存在于字典列表中，则只有匹配字典的索引 ["Count"] 应以 +=1 递增。我已经写了下面的代码，但是它不起作用。

我知道函数中的空扩展列表是一个问题，但我仍然没有让它按预期工作。我将不胜感激。

extensions = ["JPG", "XLSX", "MP3", "PDF", "EXE", "PY", "XLSX", "DOCX", "JPG", "PPTX"]


def get_extensions(extensions):
    extensionlist = []
    for item in extensions:
        extension = item.lower()
        for dictionary in extensionlist:
            if dictionary["Extension"] == extension:
                dictionary["Count"] += 1
                break
            else:
                extensionlist.append({"Extension": extension, "Count": 1})
                break
    return extensionlist


test = get_extensions(extensions)
print(test)

Answer 1

您可以使用 Counter 构建频率表然后对其进行迭代以构建您的列表:

from collections import Counter

extensions = ["JPG", "XLSX", "MP3", "PDF", "EXE", "PY", "XLSX", "DOCX", "JPG", "PPTX"]

frequencies = Counter(extensions)

# Build a list of dicts using a list comprehension. Not 
# really sure why you'd want it in this format (rather 
# than a dictionary).
output = [
    { "Extension": ext.lower(), "Count": freq }
    for ext, freq in frequencies.items()
]

如果您想使用 for 循环“手动”执行此操作，我建议采用类似的方法:首先构造一个包含频率计数扩展键的字典，然后 然后 构造列表:

frequencies = {}

for extension in extensions:
    # d.get(key, default) is like [], except it
    # returns default if key is not in d (rather than 
    # throwing a KeyError).
    frequencies[extension] = frequencies.get(extension, 0) + 1

# This is less idiomatic than the list comprehension 
# shown above, but it's the same end result.
output = []
for extension, frequency in frequencies.items():
    output.append(...)

这比你的双 for 循环更好，因为它是一次通过 extensions 然后第二次通过 frequencies。即使您当前的实现有效，您每次都会对列表进行线性扫描，您需要确定它是否已经包含特定的扩展名(因此，在最坏的情况下，您需要检查输出列表中的 1, 2, ..., n 元素是否有您的 m 扩展)。