在Python中使用for循环从列表构建字典

Question

I have multiple format files in a directory. I am trying to build a list or dictionary to group similar formatted (extension) files in python using for loop, but somehow it is not working.

Here is my sample code:

extension = ['pdf','xlsx','doc']

file_name_path=[]
file_dict ={}
for i in range(len(extension)):
    for file_name in filelst:
        if os.path.splitext(file_name)[-1] == extension[i]:
            file_name_path.append(file_name)
            file_dict[str(extension[i])]= file_name_path


file_name_path   
file_dict

where filelst is a list having all file names for example like

filelst = 
['PD_CFS_PLL_OnMonSummary_2017Q2.xlsx',
 'PD_Detailed_OMR_PLL_Lines_2017Q2.xlsx',
 'PD_Detailed_OMR_PLL_Loans_2017Q2.xlsx',
 'regexp-tip-sheet.pdf',
 'SAS statistical-business-analyst certification .pdf']

Answer 1

Alternative method to get dictionary with extensions as keys

extension = ['.pdf','.xlsx','.doc']
filelist = ['one.pdf','two.pdf','three.doc','four.xlsx'] #just for example
d = dict()
for i in extension:
    d[i] = [j for j in filelist if os.path.splitext(j)[-1].lower()==i]
print(d)

output:

{'.doc': ['three.doc'], '.xlsx': ['four.xlsx'], '.pdf': ['one.pdf', 'two.pdf']}

Note that I used dots in extension list as os.path.splitext return list with last element being '.extension' . .lower() is used to made this solution case-insensitive, strings in extension list have to contain lowercase characters only.

Answer 2

You are overwriting the dictionaries value each time the same key is seen.

Instead, use a list as the value and append. This is what defaultdict is for.

from collections import defaultdict

extension = ['pdf','xlsx','doc']

file_dict = defaultdict(list)

for file_name in filelst:
    ext = os.path.splitext(file_name)[-1].lower()
    if ext in extension:
        file_dict[ext].append(file_name)