为什么 csv.DictWriter 只返回字典的最后一行？

Question

I am really new at python and I'm writing a script to move some data to a CSV file. Data are taken from a dictionary where keys are fixed and values are functions. This is all inside a for loop iterating files from which those data are collected.

This is what I've tried:

import os, csv

for folder, files in os.walk(directory):
    
     for file in files:
           def name(file):
                name = file.split(".", 1)[0]
           def type(file):
                if file.endswith("a"):
                   return("file_a")
                elif file.endswith("b"):
                   return("file_b")
           def size(file):
                size = os.path.getsize(os.path.join(directory,file))
           
           my_dict = {"name": name(file), "type": type(file), "size": size(file)}
      
           with open("csv_file.csv", "w", newline="") as csv_file:
                fieldnames = ("name", "type", "size")
                writer = csv.DictWriter(csv_file, fieldnames=fieldnames)
                writer.writeheader()
                writer.writerows(my_dict)

Answer 1

The issue here is that writer.writeheader() overwrites the entire file. After each row, you are calling this function that overwrites the entire file, so only the last row will still be there at the end .

As mentioned in the comments, the actual issue is that you are opening the file with 'w' permissions in every iteration of the for loop, but you would want to append to the file, hence simply replacing 'w' by 'a' should give you the wanted result, however, the following solution is still a cleaner approach!

import os, csv

# export opening of the CSV file outside of the loop
# that way it will only be opened once
# use "a" for "append" instead of "w" for write/overwrite
# when you want to add data to an existing file
with open("csv_file.csv", "w", newline="") as csv_file:
    fieldnames = ("name", "type", "size")
    writer = csv.DictWriter(csv_file, fieldnames=fieldnames)
    writer.writeheader()
    
    for folder, files in os.walk(directory):
        for file in files:
           def name(file):
               name = file.split(".", 1)[0]
           def type(file):
               if file.endswith("a"):
                   return("file_a")
               elif file.endswith("b"):
                   return("file_b")
           def size(file):
                size = os.path.getsize(os.path.join(directory,file))
           
           my_dict = {"name": name(file), "type": type(file), "size": size(file)}
           writer.writerows(my_dict)