如何在Python脚本中实现CSV对比后将新数据追加至历史文件？

Got it, let's tweak your script to handle appending the new entries to allhistory.csv seamlessly. I'll show you two versions—one that stays close to your original code, and a more robust one using the csv module properly (better for edge cases like weird formatting or headers).

First version: Minimal changes to your existing code #

This keeps your original comparison logic but adds the append step without re-reading the update.csv file (more efficient):

import csv

# Compare files and collect new entries first
with open('allhistory.csv', 'r') as t1, open('filewithnewdata.csv', 'r') as t2:
    fileone = t1.readlines()
    filetwo = t2.readlines()

new_entries = [line for line in filetwo if line not in fileone]

# Write to update.csv (your original output)
with open('update.csv', 'w') as outFile:
    outFile.writelines(new_entries)

# Append new entries to allhistory.csv
if new_entries:  # Only run if there's actual new data to add
    with open('allhistory.csv', 'a', newline='') as history_file:
        history_file.writelines(new_entries)

Key tweaks here:

• Stored the new entries in a variable first so we don't have to read update.csv again later
• Used a (append) mode for allhistory.csv—this adds content to the end instead of overwriting it
• Added newline='' to avoid extra blank lines showing up on Windows (a common CSV gotcha)
• Added a check for empty new_entries to skip unnecessary file operations

Second version: More robust (using csv module properly) #

If your CSV files have headers, quoted fields, or inconsistent line endings, using the csv module's reader/writer is safer. This avoids false positives for "duplicate" lines caused by formatting differences:

import csv

def get_csv_rows(file_path):
    """Helper function to read all rows from a CSV file"""
    with open(file_path, 'r', newline='') as f:
        return list(csv.reader(f))

# Read rows from both files
history_rows = get_csv_rows('allhistory.csv')
new_data_rows = get_csv_rows('filewithnewdata.csv')

# Find unique new rows (convert to tuples since lists can't be stored in sets)
history_row_set = set(tuple(row) for row in history_rows)
new_entries = [row for row in new_data_rows if tuple(row) not in history_row_set]

# Write new entries to update.csv
with open('update.csv', 'w', newline='') as outfile:
    writer = csv.writer(outfile)
    writer.writerows(new_entries)

# Append to allhistory.csv if there's new data
if new_entries:
    with open('allhistory.csv', 'a', newline='') as history_file:
        writer = csv.writer(history_file)
        writer.writerows(new_entries)

Why this is better:

• Handles CSV-specific formatting (like fields with commas inside quotes) correctly
• Doesn't get confused by different line endings (\n vs \r\n)
• Makes it easier to handle headers if needed (e.g., you could skip the first row of filewithnewdata.csv if you don't want to re-add headers)

Just pick whichever version fits your use case—both will get the job done of appending your new entries to the history file!

内容的提问来源于stack exchange，提问作者user1959064