python - 在 Python 中写入空文件之前如何检查条件？

Question

我需要读取 data.csv 中的所有行，如果 City 列的值为 Los Angeles，那么它会将该行添加到 match.csv，但我不希望它创建带有标题的 match.csv如果 data.csv 的“城市”列中没有包含洛杉矶的行，则为空行。我该怎么做？

with open("data.csv", 'r') as input_file, open("match.csv", 'w') as output_file:
    data_reader = csv.DictReader(input_file)
    match_write = csv.DictWriter(output_file, delimiter=',', fieldnames=data_reader.fieldnames)
    match_write.writeheader()
    for row in data_reader:
        if row['City'] == "Los Angeles":
            match_write.writerow(row)

Answer 1

如果您只是不想创建 header ，并且可以使用空文件，那么在洛杉矶的第一个实例之前不要创建编写器。例如，除非有包含 Los Angeles 的行，否则不会将 header 添加到文件 match.csv 中:

match_write = None

with open("data.csv", 'r') as input_file, open("match.csv", 'w') as output_file:
    data_reader = csv.DictReader(input_file)
    for row in data_reader:
        if row['City'] == "Los Angeles":
            if match_write is None:
                match_write = csv.DictWriter(output_file, delimiter=',', fieldnames=data_reader.fieldnames)
                match_write.writeheader()
            match_write.writerow(row)

但是，如果除非存在洛杉矶实例，否则根本不应该创建 match.csv 文件，那么在第一个实例之前也不应打开该文件。

output_file = None

with open("data.csv", 'r') as input_file:
    data_reader = csv.DictReader(input_file)
    for row in data_reader:
        if row['City'] == "Los Angeles":
            if output_file is None:
                output_file = open("match.csv", 'w')
                match_write = csv.DictWriter(output_file, delimiter=',', fieldnames=data_reader.fieldnames)
                match_write.writeheader()
            match_write.writerow(row)

if output_file is not None:
    output_file.close()