如何修改Python PIL脚本仅处理PNG格式图片文件

我用从Stack Overflow找到的代码裁剪文件夹里的图片并保存到另一个文件夹，在Windows 11的Jupyter Notebook中运行。复制文件到Linux根目录子文件夹后，系统自动生成了Zone.Identifier文件，导致脚本尝试处理这些非图片文件，手动删除后脚本正常运行。

我的原代码：

from PIL import Image
import os.path

SOURCE_DIRECTORY = "/home/andy/crop/nyc/"
TARGET_DIRECTORY = "/home/andy/crop/output/nyc/"
directory_list = os.listdir(SOURCE_DIRECTORY)

def crop():
    for source_file in directory_list:
        source_path = os.path.join(SOURCE_DIRECTORY, source_file) 
        if os.path.isfile(source_path):
            raw_image = Image.open(source_path)
            file_name = os.path.basename(source_path)
            file_name, extension = os.path.splitext(file_name)

            image_cropped_top = raw_image.crop((344, 340, 474, 519))
            image_cropped_top.save(TARGET_DIRECTORY + file_name+'.png', "PNG", quality=100)

crop()

运行后报错：

---------------------------------------------------------------------------
UnidentifiedImageError                    Traceback (most recent call last)
Input In [39], in <cell line: 20>()
     17             image_cropped_top = raw_image.crop((344, 340, 474, 519))
     18             image_cropped_top.save(TARGET_DIRECTORY + file_name+'.png', "PNG", quality=100)
---> 20 crop()

Input In [39], in crop()
     11 source_path = os.path.join(SOURCE_DIRECTORY, source_file) 
     12 if os.path.isfile(source_path):
---> 13     raw_image = Image.open(source_path)
     14     file_name = os.path.basename(source_path)
     15     file_name, extension = os.path.splitext(file_name)

File ~/miniconda3/lib/python3.9/site-packages/PIL/Image.py:3147, in open(fp, mode, formats)
   3145 for message in accept_warnings:
   3146     warnings.warn(message)
---> 3147 raise UnidentifiedImageError(
   3148     "cannot identify image file %r" % (filename if filename else fp)
   3149 )

UnidentifiedImageError: cannot identify image file '/home/andy/crop/nyc/DUDE.png:Zone.Identifier'

我倾向于通过代码仅选择PNG文件进行裁剪，请问该如何操作？

修改方案 #

只需在代码中添加文件扩展名判断，只处理.png（包括大小写.PNG）格式的文件，以下是两种可行的修改方式：

方式一：循环内添加判断 #

from PIL import Image
import os.path

SOURCE_DIRECTORY = "/home/andy/crop/nyc/"
TARGET_DIRECTORY = "/home/andy/crop/output/nyc/"
directory_list = os.listdir(SOURCE_DIRECTORY)

def crop():
    for source_file in directory_list:
        source_path = os.path.join(SOURCE_DIRECTORY, source_file) 
        # 先判断是否为文件，再筛选PNG格式（不区分大小写）
        if os.path.isfile(source_path):
            _, extension = os.path.splitext(source_file)
            if extension.lower() == '.png':
                raw_image = Image.open(source_path)
                file_name = os.path.splitext(os.path.basename(source_path))[0]
                
                image_cropped_top = raw_image.crop((344, 340, 474, 519))
                # 用os.path.join拼接路径，避免跨平台问题
                image_cropped_top.save(os.path.join(TARGET_DIRECTORY, f"{file_name}.png"), "PNG", quality=100)

crop()

方式二：提前筛选PNG文件 #

直接在获取文件列表时过滤出PNG文件，循环内无需再做判断，代码更简洁：

from PIL import Image
import os.path

SOURCE_DIRECTORY = "/home/andy/crop/nyc/"
TARGET_DIRECTORY = "/home/andy/crop/output/nyc/"
# 提前筛选出目录下的PNG文件（不区分大小写）
directory_list = [
    f for f in os.listdir(SOURCE_DIRECTORY) 
    if os.path.isfile(os.path.join(SOURCE_DIRECTORY, f)) 
    and f.lower().endswith('.png')
]

def crop():
    for source_file in directory_list:
        source_path = os.path.join(SOURCE_DIRECTORY, source_file) 
        raw_image = Image.open(source_path)
        file_name = os.path.splitext(os.path.basename(source_path))[0]

        image_cropped_top = raw_image.crop((344, 340, 474, 519))
        image_cropped_top.save(os.path.join(TARGET_DIRECTORY, f"{file_name}.png"), "PNG", quality=100)

crop()

关键优化点： #

• 加入扩展名小写转换判断，兼容.PNG、.Png等大小写变体
• 用os.path.join拼接文件路径，避免手动拼接字符串出现的路径分隔符问题，提升代码跨平台性

内容的提问来源于stack exchange，提问作者Andrew D. Clay