在python中读取PNG的字节

Question

我想读取 PNG 的字节，并修改它们。我已经尝试过这段代码：

import codecs
ed = input("Would you like to encode or decode?\n(e/d)> ")
image = input("What image would you like to use?\n> ")
if(ed == 'e'):
    imagef = codecs.open(image, encoding = "hex")
    imagel = imagef.read()
    img = imagel.decode('hex')
    print(img)

但我收到以下错误：

Would you like to encode or decode?
(e/d)> e
What image would you like to use?
> i.png
Traceback (most recent call last):
  File "main.py", line 7, in <module>
    imagel = imagef.read()
  File "/usr/local/Cellar/python/3.6.5/Frameworks/Python.framework/Versions/3.6/lib/python3.6/codecs.py", line 700, in read
    return self.reader.read(size)
  File "/usr/local/Cellar/python/3.6.5/Frameworks/Python.framework/Versions/3.6/lib/python3.6/codecs.py", line 503, in read
    newchars, decodedbytes = self.decode(data, self.errors)
  File "/usr/local/Cellar/python/3.6.5/Frameworks/Python.framework/Versions/3.6/lib/python3.6/encodings/hex_codec.py", line 25, in decode
    return hex_decode(input, errors)
  File "/usr/local/Cellar/python/3.6.5/Frameworks/Python.framework/Versions/3.6/lib/python3.6/encodings/hex_codec.py", line 19, in hex_decode
    return (binascii.a2b_hex(input), len(input))

binascii.Error: Non-hexadecimal digit found

我该如何解决这个问题？

Answer 1

退一步 - 你的假设不正确

png 文件没有“十六进制内容”——它们有字节——“十六进制”不是你想的那样，它只是表示数字的另一种方式。

您的代码无法修复，因为它首先没有任何意义 - 没有什么可解码的。

程序员使用十六进制是为了方便，因为每个十六进制数字 可以表示 4 位，因此只需要两个十六进制数字来表示一个字节。然而，这只是代表。

编辑：

从 cmets 看来，您想要的是将二进制数据添加到文件末尾：

message = 'Hello World!'

with open(image, 'ab') as f: # open the file for appending, binary mode
    f.write(bytes(8))  # write 8 null bytes to the file
    f.write(message.encode('utf-8'))  # add the message, encoded to bytes
                                      # by using the utf-8 encoding

然后再次阅读消息：

with open(image, 'rb') as f: # open the file in read binary mode
    data = f.read() # read the bytes from the file to a variable

pos = data.find(bytes(8)) # locate the 8 null bytes
message = data[pos + 8:].decode('utf-8')  # decode the message