子进程“TypeError:需要一个类似字节的对象，而不是'str'"

问题描述

我正在使用 几年前提出的问题中的这段代码，但是，我认为这已经过时了.尝试运行代码，我收到上面的错误.我仍然是 Python 的新手，所以我无法从类似的问题中得到太多的澄清.有谁知道为什么会这样?

I'm using this code from a previously asked question a few years ago, however, I believe this is outdated. Trying to run the code, I receive the error above. I'm still a novice in Python, so I could not get much clarification from similar questions. Does anyone know why this is happening?

import subprocess

def getLength(filename):
  result = subprocess.Popen(["ffprobe", filename],
    stdout = subprocess.PIPE, stderr = subprocess.STDOUT)
  return [x for x in result.stdout.readlines() if "Duration" in x]

print(getLength('bell.mp4'))

追溯

Traceback (most recent call last):
  File "B:Program Filesffmpegin	est3.py", line 7, in <module>
    print(getLength('bell.mp4'))
  File "B:Program Filesffmpegin	est3.py", line 6, in getLength
    return [x for x in result.stdout.readlines() if "Duration" in x]
  File "B:Program Filesffmpegin	est3.py", line 6, in <listcomp>
    return [x for x in result.stdout.readlines() if "Duration" in x]
TypeError: a bytes-like object is required, not 'str'

推荐答案

subprocess 默认为 stdout 或 stderr 流返回 bytes 对象.这意味着您还需要在针对这些对象的操作中使用 bytes 对象."Duration" in x 使用 str 对象.使用字节文字(注意 b 前缀):

subprocess returns bytes objects for stdout or stderr streams by default. That means you also need to use bytes objects in operations against these objects. "Duration" in x uses str object. Use a bytes literal (note the b prefix):

return [x for x in result.stdout.readlines() if b"Duration" in x]

或首先解码您的数据，如果您知道使用的编码(通常是语言环境默认设置，但您可以 为子进程设置 LC_ALL 或更具体的语言环境变量):

or decode your data first, if you know the encoding used (usually, the locale default, but you could set LC_ALL or more specific locale environment variables for the subprocess):

return [x for x in result.stdout.read().decode(encoding).splitlines(True)
        if "Duration" in x]

另一种方法是通过将 encoding 参数设置为合适的编解码器来告诉 subprocess.Popen() 将数据解码为 Unicode 字符串:

The alternative is to tell subprocess.Popen() to decode the data to Unicode strings by setting the encoding argument to a suitable codec:

result = subprocess.Popen(
    ["ffprobe", filename],
    stdout=subprocess.PIPE, stderr = subprocess.STDOUT,
    encoding='utf8'
)

如果您设置 text=True(Python 3.7 及更高版本，在以前的版本中，此版本称为 universal_newlines)，您还可以使用 系统默认编解码器，和open() 调用.在这种模式下，管道默认是行缓冲的.

If you set text=True (Python 3.7 and up, in previous versions this version is called universal_newlines) you also enable decoding, using your system default codec, the same one that is used for open() calls. In this mode, the pipes are line buffered by default.

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