问题描述
根据 Python 3.5 文档，subprocess.run() 返回一个 CompletedProcess 对象，它的 stdout 成员包含一个字节序列，或者如果 run() 被调用时使用了universal_newlines=True，则为一个字符串".我只看到一个字节序列而不是一个字符串，我假设(希望)它相当于一个文本行.例如，
According to the Python 3.5 docs, subprocess.run() returns an a CompletedProcess object with a stdout member that contains "A bytes sequence, or a string if run() was called with universal_newlines=True." I'm only seeing a byte sequence and not a string, which I was assuming (hoping) would be equivalent to a text line. For example,
import pprint
import subprocess
my_data = ""
line_count = 0
proc = subprocess.run(
args = [ 'cat', 'input.txt' ],
universal_newlines = True,
stdout = subprocess.PIPE)
for text_line in proc.stdout:
my_data += text_line
line_count += 1
word_file = open('output.txt', 'w')
pprint.pprint(my_data, word_file)
pprint.pprint(line_count, word_file)
注意:这使用了 Python 3.5 中的一项新功能，该功能不会在以前的版本中运行.
Note: this uses a new feature in Python 3.5 that won't run in previous versions.
我是否需要创建自己的行缓冲逻辑，或者有没有办法让 Python 为我做这件事?
Do I need to create my own line buffering logic, or is there a way to get Python to do that for me?
推荐答案
proc.stdout 在你的情况下已经是一个字符串，运行 print(type(proc.stdout))，以确保.它包含所有子进程的输出——subprocess.run() 直到子进程死亡才返回.
proc.stdout is already a string in your case, run print(type(proc.stdout)), to make sure. It contains all subprocess' output -- subprocess.run() does not return until the child process is dead.
for text_line in proc.stdout: 不正确:for char in text_string 枚举 Python 中的字符(Unicode 代码点)，而不是行.要获取线路，请致电:
for text_line in proc.stdout: is incorrect: for char in text_string enumerates characters (Unicode codepoints) in Python, not lines. To get lines, call:
lines = result.stdout.splitlines()
如果字符串中有 Unicode 换行符，结果可能与 .split('
') 不同.
The result may be different from .split('
') if there are Unicode newlines in the string.
如果你想逐行读取输出(以避免长时间运行的进程耗尽内存):
If you want to read the output line by line (to avoid running out of memory for long-running processes):
from subrocess import Popen, PIPE
with Popen(command, stdout=PIPE, universal_newlines=True) as process:
for line in process.stdout:
do_something_with(line)
注意:process.stdout 在这种情况下是一个类似文件的对象.Popen() 不等待进程完成——Popen() 在子进程启动后立即返回.process 是一个 subprocess.Popen 实例，这里不是 CompletedProcess.
Note: process.stdout is a file-like object in this case. Popen() does not wait for the process to finish -- Popen() returns immidiately as soon as the child process is started. process is a subprocess.Popen instance, not CompletedProcess here.
如果您只需要计算输出中的行数(以 b'
' 结尾)，例如 wc -l:
If all you need is to count the number of lines (terminated by b'
') in the output, like wc -l:
from functools import partial
with Popen(command, stdout=PIPE) as process:
read_chunk = partial(process.stdout.read, 1 << 13)
line_count = sum(chunk.count(b'
') for chunk in iter(read_chunk, b''))
请参阅为什么在 C++ 中从标准输入读取行比 Python 慢得多?
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