How is subprocess.Popen used in Python?

subprocess.Popen is a function used to create a new process, allowing for the execution of specific operations by calling system commands or other executable files.

Here are some common parameters used with subprocess.Popen:

1. args: a string or a list containing strings, used to specify the command or executable file to be executed and its parameters.
2. bufsize: an optional integer parameter that can be used to set the size of the buffer, with a default value of 0 (no buffer).
3. stdin, stdout, stderr: These are used to specify the standard input, standard output, and standard error streams, respectively. They can be file objects or integers representing the corresponding file descriptors. The default value is None, indicating they inherit the corresponding stream from the parent process.
4. shell: an optional boolean parameter that specifies whether to execute the command through a shell. The default value is False.
5. Universal_newlines is an optional boolean parameter that specifies whether to convert input and output text mode to universal newlines (\n). The default value is False.

subprocess.Popen creates a Popen object, which can be used to interact with the new process using its methods and properties. Some commonly used methods and properties include:

1. interact with a child process by sending data (if available) and waiting for it to complete. The method returns a tuple containing standard output and standard error output.
2. wait(): Wait for the child process to finish and return its exit status.
3. poll(): checks if the child process has terminated. If it has, it returns the exit status; otherwise, it returns None.
4. end() : end the child process.
5. terminate(): kill the child process.
6. exit status of child process

Here is a simple example using subprocess.Popen:

import subprocess

# 执行一个命令并获取输出
result = subprocess.Popen(["ls", "-l"], stdout=subprocess.PIPE)
output = result.communicate()[0].decode("utf-8")
print(output)

# 执行一个命令并等待其完成
subprocess.Popen(["ls", "-l"]).wait()
print("Done")

In the above example, the ls -l command was first executed using subprocess.Popen, redirecting the standard output to a pipe using the stdout=subprocess.PIPE parameter. The output of the command was then obtained using the communicate() method, converted to a string, and printed out. Next, another ls -l command was executed using subprocess.Popen, and the wait() method was used to wait for the command to complete. After the command finished, “Done” was printed.