### subprocess.Popen
```python
import subprocess

process = subprocess.Popen(['echo', 'More output'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
stdout, stderr = process.communicate()
stdout, stderr
```

Input arguments is a list.
Notice `communicate()` will **block** until process was finished.
And the output string `stdout` and `stderr` is of type `byte`. You can convert the output to `string` by:
```python
new_string = stdout.decode('utf-8')
```
or use `universal_newlines=True` in `subprocess.Popen()`. Example:
```python
process = subprocess.Popen(['ping', '-c 4', 'python.org'], 
                           stdout=subprocess.PIPE,
                           universal_newlines=True)
```
The `.poll()` will return the exit code of process. If process is still running. `.poll()` will return `None`. Example:
```python
process = subprocess.Popen(['ping', '-c 4', 'python.org'], stdout=subprocess.PIPE, universal_newlines=True)

while True:
    output = process.stdout.readline()
    print(output.strip())
    # Do something else
    return_code = process.poll()
    if return_code is not None:
        print('RETURN CODE', return_code)
        # Process has finished, read rest of the output 
        for output in process.stdout.readlines():
            print(output.strip())
        break
```

-----
參考：
- [docs.python.org: `subprocess.Popen`](https://docs.python.org/3/library/subprocess.html#subprocess.Popen)

### subprocess.run
`subprocess.run()`跟`subprocess.Popen()`是一樣的行為，差別是`subprocess.run()`會在process執行完畢之後才return，也就是說流程會被block住。
`subprocess.run()`會回傳一個型別是`subprocess.CompletedProcess`的object.

-----
參考：
- [docs.python.org: _class_ `subprocess.CompletedProcess`](https://docs.python.org/3/library/subprocess.html#subprocess.CompletedProcess)