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21.01. Programming/Python/subprocess.md

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+### 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)