Chapter 19 - The subprocess Module¶
The subprocess module gives the developer the ability to start processes or programs from Python. In other words, you can start applications and pass arguments to them using the subprocess module. The subprocess module was added way back in Python 2.4 to replace the os modules set of os.popen, os.spawn and os.system calls as well as replace popen2 and the old commands module. We will be looking at the following aspects of the subprocess module:
- the call function
- the Popen class
- how to communicate with a spawned process
Let’s get started!
The call function¶
The subprocess module provides a function named call. This function allows you to call another program, wait for the command to complete and then return the return code. It accepts one or more arguments as well as the following keyword arguments (with their defaults): stdin=None, stdout=None, stderr=None, shell=False.
Let’s look at a simple example:
>>> import subprocess >>> subprocess.call("notepad.exe") 0
If you run this on a Windows machine, you should see Notepad open up. You will notice that IDLE waits for you to close Notepad and then it returns a code zero (0). This means that it completed successfully. If you receive anything except for a zero, then it usually means you have had some kind of error.
Normally when you call this function, you would want to assign the resulting return code to a variable so you can check to see if it was the result you expected. Let’s do that:
>>> code = subprocess.call("notepad.exe") >>> if code == 0: print("Success!") else: print("Error!") Success!
If you run this code, you will see that it prints out Success! to the screen. The call method also accepts arguments to be passed to the program that you’re executing. Let’s see how that works:
>>> code = subprocess.call(["ping", "www.yahoo.com"]) Pinging ds-any-fp3-real.wa1.b.yahoo.com [220.127.116.11] with 32 bytes of data: Reply from 18.104.22.168: bytes=32 time=66ms TTL=45 Reply from 22.214.171.124: bytes=32 time=81ms TTL=45 Reply from 126.96.36.199: bytes=32 time=81ms TTL=45 Reply from 188.8.131.52: bytes=32 time=69ms TTL=45 Ping statistics for 184.108.40.206: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 66ms, Maximum = 81ms, Average = 74ms >>> code 0
You will notice that in this example we are passing a list of arguments. The first item in the list is the program we want to call. Anything else in the list are arguments that we want to pass to that program. So in this example, we are executing a ping against Yahoo’s website. You will notice that the return code was zero, so everything completed successfully.
You can also execute the program using the operating system’s shell. This does add a level of abstraction to the process and it raises the possibility of security issues. Here is the Python documentation’s official warning on the matter:
Executing shell commands that incorporate unsanitized input from an untrusted source makes a program vulnerable to shell injection, a serious security flaw which can result in arbitrary command execution. For this reason, the use of shell=True is strongly discouraged in cases where the command string is constructed from external input.
The usual recommendation is not to use it if an outside process or person can modify the call’s arguments. If you’re hard-coding something yourself, then it doesn’t matter as much.
The Popen Class¶
The Popen class executes a child program in a new process. Unlike the call method, it does not wait for the called process to end unless you tell it to using by using the wait method. Let’s try running Notepad through Popen and see if we can detect a difference:
>>> program = "notepad.exe" >>> subprocess.Popen(program) <subprocess.Popen object at 0x01EE0430>
Here we create a variable called program and assign it the value of “notepad.exe”. Then we pass it to the Popen class. When you run this, you will see that it immediately returns the subprocess.Popen object and the application that was called executes. Let’s make Popen wait for the program to finish:
>>> program = "notepad.exe" >>> process = subprocess.Popen(program) >>> code = process.wait() >>> print(code) 0
When you do this in IDLE, Notepad will pop up and may be in front of your IDLE session. Just move Notepad out of the way, but do not close it! You need to tell your process to wait for you won’t be able to get the return code. Once you’ve typed that line, close Notepad and print the code out. Or you could just put all this code into a saved Python file and run that.
Note that using the wait method can cause the child process to deadlock when using the stdout/stderr=PIPE commands when the process generates enough output to block the pipe. You can use the communicate method to alleviate this situation. We’ll be looking at that method in the next section.
Now let’s try running Popen using multiple arguments:
>>> subprocess.Popen(["ls", "-l"]) <subprocess.Popen object at 0xb7451001>
If you run this code in Linux, you’ll see it print out the Popen object message and then a listing of the permissions and contents of whatever folder you ran this in. You can use the shell argument with Popen too, but the same caveats apply to Popen as they did to the call method.
Learning to Communicate¶
There are several ways to communicate with the process you have invoked. We’re just going to focus on how to use the subprocess module’s communicate method. Let’s take a look:
args = ["ping", "www.yahoo.com"] process = subprocess.Popen(args, stdout=subprocess.PIPE) data = process.communicate() print(data)
In this code example, we create an args variable to hold our list of arguments. Then we redirect standard out (stdout) to our subprocess so we can communicate with it. The communicate method itself allows us to communicate with the process we just spawned. We can actually pass input to the process using this method. But in this example, we just use communicate to read from standard out. You will notice when you run this code that communicate will wait for the process to finish and then returns a two-element tuple that contains what was in stdout and stderr. Here is the result from my run:
('Pinging ds-any-fp3-real.wa1.b.yahoo.com [220.127.116.11] with 32 bytes of data: Reply from 18.104.22.168: bytes=32 time=139ms TTL=45 Reply from 22.214.171.124: bytes=32 time=162ms TTL=45 Reply from 126.96.36.199: bytes=32 time=164ms TTL=45 Reply from 188.8.131.52: bytes=32 time=110ms TTL=45 Ping statistics for 184.108.40.206: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 110ms, Maximum = 164ms, Average = 143ms ', None)
That’s kind of ugly. Let’s make it print out the result in a more readable format, shall we?
import subprocess args = ["ping", "www.yahoo.com"] process = subprocess.Popen(args, stdout=subprocess.PIPE) data = process.communicate() for line in data: print(line)
If you run this code, you should see something like the following printed to your screen:
Pinging ds-any-fp3-real.wa1.b.yahoo.com [220.127.116.11] with 32 bytes of data: Reply from 18.104.22.168: bytes=32 time=67ms TTL=45 Reply from 22.214.171.124: bytes=32 time=68ms TTL=45 Reply from 126.96.36.199: bytes=32 time=70ms TTL=45 Reply from 188.8.131.52: bytes=32 time=69ms TTL=45 Ping statistics for 184.108.40.206: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 67ms, Maximum = 70ms, Average = 68ms None
That last line that says “None” is the result of stderr, which means that there were no errors.
At this point, you have the knowledge to use the subprocess module effectively. You can open a process in two different ways, you know how to wait for a return code, and you know how to communicate with the child process that you created.
In the next chapter, we will be looking at the sys module.